Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

cmd/jobsearch/main.go

@@ -52,17 +52,16 @@ opportunities.
 		PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
 			cli.SetupInitialLogging(cmd)
 
-			slog.Debug("root.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "root")
 			slog.Debug(
-				"command: \"" +
+				"command",
-					strings.Join(strings.Split(cmd.CommandPath(), " ")[1:], " ") +
+				"path", strings.Join(strings.Split(cmd.CommandPath(), " ")[1:], " "),
-					"\"",
 			)
 
 			return nil
 		},
 		PersistentPostRunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("root.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "root")
 
 			return opts.Finish()
 		},

cmd/mock-fts/main.go

@@ -24,6 +24,7 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"log/slog"
 	"net/http"
 	"os"
 	"os/signal"
@@ -206,15 +207,13 @@ func run(ctx context.Context, o *options) (err error) {
 	if err != nil {
 		return err
 	}
-	log.Printf(
+	slog.Info(
-		"catalog: %d buyers, %d suppliers, %d subjects, %d methods, %d classifications",
+		"catalog loaded",
-		len(
+		"buyers", len(cat.Buyers),
-			cat.Buyers,
+		"suppliers", len(cat.Suppliers),
-		),
+		"subjects", len(cat.Subjects),
-		len(cat.Suppliers),
+		"methods", len(cat.Methods),
-		len(cat.Subjects),
+		"classifications", len(cat.Classifications),
-		len(cat.Methods),
-		len(cat.Classifications),
 	)
 
 	// ----- store -----
@@ -223,7 +222,7 @@ func run(ctx context.Context, o *options) (err error) {
 		if err := os.Remove(storePath); err != nil && !errors.Is(err, os.ErrNotExist) {
 			return err
 		}
-		log.Print("store: reset")
+		slog.Info("store reset", "path", storePath)
 	}
 	store, err := mock.OpenStore(storePath)
 	if err != nil {
@@ -238,7 +237,7 @@ func run(ctx context.Context, o *options) (err error) {
 		return err
 	}
 
-	log.Printf("store: %d releases (%d generated to catch up)", store.Len(), n)
+	slog.Info("store ready", "releases", store.Len(), "generated", n)
 
 	// The generator and server share a context that also ends if the server
 	// fails, so the generator is never left running after run returns.
@@ -286,11 +285,11 @@ func run(ctx context.Context, o *options) (err error) {
 		)
 		defer cancelShutdown()
 		if err := httpSrv.Shutdown(shutdownCtx); err != nil {
-			log.Printf("shutdown: %v", err)
+			slog.Error("shutdown failed", "error", err)
 		}
 	}()
 
-	log.Printf("FTS mock listening on %s%s (query times in %s)", o.addr, mock.BasePath, loc)
+	slog.Info("mock listening", "addr", o.addr, "path", mock.BasePath, "time_zone", loc.String())
 	serveErr := httpSrv.ListenAndServe()
 
 	cancel() // stop the worker (and the shutdown goroutine) if the server failed

internal/cli/config/command.go

@@ -14,10 +14,10 @@ func NewCommand(opts *cli.Options) *cobra.Command {
 		Use:   "config",
 		Short: "Configuration management",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "config")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "config")
 		},
 	}
 	cmd.AddCommand(commandShow(opts))

internal/cli/config/get.go

@@ -15,13 +15,13 @@ func commandGet(opts *cli.Options) *cobra.Command {
 		Args:  cobra.ExactArgs(1),
 		Short: "Get a value from configuration",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.get.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "config.get")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.get.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "config.get")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("config.get.Run")
+			slog.Debug("command run", "command", "config.get")
 
 			if err := cobra.ExactArgs(1)(cmd, args); err != nil {
 				return err

internal/cli/config/init.go

@@ -19,13 +19,13 @@ func commandInit() *cobra.Command {
 		Args:  cobra.MatchAll(cobra.MinimumNArgs(0), cobra.MaximumNArgs(1)),
 		Short: "Create an empty configuration file",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.init.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "config.init")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.init.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "config.init")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("config.init.Run")
+			slog.Debug("command run", "command", "config.init")
 
 			if err := cobra.MinimumNArgs(0)(cmd, args); err != nil {
 				return err

internal/cli/config/set.go

@@ -18,13 +18,13 @@ func commandSet(opts *cli.Options) *cobra.Command {
 		Args:  cobra.MatchAll(cobra.MinimumNArgs(1), cobra.MaximumNArgs(2)),
 		Short: "Set a value to configuration",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.set.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "config.set")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.set.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "config.set")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("config.set.Run")
+			slog.Debug("command run", "command", "config.set")
 
 			if err := cobra.MinimumNArgs(1)(cmd, args); err != nil {
 				return err

internal/cli/config/show.go

@@ -14,13 +14,13 @@ func commandShow(opts *cli.Options) *cobra.Command {
 		Use:   "show",
 		Short: "Print config information",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.show.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "config.show")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("config.show.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "config.show")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("config.show.Run")
+			slog.Debug("command run", "command", "config.show")
 
 			conf, err := opts.RunSetup(cmd, true, true)
 			if err != nil {

internal/cli/confirm_action.go

@@ -3,7 +3,7 @@ package cli
 import (
 	"bufio"
 	"fmt"
-	"log"
+	"log/slog"
 	"os"
 	"strings"
 )
@@ -19,7 +19,8 @@ func Confirm(s string) bool {
 
 		response, err := reader.ReadString('\n')
 		if err != nil {
-			log.Fatal(err)
+			slog.Error("reading the answer", "error", err)
+			os.Exit(1)
 		}
 
 		response = strings.ToLower(strings.TrimSpace(response))

internal/cli/fetch/command.go

@@ -14,10 +14,10 @@ func NewCommand(opts *cli.Options) *cobra.Command {
 		Use:   "fetch",
 		Short: "Fetch current opportunities",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("fetch.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "fetch")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("fetch.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "fetch")
 		},
 	}
 	cmd.AddCommand(commandOnce(opts))

internal/cli/fetch/once.go

@@ -53,13 +53,13 @@ func commandOnce(opts *cli.Options) *cobra.Command {
 		Use:   "once",
 		Short: "Fetch current opportunities - single shot",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("fetch.once.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "fetch.once")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("fetch.once.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "fetch.once")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("fetch.once.Run")
+			slog.Debug("command run", "command", "fetch.once")
 
 			conf, err := opts.RunSetup(cmd, true, true)
 			if err != nil {

internal/cli/fetch/service.go

@@ -20,13 +20,13 @@ func commandService(opts *cli.Options) *cobra.Command {
 		Use:   "service",
 		Short: "Fetch current opportunities - background service",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("fetch.service.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "fetch.service")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("fetch.service.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "fetch.service")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("fetch.service.Run")
+			slog.Debug("command run", "command", "fetch.service")
 
 			foreground, _ := cmd.Flags().GetBool("foreground")
 

internal/cli/migrate/down.go

@@ -21,13 +21,13 @@ func commandDown(opts *cli.Options) *cobra.Command {
 confirmed by providing -yes.`,
 		Args: cobra.NoArgs,
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.down.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "migrate.down")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.down.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "migrate.down")
 		},
 		RunE: func(cmd *cobra.Command, args []string) (err error) {
-			slog.Debug("migrate.down.Run")
+			slog.Debug("command run", "command", "migrate.down")
 
 			yes, _ := cmd.Flags().GetBool("yes")
 			steps, _ := cmd.Flags().GetUint("steps")

internal/cli/migrate/migrate.go

@@ -26,13 +26,13 @@ Running without a subcommand is the same as running "migrate up": every pending
 migration is applied.`,
 		Args: cobra.NoArgs,
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "migrate")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "migrate")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("migrate.Run")
+			slog.Debug("command run", "command", "migrate")
 			return runStepUp(cmd, opts, 0, false)
 		},
 	}

internal/cli/migrate/status.go

@@ -75,13 +75,13 @@ func commandStatus(opts *cli.Options) *cobra.Command {
 		Short: "Show which migrations have been applied to the SQLite database",
 		Args:  cobra.NoArgs,
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.status.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "migrate.status")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.status.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "migrate.status")
 		},
 		RunE: func(cmd *cobra.Command, args []string) (err error) {
-			slog.Debug("migrate.status.Run")
+			slog.Debug("command run", "command", "migrate.status")
 
 			m, db, err := openMigrator(cmd, opts, false, false)
 			if err != nil {

internal/cli/migrate/up.go

@@ -46,13 +46,13 @@ func commandUp(opts *cli.Options) *cobra.Command {
 		Short: "Migrate SQLite database to the latest version, or by a number of steps",
 		Args:  cobra.NoArgs,
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.up.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "migrate.up")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("migrate.up.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "migrate.up")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("migrate.up.Run")
+			slog.Debug("command run", "command", "migrate.up")
 
 			steps, _ := cmd.Flags().GetUint("steps")
 			create, _ := cmd.Flags().GetBool("create")

internal/cli/options.go

@@ -65,7 +65,7 @@ func (o *Options) RunSetup(
 		// Check if the logging should be switched to file
 		if !terminalLog {
 			if len(conf.Logging.File) == 0 {
-				slog.Warn("No log file configured, remaining in console logging mode!")
+				slog.Warn("no log file configured; logging to the terminal")
 			} else if err := logging.LogToFile(conf.Logging.File); err != nil {
 				return nil, fmt.Errorf("opening log file: %w", err)
 			}

