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/imageprocessor/deadline_internal_test.go

@@ -28,11 +28,23 @@ func (r *readerAfterDeadline) Read(p []byte) (int, error) {
 	return r.data.Read(p)
 }
 
+// inputNotToBeRead is a Process input that fails the test when it is read.
+type inputNotToBeRead struct {
+	t *testing.T
+}
+
+func (r inputNotToBeRead) Read([]byte) (int, error) {
+	r.t.Error("Process read its input although its deadline had passed")
+
+	return 0, io.EOF
+}
+
 // TestProcessStopsWhenDeadlinePassed checks that Process returns
 // context.DeadlineExceeded, and holds no slot afterwards, when its context's
-// deadline has passed before it starts or while it reads its input. libvips
-// cannot stop an encode once it has begun, so Process must not begin one for
-// a request that has ended.
+// deadline has passed before it starts, then without reading its input, or
+// while it reads its input. govips gives pixa no way to stop libvips once an
+// encode has begun, so Process must not begin one for a request that has
+// ended.
 func TestProcessStopsWhenDeadlinePassed(t *testing.T) {
 	t.Parallel()
 
@@ -45,7 +57,11 @@ func TestProcessStopsWhenDeadlinePassed(t *testing.T) {
 			time.Now().Add(-time.Second))
 		defer cancel()
 
-		checkProcessStopsAtDeadline(ctx, t, bytes.NewReader(input))
+		// With a slot free, the select in acquireSlot takes the slot or sees
+		// the ended context at random, so Process is called many times
+		for range 100 {
+			checkProcessStopsAtDeadline(ctx, t, inputNotToBeRead{t: t})
+		}
 	})
 
 	t.Run("while Process reads its input", func(t *testing.T) {