Made mock-fts take its base URL from --addr when --base-url isn't given

cmd/mock-fts/main.go

@@ -24,6 +24,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"net"
 	"net/http"
 	"os"
 	"os/signal"
@@ -75,6 +76,12 @@ and classifications that releases are generated from is embedded in the
 build from internal/mock/findatender/data/catalog.json. The mock stops with an
 error if it is invalid.
 
+Paging: each package's uri and links.next start with --base-url, which
+clients follow as given. Without it, the base URL is taken from --addr, with
+an unspecified host (":8080", "0.0.0.0", "::") as localhost. Set it when
+clients reach the mock by another address, such as through a proxy or a
+mapped container port.
+
 Releases are generated from that catalog and appended to
 <data-dir>/releases.jsonl, so they survive restarts. A background worker
 publishes a new release every --min-gap to --max-gap. On startup the mock
@@ -119,7 +126,16 @@ func newRootCmd() *cobra.Command {
   fts-mock --data-dir ./mock-data --reset
   fts-mock --min-gap 5s --max-gap 10s --rate-limit 1000`,
 		PreRunE: func(cmd *cobra.Command, args []string) error {
-			return opts.validate()
+			err := opts.validate()
+			if err != nil {
+				return err
+			}
+
+			if opts.baseURL == "" {
+				opts.baseURL, err = baseURLFromAddr(opts.addr)
+			}
+
+			return err
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return run(cmd.Context(), opts)
@@ -133,8 +149,8 @@ func newRootCmd() *cobra.Command {
 	flags.StringVar(
 		&opts.baseURL,
 		"base-url",
-		"http://localhost:8080",
-		"public base URL used in uri and links.next",
+		"",
+		"public base URL used in uri and links.next (default from --addr)",
 	)
 	flags.StringVar(
 		&opts.dataDir,
@@ -220,6 +236,26 @@ func (o *options) validate() error {
 	return nil
 }
 
+// baseURLFromAddr returns the base URL clients reach a mock listening on
+// addr by: http://host:port, with localhost for an unspecified host. Port 0
+// is refused, as the port isn't known until the mock is listening.
+func baseURLFromAddr(addr string) (string, error) {
+	host, port, err := net.SplitHostPort(addr)
+	if err != nil {
+		return "", fmt.Errorf("--addr: %w", err)
+	}
+	if port == "0" {
+		return "", errors.New("--addr with port 0 needs --base-url")
+	}
+
+	switch host {
+	case "", "0.0.0.0", "::":
+		host = "localhost"
+	}
+
+	return "http://" + net.JoinHostPort(host, port), nil
+}
+
 // run starts the mock and blocks until ctx is cancelled or the server fails.
 func run(ctx context.Context, opts *options) (err error) {
 	location, err := time.LoadLocation(opts.timeZone) // already validated
@@ -326,6 +362,7 @@ func run(ctx context.Context, opts *options) (err error) {
 	slog.Info(
 		"mock listening",
 		"addr", opts.addr,
+		"base_url", opts.baseURL,
 		"path", mock.BasePath,
 		"time_zone", location.String(),
 	)