internal/cli/version/command.go

@@ -16,13 +16,13 @@ func NewCommand() *cobra.Command {
 		Short:        "Print version information",
 		SilenceUsage: true,
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("version.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "version")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("version.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "version")
 		},
 		Run: func(cmd *cobra.Command, args []string) {
-			slog.Debug("version.Run")
+			slog.Debug("command run", "command", "version")
 
 			fmt.Printf("JobSearch - %s\n", version.Version)
 			if version.BuildTime != "unknown" {

internal/cli/wsserve/command.go

@@ -22,13 +22,13 @@ func NewCommand(opts *cli.Options) *cobra.Command {
 		Use:   "wsserve",
 		Short: "Run JobSearch TCP Websocket server",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("wsserve.PersistentPreRun")
+			slog.Debug("command persistent pre-run", "command", "wsserve")
 		},
 		PersistentPostRun: func(cmd *cobra.Command, args []string) {
-			slog.Debug("wsserve.PersistentPostRun")
+			slog.Debug("command persistent post-run", "command", "wsserve")
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
-			slog.Debug("wsserve.Run")
+			slog.Debug("command run", "command", "wsserve")
 
 			addrOverride, _ := cmd.Flags().GetString("listen")
 			hostOverride, _ := cmd.Flags().GetString("host")
@@ -74,8 +74,8 @@ func NewCommand(opts *cli.Options) *cobra.Command {
 				stop()
 			}()
 
-			hub := websocket.NewHub(conf.Server.WebSocket, slog.Default())
+			hub := websocket.NewHub(conf.Server.WebSocket)
-			svr := tcpserver.New(&conf.Server, hub, slog.Default())
+			svr := tcpserver.New(&conf.Server, hub)
 
 			return svr.Run(ctx)
 		},

internal/config/config.go

@@ -70,11 +70,11 @@ func LoadFromViper(conf *JobSearchConfig) {
 		}
 		value, ok := viperValue(key, keyType)
 		if !ok {
-			slog.Error(fmt.Sprintf("unknown key type: %s", keyType))
+			slog.Error("unknown config key type", "key", key, "kind", keyType.String())
 			continue
 		}
 		if err := conf.SetValue(key, value); err != nil {
-			slog.Warn("ignoring invalid config value", "key", key, "err", err)
+			slog.Warn("ignoring invalid config value", "key", key, "error", err)
 		}
 	}
 }

internal/config/debug.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"context"
 	"fmt"
 	"log/slog"
 	"os"
@@ -9,41 +10,81 @@ import (
 	"strings"
 
 	"github.com/spf13/viper"
+
+	"git.avsdev.uk/Go/job-search/internal/logging"
 )
 
-// printEnv logs the JOBSEARCH* environment variables at debug level.
+// printEnv logs each JOBSEARCH* environment variable at debug level. On a
+// terminal it logs one line and prints the variables under it as
+// "name=value" lines; elsewhere each variable is a log entry of its own.
 func printEnv() {
-	slog.Debug("Environment:")
+	logger := slog.Default()
-	env := os.Environ()
+	terminal := logging.WritesToTerminal(logger.Handler())
-	slices.Sort(env)
+	if terminal && !logger.Enabled(context.Background(), slog.LevelDebug) {
-	for _, e := range env {
+		return
-		pair := strings.SplitN(e, "=", 2)
+	}
-		if strings.HasPrefix(pair[0], EnvPrefix) {
+
-			slog.Debug("\t" + pair[0] + "=" + pair[1])
+	var variables []string
+	for _, variable := range os.Environ() {
+		if strings.HasPrefix(variable, EnvPrefix) {
+			variables = append(variables, variable)
 		}
 	}
+	slices.Sort(variables)
+
+	if terminal {
+		logger.Debug("environment variables", "count", len(variables))
+	}
+	for _, variable := range variables {
+		if terminal {
+			logging.PrintToTerminal("\t" + variable)
+			continue
+		}
+		name, value, _ := strings.Cut(variable, "=")
+		logger.Debug("environment variable", "name", name, "value", value)
+	}
 }
 
 // printLoadedConfig logs every key set by the config file or an environment
-// variable (see isExplicitlySet), with its value and type, at debug level.
+// variable (see isExplicitlySet), with its value and type, at debug level. On
+// a terminal it logs one line and prints the values under it as
+// "key = value <type>" lines; elsewhere each value is a log entry of its own.
 func printLoadedConfig() {
-	slog.Debug("Loaded config:")
+	logger := slog.Default()
-	keys := viper.AllKeys()
+	terminal := logging.WritesToTerminal(logger.Handler())
-	for i := range keys {
+	if terminal && !logger.Enabled(context.Background(), slog.LevelDebug) {
-		k := keys[i]
+		return
-		if isExplicitlySet(k) {
+	}
-			v := viper.Get(k)
+
-			t := reflect.TypeOf(v)
+	var keys []string
-			slog.Debug(
+	for _, key := range viper.AllKeys() {
-				"\t" + k + " = " + fmt.Sprintf("%#v", v) +
+		if isExplicitlySet(key) {
-					" <" + t.Kind().String() + ">",
+			keys = append(keys, key)
-			)
 		}
 	}
+
+	if terminal {
+		logger.Debug("loaded config values", "count", len(keys))
+	}
+	for _, key := range keys {
+		value := viper.Get(key)
+		kind := reflect.TypeOf(value).Kind().String()
+		if terminal {
+			logging.PrintToTerminal(
+				fmt.Sprintf("\t%s = %#v <%s>", key, value, kind),
+			)
+			continue
+		}
+		logger.Debug(
+			"loaded config value",
+			"key", key,
+			"value", value,
+			"type", kind,
+		)
+	}
 }
 
 // printConfig logs conf at debug level.
 func printConfig(conf JobSearchConfig) {
-	slog.Debug("Config:")
 	conf.PrintLog(slog.LevelDebug)
 }

internal/config/load.go

@@ -68,14 +68,14 @@ func loadConfig(cfgFile string) error {
 		if _, ok := err.(viper.ConfigFileNotFoundError); ok {
 			// Indicates an explicitly set config file is not found or
 			// that no config file was found in any search path
-			slog.Warn("No config file found!")
+			slog.Warn("no config file found; using defaults")
 		} else {
 			// Config file was found but another error was produced
 			return err
 		}
 	} else {
 		// Config file found and successfully parsed
-		slog.Info("Using config file: " + viper.ConfigFileUsed())
+		slog.Info("using config file", "file", viper.ConfigFileUsed())
 	}
 	return nil
 }

internal/config/load_test.go

@@ -93,7 +93,7 @@ func TestInitNoConfigFile(t *testing.T) {
 	if err := Init("", &conf); err != nil {
 		t.Fatal(err)
 	}
-	if !strings.Contains(logs.String(), "No config file found") {
+	if !strings.Contains(logs.String(), "no config file found") {
 		t.Errorf("no warning logged: %s", logs)
 	}
 	if conf.Logging.Level != "warn" {
@@ -116,7 +116,7 @@ func TestInitKeepsCommandLineLevel(t *testing.T) {
 				t.Errorf("Logging.Level = %q; want %s kept", conf.Logging.Level, level)
 			}
 			// At debug, the environment and loaded config are logged.
-			gotDebug := strings.Contains(logs.String(), "Loaded config:")
+			gotDebug := strings.Contains(logs.String(), "loaded config value")
 			if gotDebug != (level == "debug") {
 				t.Errorf("debug output logged = %v at level %s: %s", gotDebug, level, logs)
 			}
@@ -241,9 +241,9 @@ func TestPrintLoadedConfig(t *testing.T) {
 
 	out := logs.String()
 	for _, want := range []string{
-		`logging.level = \"info\" <string>`,
+		`key=logging.level value=info type=string`,
-		`server.websocket.connection_limit.enabled = true <bool>`,
+		`key=server.websocket.connection_limit.enabled value=true type=bool`,
-		`database.file = \"/tmp/js.db\" <string>`,
+		`key=database.file value=/tmp/js.db type=string`,
 	} {
 		if !strings.Contains(out, want) {
 			t.Errorf("missing %q in:\n%s", want, out)

internal/config/print.go

@@ -1,10 +1,13 @@
 package config
 
 import (
+	"context"
 	"fmt"
 	"log/slog"
 	"maps"
 	"slices"
+
+	"git.avsdev.uk/Go/job-search/internal/logging"
 )
 
 // PrintTerminal prints the configuration to stdout as String formats it.
@@ -23,31 +26,33 @@ func (conf JobSearchConfig) PrintYaml() {
 }
 
