internal/recommendation/library_scale.go

@@ -118,8 +118,7 @@ func CountLibraryTracks(ctx context.Context, q *dbq.Queries) (int64, error) {
 // per request.
 const librarySizeTTL = 5 * time.Minute
 
-// librarySizeTimeout bounds the count itself. Rule 156: the caller is a user
-// waiting on a radio, and a pool-sizing hint is never worth hanging for.
+// librarySizeTimeout bounds the count itself — see loadWithDeadline.
 const librarySizeTimeout = 3 * time.Second
 
 // LibrarySize memoises the library track count.
@@ -142,7 +141,22 @@ func NewLibrarySize(now func() time.Time) *LibrarySize {
 }
 
 // Get returns the cached size, refreshing through load when stale.
+//
+// A NIL RECEIVER IS VALID and means "no cache": the count still runs, it is
+// just not memoised. That is deliberate rather than defensive habit. The api
+// handlers struct is built directly by a dozen tests that cannot know about
+// every field, and a nil here previously panicked inside a radio request —
+// turning a missing pool-sizing HINT into a 500. Uncached-but-correct is the
+// right failure for something whose whole contract is that it degrades.
 func (l *LibrarySize) Get(ctx context.Context, load func(context.Context) (int64, error)) int64 {
+	if l == nil {
+		n, err := loadWithDeadline(ctx, load)
+		if err != nil {
+			return 0 // scales to the base limits
+		}
+		return n
+	}
+
 	l.mu.Lock()
 	defer l.mu.Unlock()
 
@@ -154,9 +168,7 @@ func (l *LibrarySize) Get(ctx context.Context, load func(context.Context) (int64
 		return l.size
 	}
 
-	cctx, cancel := context.WithTimeout(ctx, librarySizeTimeout)
-	defer cancel()
-	n, err := load(cctx)
+	n, err := loadWithDeadline(ctx, load)
 	if err != nil {
 		// Keep the last known value and re-try at the next call rather than
 		// stamping `at`, so a transient failure does not pin a stale number
@@ -166,3 +178,11 @@ func (l *LibrarySize) Get(ctx context.Context, load func(context.Context) (int64
 	l.size, l.at = n, now()
 	return l.size
 }
+
+// loadWithDeadline bounds the count. Rule 156: the caller is a user waiting
+// on a radio, and a pool-sizing hint is never worth hanging for.
+func loadWithDeadline(ctx context.Context, load func(context.Context) (int64, error)) (int64, error) {
+	cctx, cancel := context.WithTimeout(ctx, librarySizeTimeout)
+	defer cancel()
+	return load(cctx)
+}

internal/recommendation/library_scale_test.go

@@ -175,3 +175,42 @@ func TestLibrarySize_RetriesAfterAFailedRefresh(t *testing.T) {
 		t.Errorf("got %d after a failed refresh, want 900 — the failure pinned a stale value", got)
 	}
 }
+
+// A nil cache must not panic, and must still be CORRECT — uncached, not
+// broken.
+//
+// This is not a hypothetical hardening. internal/api builds its handlers
+// struct directly in a dozen tests, none of which know about every field, so
+// librarySize arrives nil there. The first version of this panicked inside
+// handleRadio and took down TestHandleRadio_ColdStart_OnlySeedReturned with a
+// SIGSEGV — turning a missing pool-sizing HINT into a request-killing crash,
+// which is the opposite of what a value that "degrades rather than fails" is
+// supposed to do.
+func TestLibrarySize_NilReceiverStillCounts(t *testing.T) {
+	var c *LibrarySize // deliberately not constructed
+
+	calls := 0
+	got := c.Get(context.Background(), func(context.Context) (int64, error) {
+		calls++
+		return 40_000, nil
+	})
+	if got != 40_000 {
+		t.Errorf("nil cache returned %d, want 40000 — it should still count, just not memoise", got)
+	}
+	if calls != 1 {
+		t.Errorf("nil cache called the loader %d times, want 1", calls)
+	}
+
+	// Uncached: a second call counts again rather than reusing anything.
+	c.Get(context.Background(), func(context.Context) (int64, error) { calls++; return 40_000, nil })
+	if calls != 2 {
+		t.Errorf("nil cache memoised across calls (%d loads); it has nowhere to store a value", calls)
+	}
+
+	// And it still degrades on error rather than panicking.
+	if got := c.Get(context.Background(), func(context.Context) (int64, error) {
+		return 0, errors.New("db is down")
+	}); got != 0 {
+		t.Errorf("nil cache returned %d on a failed count, want 0 (base limits)", got)
+	}
+}