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/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)
 		}
 	}