 // PrintLog logs each configuration key and value, sorted by key, at atLevel:
-// debug, warn, error, or info for any other level.
+// debug, warn, error, or info for any other level. On a terminal it logs one
+// line and prints the values under it as "key = value" lines; elsewhere each
+// value is a log entry of its own.
 func (conf JobSearchConfig) PrintLog(atLevel slog.Level) {
-	var logFn func(args ...interface{})
-
 	switch atLevel {
-	case slog.LevelDebug:
+	case slog.LevelDebug, slog.LevelWarn, slog.LevelError:
-		logFn = func(args ...interface{}) {
-			slog.Debug(fmt.Sprint(args...))
-		}
-	case slog.LevelError:
-		logFn = func(args ...interface{}) {
-			slog.Error(fmt.Sprint(args...))
-		}
-	case slog.LevelWarn:
-		logFn = func(args ...interface{}) {
-			slog.Warn(fmt.Sprint(args...))
-		}
 	default:
-		logFn = func(args ...interface{}) {
+		atLevel = slog.LevelInfo
-			slog.Info(fmt.Sprint(args...))
-		}
 	}
 
+	ctx := context.Background()
+	logger := slog.Default()
 	flat := conf.Flatten()
-	for _, k := range slices.Sorted(maps.Keys(flat)) {
+	keys := slices.Sorted(maps.Keys(flat))
-		logFn("\t", k, " = ", flat[k])
+
+	if logging.WritesToTerminal(logger.Handler()) {
+		if !logger.Enabled(ctx, atLevel) {
+			return
+		}
+		logger.Log(ctx, atLevel, "config values", "count", len(keys))
+		for _, key := range keys {
+			logging.PrintToTerminal(fmt.Sprintf("\t%s = %v", key, flat[key]))
+		}
+		return
+	}
+
+	for _, key := range keys {
+		logger.Log(ctx, atLevel, "config value", "key", key, "value", flat[key])
 	}
 }

internal/config/print_test.go

@@ -108,15 +108,15 @@ func TestPrintLog(t *testing.T) {
 				t.Errorf("PrintLog(%v) line %q; want %s", tt.at, l, tt.want)
 			}
 		}
-		if !strings.Contains(logs.String(), "logging.level = warn") {
+		if !strings.Contains(logs.String(), "key=logging.level value=warn") {
 			t.Errorf("PrintLog(%v) missing logging.level:\n%s", tt.at, logs)
 		}
-		// Each line is msg="\t<key> = <value>".
+		// Each line is msg="config value" key=<key> value=<value>.
 		flat := conf.Flatten()
 		var keys []string
 		for _, l := range lines {
-			_, msg, _ := strings.Cut(l, `msg="\t`)
+			_, rest, _ := strings.Cut(l, ` key=`)
-			key, _, _ := strings.Cut(msg, " = ")
+			key, _, _ := strings.Cut(rest, " ")
 			if _, ok := flat[key]; !ok {
 				t.Errorf("PrintLog(%v) line %q: no key", tt.at, l)
 			}

internal/fetcher/fetcher.go

@@ -14,16 +14,60 @@ import (
 
 var ErrNoNewReleases = fmt.Errorf("%w: no new releases", util.ErrWorkerNoWork)
 
+// Option configures Run.
+type Option func(*options)
+
+// options holds the settings Options change.
+type options struct {
+	log *slog.Logger
+}
+
+// WithLogger makes Run, its workers and their clients log to logger instead
+// of slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(opts *options) {
+		opts.log = logger
+	}
+}
+
+// logger returns the logger Run logs to: the given one, or slog.Default().
+// It isn't tagged here; each worker adds its source.
+func (opts options) logger() *slog.Logger {
+	if opts.log == nil {
+		return slog.Default()
+	}
+	return opts.log
+}
+
+// runFunc runs one source's worker until ctx is cancelled, logging to
+// logger.
+type runFunc func(
+	ctx context.Context,
+	logger *slog.Logger,
+	conf *config.JobSearchConfig,
+	since time.Time,
+) error
+
 // Run starts a worker for each enabled source and blocks until every worker
 // has stopped. Workers stop when ctx is cancelled, or when any worker stops
 // with an error, which starts a shutdown of all of them. The errors are
-// logged as they happen and returned joined; nil means a clean shutdown.
+// logged as they happen and returned joined; nil means a clean shutdown. It
+// logs to slog.Default() unless given WithLogger.
 func Run(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context,
+	conf *config.JobSearchConfig,
+	since time.Time,
+	opts ...Option,
 ) error {
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
+	var settings options
+	for _, opt := range opts {
+		opt(&settings)
+	}
+	logger := settings.logger().With("component", "fetcher")
+
 	var (
 		wg   sync.WaitGroup
 		mu   sync.Mutex
@@ -32,13 +76,13 @@ func Run(
 
 	start := func(
 		name string,
-		run func(context.Context, *config.JobSearchConfig, time.Time) error,
+		run runFunc,
 	) {
 		wg.Add(1)
 		go func() {
 			defer wg.Done()
-			if err := run(ctx, conf, since); err != nil {
+			if err := run(ctx, logger, conf, since); err != nil {
-				slog.Error("source stopped, shutting down", "source", name, "err", err)
+				logger.Error("source stopped, shutting down", "source", name, "error", err)
 				mu.Lock()
 				errs = append(errs, fmt.Errorf("%s: %w", name, err))
 				mu.Unlock()

internal/fetcher/fetcher_test.go

@@ -62,7 +62,7 @@ func confFor(apiRoot string) *config.JobSearchConfig {
 }
 
 func TestFetchFunctions(t *testing.T) {
-	fetchers := map[string]func(context.Context, *config.JobSearchConfig, time.Time) error{
+	fetchers := map[string]runFunc{
 		"FindAOpportunity": fetchFindAOpportunity,
 		"FTSMock":          fetchFTSMock,
 	}
@@ -91,7 +91,9 @@ func TestFetchFunctions(t *testing.T) {
 		for _, tc := range tests {
 			t.Run(fname+"/"+tc.name, func(t *testing.T) {
 				srv, hits := ftsServer(t, tc.status, tc.body)
-				err := fetch(context.Background(), confFor(srv.URL), time.Now().Add(-time.Hour))
+				err := fetch(
+					context.Background(), slog.Default(), confFor(srv.URL), time.Now().Add(-time.Hour),
+				)
 				if !tc.check(err) {
 					t.Fatalf("err = %v, want %s", err, tc.wantErr)
 				}
@@ -110,11 +112,11 @@ func TestFetchUsesConfiguredAPIRoot(t *testing.T) {
 	conf.Sources.FindATender.APIRoot = fat.URL
 	conf.Sources.FTSMock.APIRoot = mock.URL
 
-	fetchFindAOpportunity(context.Background(), conf, time.Time{})
+	fetchFindAOpportunity(context.Background(), slog.Default(), conf, time.Time{})
 	if fatHits.Load() != 1 || mockHits.Load() != 0 {
 		t.Fatalf("FindAOpportunity hits: fat=%d mock=%d", fatHits.Load(), mockHits.Load())
 	}
-	fetchFTSMock(context.Background(), conf, time.Time{})
+	fetchFTSMock(context.Background(), slog.Default(), conf, time.Time{})
 	if fatHits.Load() != 1 || mockHits.Load() != 1 {
 		t.Fatalf("FTSMock hits: fat=%d mock=%d", fatHits.Load(), mockHits.Load())
 	}
@@ -126,7 +128,8 @@ func TestFetchCancelled(t *testing.T) {
 	// Unroutable root: the request should never be waited on.
 	conf := confFor("http://192.0.2.1:9")
 	start := time.Now()
-	if err := fetchFTSMock(ctx, conf, time.Time{}); !errors.Is(err, context.Canceled) {
+	err := fetchFTSMock(ctx, slog.Default(), conf, time.Time{})
+	if !errors.Is(err, context.Canceled) {
 		t.Fatalf("err = %v, want context.Canceled", err)
 	}
 	if d := time.Since(start); d > time.Second {
@@ -163,7 +166,7 @@ func TestRunSleeperShutdown(t *testing.T) {
 		ctx, cancel := context.WithCancel(context.Background())
 		done := make(chan struct{})
 		go func() {
-			runSleeper(ctx, conf, time.Time{})
+			runSleeper(ctx, slog.Default(), conf, time.Time{})
 			close(done)
 		}()
 		time.Sleep(10 * time.Millisecond) // let the first sleep start
@@ -184,13 +187,13 @@ func TestRunSleeperShutdown(t *testing.T) {
 
 func TestRunWorkersRejectZeroInterval(t *testing.T) {
 	conf := &config.JobSearchConfig{}
-	runs := map[string]func(context.Context, *config.JobSearchConfig, time.Time) error{
+	runs := map[string]runFunc{
 		"FindAOpportunity": runFindAOpportunity,
 		"FTSMock":          runFTSMock,
 		"Sleeper":          runSleeper,
 	}
 	for name, run := range runs {
-		if err := run(context.Background(), conf, time.Time{}); err == nil {
+		if err := run(context.Background(), slog.Default(), conf, time.Time{}); err == nil {
 			t.Errorf("%s: expected an error for a zero interval", name)
 		}
 	}

internal/fetcher/worker_findatender.go

@@ -3,7 +3,6 @@ package fetcher
 import (
 	"context"
 	"errors"
-	"fmt"
 	"log/slog"
 	"time"
 
