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.

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) {
-	s := &Store{latest: map[string]int{}, lastNum: map[int]int{}, openPos: map[string]int{}}
+// It logs to slog.Default() unless given WithLogger.
+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
-// blank or can't be decoded. A missing file holds no releases.
-func readReleases(path string) ([]ocds.Release, error) {
+// readReleases reads the releases stored at path, skipping blank lines and
+// lines that can't be decoded (which are logged to logger). A missing file
+// 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)