Test the 503 for a request waiting on a transcode stopped at a deadline

A request that waited for another request's transcode, stopped at that
request's deadline, must get ErrProcessingTimedOut, which both image
routes answer 503. Declares that error so the tests build. Also tests
that Process, past its deadline before it starts, never reads its input.

Model: opus-5-5

internal/imgcache/concurrent_misses_internal_test.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/getsentry/sentry-go"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/magic"
 )
 
@@ -435,6 +436,47 @@ func TestService_Get_ReturnsByItsDeadline(t *testing.T) {
 	}
 }
 
+// TestService_Get_WaiterGetsProcessingTimedOut has a request with no deadline
+// join the held fetch of a first request with a short one. With the only
+// processing slot held, the first request's transcode waits for it until the
+// first request's deadline ends, and stops then. The waiting request, whose
+// own context has not ended, must get ErrProcessingTimedOut, which the image
+// routes answer 503 as when pixa is busy.
+func TestService_Get_WaiterGetsProcessingTimedOut(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, fetcher := setupHeldFetchService(t)
+	svc.processor = imageprocessor.New(
+		imageprocessor.Params{MaxConcurrentProcessing: 1},
+	)
+
+	holdProcessingSlot(t, svc.processor)
+
+	const firstTimeout = 500 * time.Millisecond
+
+	firstCtx, cancelFirst := context.WithTimeout(t.Context(), firstTimeout)
+	defer cancelFirst()
+
+	first := startGet(firstCtx, svc, photoVariant(fixtures, 85, FitCover))
+
+	<-fetcher.started
+
+	waiter := startGet(t.Context(), svc, photoVariant(fixtures, 85, FitCover))
+
+	time.Sleep(arrivalWait)
+	fetcher.releaseFetches()
+
+	t.Logf("first request: Get() error = %v", (<-first).err)
+
+	got := <-waiter
+	t.Logf("waiting request: Get() error = %v", got.err)
+
+	if !errors.Is(got.err, ErrProcessingTimedOut) {
+		t.Errorf("waiting request: Get() error = %v, want %v",
+			got.err, ErrProcessingTimedOut)
+	}
+}
+
 // panickingFetcher panics on every fetch.
 type panickingFetcher struct{}