@@ -16,13 +15,14 @@ import (
 // runFindAOpportunity polls Find a Tender every configured interval until ctx
 // is cancelled.
 func runFindAOpportunity(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context, logger *slog.Logger, conf *config.JobSearchConfig, since time.Time,
 ) error {
 	w := util.Worker{
+		Logger:   logger,
 		Name:     "FindAOpportunity",
 		Interval: conf.Sources.FindATender.Interval,
 		Task: func(ctx context.Context, since time.Time) error {
-			return fetchFindAOpportunity(ctx, conf, since)
+			return fetchFindAOpportunity(ctx, logger, conf, since)
 		},
 	}
 	return w.Run(ctx, since)
@@ -32,10 +32,10 @@ func runFindAOpportunity(
 // Tender. Finding nothing returns ErrNoNewReleases, so the worker still moves
 // its checkpoint on.
 func fetchFindAOpportunity(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context, logger *slog.Logger, conf *config.JobSearchConfig, since time.Time,
 ) error {
 	opportunities, err := util.AwaitTask(ctx, func() ([]*types.Opportunity, error) {
-		ft := findatender.New(conf, conf.Sources.FindATender.APIRoot)
+		ft := findatender.New(conf, conf.Sources.FindATender.APIRoot, findatender.WithLogger(logger))
 		defer ft.Shutdown()
 		return ft.With(ctx).FetchAll(since, true)
 	})
@@ -46,7 +46,7 @@ func fetchFindAOpportunity(
 		return err
 	}
 
-	slog.Info(fmt.Sprintf("Found %d opportunities", len(opportunities)))
+	logger.Info("fetched opportunities", "count", len(opportunities))
 
 	return nil
 }

internal/fetcher/worker_ftsmock.go

@@ -3,7 +3,6 @@ package fetcher
 import (
 	"context"
 	"errors"
-	"fmt"
 	"log/slog"
 	"time"
 
@@ -16,13 +15,14 @@ import (
 // runFTSMock polls the mock Find a Tender server every configured interval
 // until ctx is cancelled.
 func runFTSMock(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context, logger *slog.Logger, conf *config.JobSearchConfig, since time.Time,
 ) error {
 	w := util.Worker{
+		Logger:   logger,
 		Name:     "FTSMock",
 		Interval: conf.Sources.FTSMock.Interval,
 		Task: func(ctx context.Context, since time.Time) error {
-			return fetchFTSMock(ctx, conf, since)
+			return fetchFTSMock(ctx, logger, conf, since)
 		},
 	}
 	return w.Run(ctx, since)
@@ -32,10 +32,10 @@ func runFTSMock(
 // Tender server. Finding nothing returns ErrNoNewReleases, so the worker still
 // moves its checkpoint on.
 func fetchFTSMock(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context, logger *slog.Logger, conf *config.JobSearchConfig, since time.Time,
 ) error {
 	opportunities, err := util.AwaitTask(ctx, func() ([]*types.Opportunity, error) {
-		ft := findatender.New(conf, conf.Sources.FTSMock.APIRoot)
+		ft := findatender.New(conf, conf.Sources.FTSMock.APIRoot, findatender.WithLogger(logger))
 		defer ft.Shutdown()
 		return ft.With(ctx).FetchAll(since, true)
 	})
@@ -46,7 +46,7 @@ func fetchFTSMock(
 		return err
 	}
 
-	slog.Info(fmt.Sprintf("Found %d opportunities", len(opportunities)))
+	logger.Info("fetched opportunities", "count", len(opportunities))
 
 	return nil
 }

internal/fetcher/worker_sleeper.go

@@ -2,6 +2,7 @@ package fetcher
 
 import (
 	"context"
+	"log/slog"
 	"time"
 
 	"git.avsdev.uk/Go/job-search/internal/config"
@@ -14,10 +15,11 @@ const sleepDuration = 60 * time.Second
 // runSleeper runs the sleeper test source until ctx is cancelled: every
 // configured interval it simulates work by sleeping for sleepDuration.
 func runSleeper(
-	ctx context.Context, conf *config.JobSearchConfig, since time.Time,
+	ctx context.Context, logger *slog.Logger, conf *config.JobSearchConfig, since time.Time,
 ) error {
 	blockShutdown := conf.Sources.Sleeper.BlockShutdown
 	w := util.Worker{
+		Logger:   logger,
 		Name:     "Sleeper",
 		Interval: conf.Sources.Sleeper.Interval,
 		Task: func(taskCtx context.Context, since time.Time) error {

internal/findatender/api.go

@@ -226,7 +226,9 @@ func missingIDsError(haveOCID, haveID bool) error {
 // releases, etc).
 //
 // This method takes ownership of the response body and closes it when done.
-func safeUnpackReleases(response *resty.Response) (*ocds.ReleasePackage, error) {
+func safeUnpackReleases(
+	logger *slog.Logger, response *resty.Response,
+) (*ocds.ReleasePackage, error) {
 	var pkg safeReleasePackage
 
 	// Take ownership of the response body.
@@ -263,9 +265,12 @@ func safeUnpackReleases(response *resty.Response) (*ocds.ReleasePackage, error)
 		err := unmarshalLenientDates(rawRel, &release)
 		if err != nil {
 			ocid, release_id, _ := releaseIDs(rawRel)
-			slog.Warn(fmt.Sprintf(
+			logger.Warn(
-				"Skipping release (%s/%s) due to error: %s", ocid, release_id, err,
+				"skipping unreadable release",
-			))
+				"ocid", ocid,
+				"release_id", release_id,
+				"error", err,
+			)
 			continue
 		}
 		out.Releases = append(out.Releases, release)
@@ -303,8 +308,9 @@ func closeBody(res *resty.Response) {
 // response status.
 func fetchURI(ft *FindATender, uri string) (*resty.Response, error) {
 	withTrace := ft.config.Logging.LogLevel() <= slog.LevelDebug
+	logger := ft.logger()
 
-	slog.Info("findTender.resty", "request_uri", uri)
+	logger.Info("fetching", "uri", uri)
 
 	ctx := ft.ctx
 	if ctx == nil {
@@ -316,23 +322,31 @@ func fetchURI(ft *FindATender, uri string) (*resty.Response, error) {
 	res, err := req.SetTrace(withTrace).Get(uri)
 
 	if withTrace {
-		if logging.IsJSONHandler(slog.Default().Handler()) {
+		logTrace(logger, res)
-			slog.Debug(
-				"Resty",
-				"trace", res.Request.TraceInfo().JSON(),
-				"status", res.Status(),
-				"response", res,
-			)
-		} else {
-			slog.Debug("Resty " + res.Request.TraceInfo().String())
-			slog.Debug("Resty status: " + res.Status())
-			slog.Debug("Resty response: " + res.String())
-		}
 	}
 
 	return res, err
 }
 
+// logTrace logs res's trace, status and body at debug level. On a terminal
+// the trace and body are printed as they are, as a field would escape them
+// into one line; elsewhere they are fields, the trace as JSON for the JSON
+// (file) logger.
+func logTrace(logger *slog.Logger, res *resty.Response) {
+	if logging.WritesToTerminal(logger.Handler()) {
+		logger.Debug("fetched", "status", res.Status())
+		logging.PrintToTerminal(res.Request.TraceInfo().String())
+		logging.PrintToTerminal(res.String())
+		return
+	}
+
+	trace := res.Request.TraceInfo().String()
+	if logging.IsJSONHandler(logger.Handler()) {
+		trace = res.Request.TraceInfo().JSON()
+	}
+	logger.Debug("fetched", "trace", trace, "status", res.Status(), "response", res.String())
+}
+
 // fetchURI performs a fetch against the API. If the API returns with a rate limit
 // reached response and allowRateLimit is true, then wait and retry like a good
 // client. If allowRateLimit is false, return to the caller informing them of
@@ -365,10 +379,9 @@ func (ft *FindATender) fetchURI(
 			return nil, fmt.Errorf("unknown retry-after value: %w", err)
 		}
 
-		slog.Warn(fmt.Sprintf(
+		retryAfter := time.Duration(retrySec) * time.Second
-			"API rate limit hit, waiting as requested for %d seconds", retrySec,
+		ft.logger().Warn("api rate limit hit, waiting as asked", "retry_after", retryAfter)
-		))
+		if rateSleep(ctx, retryAfter) != nil {
-		if rateSleep(ctx, time.Duration(retrySec)*time.Second) != nil {
 			return nil, ErrFetchCancelled
 		}
 
@@ -432,7 +445,7 @@ func (ft *FindATender) fetchOnce(opts RequestOptions) ([]ocds.Release, error) {
 		return nil, err
 	}
 
-	pkg, err := safeUnpackReleases(res)
+	pkg, err := safeUnpackReleases(ft.logger(), res)
 	if err != nil {
 		return nil, err
 	}
@@ -454,7 +467,7 @@ func (ft *FindATender) fetchAll(opts RequestOptions) ([]ocds.Release, error) {
 		return nil, err
 	}
 
-	pkg, err := safeUnpackReleases(res)
+	pkg, err := safeUnpackReleases(ft.logger(), res)
 	if err != nil {
 		return nil, err
 	}
@@ -466,9 +479,7 @@ func (ft *FindATender) fetchAll(opts RequestOptions) ([]ocds.Release, error) {
 	for {
 		if pkg.Links != nil && len(pkg.Links.Next) > 0 {
 			if seen[pkg.Links.Next] {
-				slog.Warn(fmt.Sprintf(
+				ft.logger().Warn("stopping at repeated next link", "link", pkg.Links.Next)
-					"Stopping at repeated next link: %s", pkg.Links.Next,
-				))
 				break
 			}
 			seen[pkg.Links.Next] = true
@@ -482,7 +493,7 @@ func (ft *FindATender) fetchAll(opts RequestOptions) ([]ocds.Release, error) {
 			return nil, err
 		}
 
-		pkg, err = safeUnpackReleases(res)
+		pkg, err = safeUnpackReleases(ft.logger(), res)
 		if err != nil {
 			return nil, err
 		}

internal/findatender/api_test.go

@@ -312,9 +312,14 @@ func TestFetchURIDebugTrace(t *testing.T) {
 			if len(releases) != 1 {
 				t.Errorf("got %d releases, want 1", len(releases))
 			}
-			if !strings.Contains(logs.String(), "Resty") {
+			if !strings.Contains(logs.String(), "msg=fetched") &&
+				!strings.Contains(logs.String(), `"msg":"fetched"`) {
 				t.Errorf("no trace logged: %s", logs.String())
 			}
+			// Every field must be loggable (a *resty.Response isn't, as JSON).
+			if strings.Contains(logs.String(), "!ERROR") {
+				t.Errorf("a field could not be logged: %s", logs.String())
+			}
 		})
 	}
 }
@@ -387,7 +392,7 @@ func TestSafeUnpackReleases(t *testing.T) {
 			{"ocid": "ocds-3", "id": "3", "date": "2026-10-08", "tag": [], "initiationType": "tender"}
 		]
 	}`
-	pkg, err := safeUnpackReleases(responseFor(t, body))
+	pkg, err := safeUnpackReleases(slog.Default(), responseFor(t, body))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -424,7 +429,7 @@ func TestSafeUnpackReleasesPublishedDate(t *testing.T) {
 			if tt.date != "" {
 				body = `{"publishedDate":` + tt.date + `,"releases":[]}`
 			}
-			pkg, err := safeUnpackReleases(responseFor(t, body))
+			pkg, err := safeUnpackReleases(slog.Default(), responseFor(t, body))
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -448,7 +453,7 @@ func TestSafeUnpackReleasesInvalidJSON(t *testing.T) {
 		"publisher array": `{"publisher": []}`,
 	} {
 		t.Run(name, func(t *testing.T) {
-			if _, err := safeUnpackReleases(responseFor(t, body)); err == nil {
+			if _, err := safeUnpackReleases(slog.Default(), responseFor(t, body)); err == nil {
 				t.Error("expected a decode error")
 			}
 		})

internal/findatender/findatender.go

@@ -18,21 +18,45 @@ type FindATender struct {
 	config     *config.JobSearchConfig
 	apiRoot    string
 	ctx        context.Context
+	log        *slog.Logger
+}
+
+// Option configures a FindATender client.
+type Option func(*FindATender)
+
+// WithLogger makes the client log to logger instead of slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(ft *FindATender) {
+		ft.log = logger
+	}
 }
 
 // New returns a client for the OCDS API at apiRoot. Requests time out after
-// 60 seconds and are retried once. Call Shutdown when finished with it.
+// 60 seconds and are retried once. Call Shutdown when finished with it. It
-func New(conf *config.JobSearchConfig, apiRoot string) *FindATender {
+// logs to slog.Default() unless given WithLogger.
+func New(conf *config.JobSearchConfig, apiRoot string, opts ...Option) *FindATender {
 	ft := &FindATender{
 		config:  conf,
 		apiRoot: apiRoot,
 	}
+	for _, opt := range opts {
+		opt(ft)
+	}
 	ft.httpClient = resty.New().
 		SetTimeout(60 * time.Duration(time.Second)).
 		SetRetryCount(1)
 	return ft
 }
 
+// logger returns the logger the client logs to, tagged with its component.
+func (ft *FindATender) logger() *slog.Logger {
+	log := ft.log
+	if log == nil {
+		log = slog.Default()
+	}
+	return log.With("component", "findatender")
+}
+
 // ReloadConfig replaces the configuration the client reads, such as the log
 // level. The HTTP client and API root set by New are not changed.
 func (ft *FindATender) ReloadConfig(conf *config.JobSearchConfig) {
@@ -55,7 +79,7 @@ func (ft *FindATender) With(ctx context.Context) *FindATender {
 // logged, as there is nothing a caller could do about it.
 func (ft *FindATender) Shutdown() {
 	if err := ft.httpClient.Close(); err != nil {
-		slog.Warn("findatender: closing HTTP client", "error", err)
+		ft.logger().Warn("closing http client failed", "error", err)
 	}
 }
 

internal/logging/logging.go

@@ -2,6 +2,7 @@ package logging
 
 import (
 	"errors"
+	"fmt"
 	"log/slog"
 	"os"
 	"path/filepath"
@@ -19,6 +20,14 @@ var (
 	// fileLogger is the log file opened by LogToFile, or nil when logging
 	// to the terminal.
 	fileLogger *os.File
+	// terminal is where LogToTerminal and PrintToTerminal write.
+	terminal = os.Stdout
+	// isTerminal reports whether file is an interactive terminal rather than
+	// a file or pipe. Tests replace it.
+	isTerminal = func(file *os.File) bool {
+		info, err := file.Stat()
+		return err == nil && info.Mode()&os.ModeCharDevice != 0
+	}
 )
 
 // SetLevel sets the logging level without logging the change.
@@ -31,7 +40,7 @@ func SetLevel(level slog.Level) {
 func EnableDebug() {
 	if logLevel.Level() > slog.LevelDebug {
 		logLevel.Set(slog.LevelDebug)
-		slog.Debug("Debug logging enabled")
+		slog.Debug("debug logging enabled")
 	}
 }
 
@@ -71,15 +80,59 @@ func Shutdown() error {
 }
 
 // LogToTerminal sends all logging to stdout as coloured text, replacing the
-// default slog logger.
+// default slog logger. When stdout is an interactive terminal, loggers
+// derived from it report so to WritesToTerminal.
 func LogToTerminal() {
 	opts := &tint.Options{
 		Level:      &logLevel,
 		TimeFormat: "*",
 	}
-	handler := tint.NewTextHandler(os.Stdout, opts)
+	handler := tint.NewTextHandler(terminal, opts)
-	logger := slog.New(handler)
+	if isTerminal(terminal) {
-	slog.SetDefault(logger)
+		handler = terminalHandler{
+			Handler: handler,
+		}
+	}
+
+	slog.SetDefault(slog.New(handler))
+}
+
+// WritesToTerminal reports whether handler writes to an interactive
+// terminal, where multi-line text (such as a trace) reads best printed as
+// it is with PrintToTerminal, rather than escaped into a field.
+func WritesToTerminal(handler slog.Handler) bool {
+	_, ok := handler.(terminalHandler)
+	return ok
+}
+
+// PrintToTerminal writes text to the terminal as it is, on lines of its own.
+// It is for detail that a handler would escape into one line; check
+// WritesToTerminal first.
+func PrintToTerminal(text string) {
+	fmt.Fprintln(terminal, text) //nolint:errcheck // nothing to do if the terminal has gone
+}
+
+// ---------- terminalHandler ----------
+
+// terminalHandler marks the handler LogToTerminal installs on an interactive
+// terminal. WithAttrs and WithGroup keep the mark, so a logger derived with
+// With still reports to WritesToTerminal.
+type terminalHandler struct {
+	slog.Handler
+}
+
+// WithAttrs implements slog.Handler, keeping the terminal mark.
+func (h terminalHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	return terminalHandler{
+		Handler: h.Handler.WithAttrs(attrs),
+	}
+}
+
+// WithGroup implements slog.Handler, keeping the terminal mark.
+func (h terminalHandler) WithGroup(name string) slog.Handler {
+	return terminalHandler{
+		Handler: h.Handler.WithGroup(name),
+	}
 }
 
 // IsJSONHandler reports whether h writes JSON, as the file logger does.

internal/logging/logging_test.go

@@ -18,9 +18,11 @@ func saveGlobals(t *testing.T) {
 	t.Helper()
 	level := logLevel.Level()
 	def := slog.Default()
+	out, detect := terminal, isTerminal
 	t.Cleanup(func() {
 		logLevel.Set(level)
 		slog.SetDefault(def)
+		terminal, isTerminal = out, detect
 		Shutdown()
 	})
 }
@@ -155,3 +157,52 @@ func TestLogToTerminal(t *testing.T) {
 		t.Fatal("terminal handler ignores the package level")
 	}
 }
+
+// On an interactive terminal the handler, and loggers derived from it, report
+// that they write to the terminal; when stdout is a file or pipe they don't.
+func TestWritesToTerminal(t *testing.T) {
+	for _, interactive := range []bool{true, false} {
+		saveGlobals(t)
+		isTerminal = func(*os.File) bool {
+			return interactive
+		}
+
+		LogToTerminal()
+
+		derived := slog.Default().With("component", "test").WithGroup("g")
+		for name, h := range map[string]slog.Handler{
+			"default": slog.Default().Handler(),
+			"derived": derived.Handler(),
+		} {
+			if got := WritesToTerminal(h); got != interactive {
+				t.Errorf(
+					"interactive %v, %s: WritesToTerminal = %v",
+					interactive, name, got,
+				)
+			}
+		}
+	}
+	if WritesToTerminal(slog.DiscardHandler) {
+		t.Error("another handler reported as the terminal")
+	}
+}
+
+func TestPrintToTerminal(t *testing.T) {
+	saveGlobals(t)
+	out, err := os.Create(filepath.Join(t.TempDir(), "terminal"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer out.Close()
+	terminal = out
+
+	PrintToTerminal("line one\nline two")
+
+	got, err := os.ReadFile(out.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "line one\nline two\n" {
+		t.Errorf("printed %q; want the text as it is, then a newline", got)
+	}
+}

internal/mock/findatender/generator.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"fmt"
 	"hash/fnv"
-	"log"
 	"math"
 	"math/rand"
 	"time"
@@ -21,15 +20,18 @@ type Generator struct {
 	cat   *Catalog
 	store *Store
 	rnd   *rand.Rand
+	opts  options
 }
 
 // NewGenerator returns a generator drawing from cat and storing releases in
-// store. The same seed and store contents give the same releases.
+// store. The same seed and store contents give the same releases. It logs to
-func NewGenerator(cat *Catalog, store *Store, seed int64) *Generator {
+// slog.Default() unless given WithLogger.
+func NewGenerator(cat *Catalog, store *Store, seed int64, opts ...Option) *Generator {
 	return &Generator{
 		cat:   cat,
 		store: store,
 		rnd:   rand.New(rand.NewSource(seed)), //nolint:gosec // seeded for repeatable mock data
+		opts:  applyOptions(opts),
 	}
 }
 
@@ -235,9 +237,10 @@ func (g *Generator) CatchUp(
 
 // Run publishes a new release after a random gap between min and max until ctx ends.
 func (g *Generator) Run(ctx context.Context, min, max time.Duration) {
+	logger := g.opts.logger("generator")
 	for {
 		d := g.gap(min, max)
-		log.Printf("generator: next release in %s", d.Round(time.Second))
+		logger.Info("next release", "in", d.Round(time.Second))
 
 		timer := time.NewTimer(d)
 		select {
@@ -249,11 +252,11 @@ func (g *Generator) Run(ctx context.Context, min, max time.Duration) {
 
 		r := g.Make(time.Now())
 		if err := g.store.Append(r); err != nil {
-			log.Printf("generator: storing release: %v", err)
+			logger.Error("storing release failed", "error", err)
 			continue
 		}
 
-		log.Printf("generator: published %s (%s) %s", r.ID, r.Tag[0], r.OCID)
+		logger.Info("published release", "release_id", r.ID, "tag", r.Tag[0], "ocid", r.OCID)
 	}
 }
 

internal/mock/findatender/handler.go

@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"log"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -27,17 +26,20 @@ type server struct {
 	store   *Store
 	baseURL string
 	loc     *time.Location // zone for updatedFrom/updatedTo without an offset; nil means UTC
+	opts    options
 }
 
 // NewHandler serves release packages from store. baseURL is the public base
 // URL used in each package's uri and links.next. loc is the time zone that
 // updatedFrom and updatedTo are read in when they carry no offset, as the
 // documented yyyy-MM-ddTHH:mm:ss format doesn't (Europe/London for FTS).
-// Stored release dates are UTC; comparisons are on absolute instants.
+// Stored release dates are UTC; comparisons are on absolute instants. It
+// logs to slog.Default() unless given WithLogger.
 func NewHandler(
-	store *Store, baseURL string, loc *time.Location,
+	store *Store, baseURL string, loc *time.Location, opts ...Option,
 ) http.HandlerFunc {
-	return (&server{store: store, baseURL: baseURL, loc: loc}).handle
+	s := &server{store: store, baseURL: baseURL, loc: loc, opts: applyOptions(opts)}
+	return s.handle
 }
 
 // encodeCursor makes the opaque cursor for a next link that resumes at store
@@ -134,7 +136,7 @@ func (s *server) handle(w http.ResponseWriter, req *http.Request) {
 	enc.SetIndent("", "  ")
 	enc.SetEscapeHTML(false)
 	if err := enc.Encode(pkg); err != nil {
-		log.Printf("handler: writing response: %v", err)
+		s.opts.logger("handler").Warn("writing response failed", "error", err)
 	}
 }
 

internal/mock/findatender/options.go

@@ -0,0 +1,40 @@
+package mock
+
+import (
+	"log/slog"
+)
+
+// Option configures a Store, Generator or handler.
+type Option func(*options)
+
+// options holds the settings Options change.
+type options struct {
+	log *slog.Logger
+}
+
+// WithLogger makes a Store, Generator or handler log to logger instead of
+// slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(opts *options) {
+		opts.log = logger
+	}
+}
+
+// applyOptions returns the settings opts make.
+func applyOptions(opts []Option) options {
+	var settings options
+	for _, opt := range opts {
+		opt(&settings)
+	}
+	return settings
+}
+
+// logger returns the logger to use, tagged with component: the given one, or
+// slog.Default(), looked up now.
+func (opts options) logger(component string) *slog.Logger {
+	log := opts.log
+	if log == nil {
+		log = slog.Default()
+	}
+	return log.With("component", component)
+}

internal/mock/findatender/store.go

@@ -6,7 +6,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"log"
+	"log/slog"
 	"math/rand"
 	"os"
 	"path/filepath"
@@ -33,19 +33,27 @@ type Store struct {
 	// removing and picking one at random are all constant time.
 	open    []string
 	openPos map[string]int
+
+	opts options
 }
 
 // OpenStore loads the releases stored at path, creating the file and its
 // directory if needed, and opens it for appending. Lines that can't be read
 // are skipped, releases are put back in date order if needed, and a record
 // left incomplete by a crash is ended so new ones start on a fresh line.
-func OpenStore(path string) (*Store, error) {
+// It logs to slog.Default() unless given WithLogger.
-	s := &Store{latest: map[string]int{}, lastNum: map[int]int{}, openPos: map[string]int{}}
+func OpenStore(path string, opts ...Option) (*Store, error) {
+	s := &Store{
+		latest:  map[string]int{},
+		lastNum: map[int]int{},
+		openPos: map[string]int{},
+		opts:    applyOptions(opts),
+	}
 	if err := os.MkdirAll(filepath.Dir(path), 0o750); err != nil {
 		return nil, err
 	}
 
-	releases, err := readReleases(path)
+	releases, err := readReleases(s.opts.logger("store"), path)
 	if err != nil {
 		return nil, err
 	}
@@ -67,9 +75,10 @@ func OpenStore(path string) (*Store, error) {
 	return s, nil
 }
 
-// readReleases reads the releases stored at path, skipping lines that are
+// readReleases reads the releases stored at path, skipping blank lines and
-// blank or can't be decoded. A missing file holds no releases.
+// lines that can't be decoded (which are logged to logger). A missing file
-func readReleases(path string) ([]ocds.Release, error) {
+// holds no releases.
+func readReleases(logger *slog.Logger, path string) ([]ocds.Release, error) {
 	in, err := os.Open(filepath.Clean(path))
 	if os.IsNotExist(err) {
 		return nil, nil
@@ -89,7 +98,7 @@ func readReleases(path string) ([]ocds.Release, error) {
 		}
 		var r ocds.Release
 		if err := json.Unmarshal(b, &r); err != nil {
-			log.Printf("store: skipping unreadable line %d: %v", line, err)
+			logger.Warn("skipping unreadable line", "line", line, "error", err)
 			continue
 		}
 		releases = append(releases, r)

internal/util/worker.go

@@ -36,6 +36,9 @@ type Worker struct {
 	ShutdownTimeout time.Duration
 	// Task is the work to perform on each tick.
 	Task Task
+	// Logger is what the worker logs to; nil means slog.Default(), looked up
+	// when Run starts.
+	Logger *slog.Logger
 }
 
 // Run executes Task immediately, then once per Interval, until ctx is
@@ -55,7 +58,11 @@ func (w Worker) Run(ctx context.Context, since time.Time) error {
 		timeout = DefaultWorkerShutdownTimeout
 	}
 
-	logger := slog.With("source", w.Name)
+	logger := w.Logger
+	if logger == nil {
+		logger = slog.Default()
+	}
+	logger = logger.With("source", w.Name)
 	logger.Info("starting worker", "since", since, "interval", w.Interval)
 
 	// workCtx is NOT cancelled immediately when ctx is. It gives an in-flight
@@ -123,7 +130,7 @@ func (w Worker) execute(
 		logger.Warn("task cancelled", "elapsed", elapsed)
 		return since
 	default:
-		logger.Error("task failed", "err", err, "elapsed", elapsed)
+		logger.Error("task failed", "error", err, "elapsed", elapsed)
 		return since
 	}
 }

internal/websocket/client.go

@@ -106,7 +106,7 @@ func (c *client) run(ctx context.Context) {
 		// consumer), after a failed write or ping, or by the caller.
 		c.conn.CloseNow() //nolint:errcheck,gosec // finished with; it may already be closed
 	default:
-		c.hub.log.Debug("read error", "client", c.id, "error", err)
+		c.hub.logger().Debug("read error", "client", c.id, "error", err)
 		c.conn.CloseNow() //nolint:errcheck,gosec // the connection has failed anyway
 	}
 
@@ -178,7 +178,7 @@ func (c *client) writeLoop(ctx context.Context) {
 			err := c.conn.Write(writeCtx, websocket.MessageText, msg)
 			cancel()
 			if err != nil {
-				c.hub.log.Debug("write failed", "client", c.id, "error", err)
+				c.hub.logger().Debug("write failed", "client", c.id, "error", err)
 				c.cancel()
 				return
 			}
@@ -215,7 +215,7 @@ func (c *client) pingLoop(ctx context.Context) {
 			err := c.conn.Ping(pingCtx)
 			cancel()
 			if err != nil {
-				c.hub.log.Debug("ping failed", "client", c.id, "error", err)
+				c.hub.logger().Debug("ping failed", "client", c.id, "error", err)
 				c.conn.CloseNow() //nolint:errcheck,gosec // dropping the client anyway
 				c.cancel()
 				return
@@ -240,11 +240,11 @@ func (c *client) trySend(msg []byte) bool {
 func (c *client) sendEnvelope(env Envelope) {
 	encoded, err := json.Marshal(env)
 	if err != nil {
-		c.hub.log.Error("marshal", "client", c.id, "error", err)
+		c.hub.logger().Error("marshal", "client", c.id, "error", err)
 		return
 	}
 	if !c.trySend(encoded) {
-		c.hub.log.Warn("dropping slow client", "client", c.id, "ip", c.ip)
+		c.hub.logger().Warn("dropping slow client", "client", c.id, "ip", c.ip)
 		go c.close(websocket.StatusPolicyViolation, "slow consumer")
 	}
 }
@@ -273,14 +273,18 @@ func (c *client) close(code websocket.StatusCode, reason string) {
 // level, with its size; at most logMessageLimit bytes of it are logged. It
 // does nothing unless debug logging is enabled.
 func (c *client) logMessage(ctx context.Context, event string, data []byte) {
-	if !c.hub.log.Enabled(ctx, slog.LevelDebug) {
+	logger := c.hub.logger()
+	if !logger.Enabled(ctx, slog.LevelDebug) {
 		return
 	}
 	logged, truncated := data, false
 	if len(logged) > logMessageLimit {
 		logged, truncated = logged[:logMessageLimit], true
 	}
-	c.hub.log.LogAttrs(ctx, slog.LevelDebug, event,
+	logger.LogAttrs(
+		ctx,
+		slog.LevelDebug,
+		event,
 		slog.String("client", c.id),
 		slog.Int("bytes", len(data)),
 		slog.String("message", string(logged)),

internal/websocket/client_test.go

@@ -44,7 +44,7 @@ func (f *fakeConn) CloseNow() error { return nil }
 // newTestClient returns a client of a new hub on f, with no loops running,
 // and its context.
 func newTestClient(f *fakeConn) (*client, context.Context) {
-	h := NewHub(config.JSCServerWebSocket{}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{}, WithLogger(discardLog))
 	ctx, cancel := context.WithCancel(context.Background())
 	return newClient(h, f, "192.0.2.1", cancel), ctx
 }
@@ -114,7 +114,7 @@ func TestClientMessageRateLimit(t *testing.T) {
 		{config.JSCRateLimitRule{Enabled: true, PerSecond: 1, Burst: 1}, true},
 	}
 	for _, tt := range tests {
-		h := NewHub(config.JSCServerWebSocket{MessageRateLimit: tt.rule}, discardLog)
+		h := NewHub(config.JSCServerWebSocket{MessageRateLimit: tt.rule}, WithLogger(discardLog))
 		c := newClient(h, newFakeConn(), "", func() {})
 		c.limiter.Allow()
 		if limited := !c.limiter.Allow(); limited != tt.limited {
@@ -193,7 +193,7 @@ func jsonLog(level slog.Level) (*slog.Logger, *syncBuffer) {
 // At debug level, messages received from and sent to a client are logged.
 func TestLogMessages(t *testing.T) {
 	log, buf := jsonLog(slog.LevelDebug)
-	h := NewHub(config.JSCServerWebSocket{WriteTimeout: time.Second}, log)
+	h := NewHub(config.JSCServerWebSocket{WriteTimeout: time.Second}, WithLogger(log))
 	peer, _ := pipeClient(t, h)
 
 	ping := `{"type":"ping","id":"1"}`
@@ -235,7 +235,7 @@ func TestLogMessages(t *testing.T) {
 // unless debug logging is on.
 func TestLogMessageLimit(t *testing.T) {
 	log, buf := jsonLog(slog.LevelDebug)
-	h := NewHub(config.JSCServerWebSocket{}, log)
+	h := NewHub(config.JSCServerWebSocket{}, WithLogger(log))
 	c := newClient(h, newFakeConn(), "", func() {})
 	long := strings.Repeat("x", logMessageLimit+100)
 	c.logMessage(context.Background(), "message sent", []byte(long))
@@ -251,7 +251,7 @@ func TestLogMessageLimit(t *testing.T) {
 	}
 
 	log, buf = jsonLog(slog.LevelInfo)
-	c.hub = NewHub(config.JSCServerWebSocket{}, log)
+	c.hub = NewHub(config.JSCServerWebSocket{}, WithLogger(log))
 	c.logMessage(context.Background(), "message sent", []byte("x"))
 	if entries := buf.entries(t); len(entries) != 0 {
 		t.Errorf("logged %v at info level", entries)

internal/websocket/http.go

@@ -22,7 +22,7 @@ func (h *Hub) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		CompressionMode: mode,
 	})
 	if err != nil {
-		h.log.Debug("upgrade failed", "error", err)
+		h.logger().Debug("upgrade failed", "error", err)
 		return // Accept has already written the HTTP error.
 	}
 	defer conn.CloseNow() //nolint:errcheck // finished with; it may already be closed
@@ -39,7 +39,7 @@ func (h *Hub) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 	defer h.unregister(c)
 
-	h.log.Info("client connected", "client", c.id, "ip", ip)
+	h.logger().Info("client connected", "client", c.id, "ip", ip)
 	c.run(ctx)
-	h.log.Info("client disconnected", "client", c.id, "ip", ip)
+	h.logger().Info("client disconnected", "client", c.id, "ip", ip)
 }

internal/websocket/http_test.go

@@ -46,7 +46,7 @@ func dialWS(t *testing.T, ctx context.Context, url string) *ws.Conn {
 // A client upgraded over HTTP gets replies. A max_message_size of 0 uses the
 // default limit rather than refusing every message.
 func TestHTTPMessages(t *testing.T) {
-	h := NewHub(config.JSCServerWebSocket{WriteTimeout: time.Second}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{WriteTimeout: time.Second}, WithLogger(discardLog))
 	url := wsHub(t, h)
 	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
 	defer cancel()
@@ -98,7 +98,7 @@ func TestPushBadPayload(t *testing.T) {
 
 // A binary frame closes the connection with 1003 (unsupported data).
 func TestHTTPBinaryFrame(t *testing.T) {
-	h := NewHub(config.JSCServerWebSocket{WriteTimeout: time.Second}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{WriteTimeout: time.Second}, WithLogger(discardLog))
 	url := wsHub(t, h)
 	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
 	defer cancel()
@@ -120,7 +120,7 @@ func TestHTTPMessageRateLimit(t *testing.T) {
 		WriteTimeout:     time.Second,
 		MessageRateLimit: config.JSCRateLimitRule{Enabled: true, PerSecond: 1, Burst: 1},
 	}
-	h := NewHub(conf, discardLog)
+	h := NewHub(conf, WithLogger(discardLog))
 	url := wsHub(t, h)
 	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
 	defer cancel()
@@ -147,7 +147,7 @@ func TestHTTPMessageRateLimit(t *testing.T) {
 // A request that isn't a WebSocket upgrade is refused by the upgrade and no
 // client is registered.
 func TestHTTPNotAnUpgrade(t *testing.T) {
-	h := NewHub(config.JSCServerWebSocket{}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{}, WithLogger(discardLog))
 	srv := httptest.NewServer(h)
 	defer srv.Close()
 	res, err := http.Get(srv.URL)
@@ -166,7 +166,7 @@ func TestHTTPNotAnUpgrade(t *testing.T) {
 // After Shutdown, an upgrade succeeds but the client is closed at once with
 // 1001 (going away).
 func TestHTTPRefusedAfterShutdown(t *testing.T) {
-	h := NewHub(config.JSCServerWebSocket{}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{}, WithLogger(discardLog))
 	url := wsHub(t, h)
 	if err := h.Shutdown(context.Background()); err != nil {
 		t.Fatal(err)

internal/websocket/hub.go

@@ -27,13 +27,35 @@ type Hub struct {
 // max_message_size is not set.
 const defaultMaxMessageSize = 32 << 10
 
-// NewHub creates a hub.
+// Option configures a Hub.
-func NewHub(conf config.JSCServerWebSocket, log *slog.Logger) *Hub {
+type Option func(*Hub)
-	return &Hub{
+
+// WithLogger makes the hub log to logger instead of slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(h *Hub) {
+		h.log = logger
+	}
+}
+
+// NewHub creates a hub. It logs to slog.Default() unless given WithLogger.
+func NewHub(conf config.JSCServerWebSocket, opts ...Option) *Hub {
+	hub := &Hub{
 		conf:    conf,
-		log:     log.With("component", "ws"),
 		clients: make(map[*client]struct{}),
 	}
+	for _, opt := range opts {
+		opt(hub)
+	}
+	return hub
+}
+
+// logger returns the logger the hub logs to, tagged with its component.
+func (h *Hub) logger() *slog.Logger {
+	log := h.log
+	if log == nil {
+		log = slog.Default()
+	}
+	return log.With("component", "ws")
 }
 
 // Push sends a message of type msgType to every client. The payload is
@@ -45,7 +67,7 @@ func (h *Hub) Push(msgType string, payload any) {
 	if payload != nil {
 		var err error
 		if rawPayload, err = json.Marshal(payload); err != nil {
-			h.log.Error("marshal push payload", "type", msgType, "error", err)
+			h.logger().Error("marshal push payload", "type", msgType, "error", err)
 			return
 		}
 	}
@@ -55,7 +77,7 @@ func (h *Hub) Push(msgType string, payload any) {
 		Payload: rawPayload,
 	})
 	if err != nil {
-		h.log.Error("marshal push", "type", msgType, "error", err)
+		h.logger().Error("marshal push", "type", msgType, "error", err)
 		return
 	}
 
@@ -69,7 +91,7 @@ func (h *Hub) Push(msgType string, payload any) {
 	h.m.RUnlock()
 
 	for _, client := range slow {
-		h.log.Warn("dropping slow client", "client", client.id, "ip", client.ip)
+		h.logger().Warn("dropping slow client", "client", client.id, "ip", client.ip)
 		go client.close(ws.StatusPolicyViolation, "slow consumer")
 	}
 }

internal/websocket/hub_test.go

@@ -32,7 +32,7 @@ func pipeClient(t *testing.T, h *Hub) (net.Conn, chan struct{}) {
 // once and returns ctx's error.
 func TestShutdownTimeout(t *testing.T) {
 	// No write timeout: the client's goodbye blocks until it is cut short.
-	h := NewHub(config.JSCServerWebSocket{}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{}, WithLogger(discardLog))
 	_, done := pipeClient(t, h)
 
 	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Millisecond)
@@ -49,7 +49,7 @@ func TestShutdownTimeout(t *testing.T) {
 
 // Shutdown with no clients returns at once, even with an expired ctx.
 func TestShutdownNoClients(t *testing.T) {
-	h := NewHub(config.JSCServerWebSocket{}, discardLog)
+	h := NewHub(config.JSCServerWebSocket{}, WithLogger(discardLog))
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 	for range 20 {
@@ -64,7 +64,7 @@ func TestPingFailureDropsClient(t *testing.T) {
 	h := NewHub(config.JSCServerWebSocket{
 		PingInterval: 10 * time.Millisecond,
 		PongTimeout:  20 * time.Millisecond,
-	}, discardLog)
+	}, WithLogger(discardLog))
 	_, done := pipeClient(t, h)
 
 	select {
@@ -85,7 +85,7 @@ func TestMaxMessageSize(t *testing.T) {
 		{config.JSCServerWebSocket{MaxMessageSize: 100}, 100},
 	}
 	for _, tt := range tests {
-		h := NewHub(tt.conf, discardLog)
+		h := NewHub(tt.conf, WithLogger(discardLog))
 		if got := h.maxMessageSize(); got != tt.want {
 			t.Errorf("maxMessageSize(%d) = %d; want %d", tt.conf.MaxMessageSize, got, tt.want)
 		}

internal/websocket/tcp.go

@@ -71,10 +71,10 @@ func (h *Hub) ServeTCP(ctx context.Context, newConn net.Conn) {
 	}
 	defer h.unregister(client)
 
-	h.log.Info("client connected", "client", client.id, "ip", ip, "transport", "tcp")
+	h.logger().Info("client connected", "client", client.id, "ip", ip, "transport", "tcp")
 	client.run(ctx)
 	conn.CloseNow() //nolint:errcheck,gosec // finished with; it may already be closed
-	h.log.Info("client disconnected", "client", client.id, "ip", ip, "transport", "tcp")
+	h.logger().Info("client disconnected", "client", client.id, "ip", ip, "transport", "tcp")
 }
 
 // ---------- tcpConn ----------

internal/websocket/tcp_test.go

@@ -31,7 +31,7 @@ func tcpHub(t *testing.T, mutate func(*config.JSCServerWebSocket)) (*Hub, string
 	if mutate != nil {
 		mutate(&conf)
 	}
-	h := NewHub(conf, discardLog)
+	h := NewHub(conf, WithLogger(discardLog))
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		t.Fatal(err)
@@ -243,7 +243,9 @@ func TestTCPPing(t *testing.T) {
 // A client that doesn't read is dropped as a slow consumer once its queue is
 // full. net.Pipe has no buffer, so writes to it block until the peer reads.
 func TestTCPSlowConsumer(t *testing.T) {
-	h := NewHub(config.JSCServerWebSocket{WriteTimeout: 50 * time.Millisecond}, discardLog)
+	h := NewHub(
+		config.JSCServerWebSocket{WriteTimeout: 50 * time.Millisecond}, WithLogger(discardLog),
+	)
 	server, peer := net.Pipe()
 	t.Cleanup(func() { server.Close(); peer.Close() })
 	done := make(chan struct{})

internal/websocket/tcpserver/server.go

@@ -33,15 +33,38 @@ type Server struct {
 	hub  *websocket.Hub
 }
 
+// Option configures a Server.
+type Option func(*Server)
+
+// WithLogger makes the server log to logger instead of slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(s *Server) {
+		s.log = logger
+	}
+}
+
 // New builds a server for hub's plain TCP clients. Run starts it; it shuts
 // hub down when it stops, so hub should not be shared with another server.
-// Push to hub to reach the server's clients.
+// Push to hub to reach the server's clients. It logs to slog.Default()
-func New(conf *config.JSCServer, hub *websocket.Hub, log *slog.Logger) *Server {
+// unless given WithLogger.
-	return &Server{
+func New(conf *config.JSCServer, hub *websocket.Hub, opts ...Option) *Server {
+	server := &Server{
 		conf: conf,
-		log:  log,
 		hub:  hub,
 	}
+	for _, opt := range opts {
+		opt(server)
+	}
+	return server
+}
+
+// logger returns the logger the server logs to, tagged with its component.
+func (s *Server) logger() *slog.Logger {
+	log := s.log
+	if log == nil {
+		log = slog.Default()
+	}
+	return log.With("component", "tcp")
 }
 
 // Run listens on the configured host and port and serves plain TCP clients
@@ -71,14 +94,14 @@ func (s *Server) serve(ctx context.Context, listen net.Listener) error {
 	})
 	defer stop()
 
-	s.log.Info("listening", "addr", listen.Addr().String())
+	s.logger().Info("listening", "addr", listen.Addr().String())
 	acceptErr := s.accept(ctx, connCtx, listen)
 	if ctx.Err() != nil {
 		acceptErr = nil // cancelling ctx is how the server is stopped
 	}
 	listen.Close() //nolint:errcheck,gosec // closed already when ctx was cancelled
 
-	s.log.Info("shutting down")
+	s.logger().Info("shutting down")
 	shutdownCtx := context.Background()
 	if s.conf.Timeouts.ShutdownTimeout > 0 {
 		var cancel context.CancelFunc
@@ -117,7 +140,7 @@ func (s *Server) accept(
 
 			// Back off and retry e.g. when out of file descriptors.
 			delay = min(max(2*delay, 5*time.Millisecond), time.Second)
-			s.log.Warn("tcp accept failed", "error", err, "retry_in", delay)
+			s.logger().Warn("tcp accept failed", "error", err, "retry_in", delay)
 
 			select {
 			case <-time.After(delay):
@@ -131,7 +154,7 @@ func (s *Server) accept(
 		if limiter != nil {
 			ip := remoteIP(newConn)
 			if allowed, _ := limiter.Allow(ip); !allowed {
-				s.log.Warn("tcp connection rate limited", "ip", ip)
+				s.logger().Warn("tcp connection rate limited", "ip", ip)
 				go reject(newConn, "rate limited")
 				continue
 			}

internal/websocket/tcpserver/server_test.go

@@ -36,8 +36,8 @@ func testConf(mutate func(*config.JSCServer)) *config.JSCServer {
 
 // newServer returns a server for conf with its own hub.
 func newServer(conf *config.JSCServer) (*Server, *websocket.Hub) {
-	h := websocket.NewHub(conf.WebSocket, discardLog)
+	h := websocket.NewHub(conf.WebSocket, websocket.WithLogger(discardLog))
-	return New(conf, h, discardLog), h
+	return New(conf, h, WithLogger(discardLog)), h
 }
 
 // startServer serves on a local port, returning the server's hub, its