Serve both image routes with one function and one error mapping (closes #316)

HandleImage parses the path and verifies the signature, and HandleImageEnc
decrypts the token. Each then calls serveImage, which chooses the format for
auto, gets the image and serves it: the headers, ETag and 304, HEAD, the copy
and the log line. respondImageError answers the image service's errors on both
routes, and handleImageError is gone. /v1/image/ now answers 504 when the
upstream sends no response headers within upstream_fetch_timeout, as README.md
documented for /v1/e/. /v1/e/ now logs these errors at error level, as
/v1/image/ does. README.md's route tables list 504 for /v1/image/. A new test
sends the same upstream fetch errors through both routes.

Model: opus-5-5

internal/handlers/server_busy_internal_test.go

@@ -15,9 +15,10 @@ import (
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
-// TestServerBusyAnswers503 checks that both image routes answer 503 with
-// a clear error when the image service gives up waiting for a free upstream
-// connection or processing slot, wrapped as the service wraps them.
+// TestServerBusyAnswers503 checks that respondImageError, which answers the
+// errors of both image routes, answers 503 with a clear error when the image
+// service gives up waiting for a free upstream connection or processing slot,
+// wrapped as the service wraps them.
 func TestServerBusyAnswers503(t *testing.T) {
 	t.Parallel()
 
@@ -30,35 +31,28 @@ func TestServerBusyAnswers503(t *testing.T) {
 		fmt.Errorf("upstream fetch failed: %w", httpfetcher.ErrTooManyConnections),
 		fmt.Errorf("image processing failed: %w", imageprocessor.ErrTooManyImages),
 	} {
-		plain := httptest.NewRecorder()
-		h.respondImageError(plain, req, err)
+		rec := httptest.NewRecorder()
+		h.respondImageError(rec, req, err)
 
-		encrypted := httptest.NewRecorder()
-		h.handleImageError(encrypted, err)
+		if rec.Code != http.StatusServiceUnavailable {
+			t.Errorf("for %v: status = %d, want %d",
+				err, rec.Code, http.StatusServiceUnavailable)
+		}
 
-		for route, rec := range map[string]*httptest.ResponseRecorder{
-			"/v1/image/": plain, "/v1/e/": encrypted,
-		} {
-			if rec.Code != http.StatusServiceUnavailable {
-				t.Errorf("%s for %v: status = %d, want %d",
-					route, err, rec.Code, http.StatusServiceUnavailable)
-			}
-
-			if !strings.Contains(rec.Body.String(), "server busy, try again later") {
-				t.Errorf("%s for %v: body = %q, want the server busy error",
-					route, err, rec.Body.String())
-			}
+		if !strings.Contains(rec.Body.String(), "server busy, try again later") {
+			t.Errorf("for %v: body = %q, want the server busy error",
+				err, rec.Body.String())
 		}
 	}
 }
 
-// TestProcessingTimedOutAnswers503 checks that respondImageError and
-// handleImageError, which answer the errors of the two image routes, answer
-// 503 with the server busy error when the image service reports that
-// processing stopped at the deadline of the request doing it, wrapped as the
-// service wraps it. A request that waited for that processing gets this error
-// while its own deadline may have time left. The request's source only goes
-// into a log line, so it is left empty.
+// TestProcessingTimedOutAnswers503 checks that respondImageError, which
+// answers the errors of both image routes, answers 503 with the server busy
+// error when the image service reports that processing stopped at the
+// deadline of the request doing it, wrapped as the service wraps it. A request
+// that waited for that processing gets this error while its own deadline may
+// have time left. The request's source only goes into a log line, so it is
+// left empty.
 func TestProcessingTimedOutAnswers503(t *testing.T) {
 	t.Parallel()
 
@@ -66,25 +60,15 @@ func TestProcessingTimedOutAnswers503(t *testing.T) {
 	err := fmt.Errorf("%w: failed to encode: %w",
 		imgcache.ErrProcessingTimedOut, context.DeadlineExceeded)
 
-	plain := httptest.NewRecorder()
-	h.respondImageError(plain, &imagerequest.Request{}, err)
+	rec := httptest.NewRecorder()
+	h.respondImageError(rec, &imagerequest.Request{}, err)
+	t.Logf("status = %d, body = %q", rec.Code, rec.Body.String())
 
-	encrypted := httptest.NewRecorder()
-	h.handleImageError(encrypted, err)
+	if rec.Code != http.StatusServiceUnavailable {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusServiceUnavailable)
+	}
 
-	for name, rec := range map[string]*httptest.ResponseRecorder{
-		"respondImageError": plain, "handleImageError": encrypted,
-	} {
-		t.Logf("%s: status = %d, body = %q", name, rec.Code, rec.Body.String())
-
-		if rec.Code != http.StatusServiceUnavailable {
-			t.Errorf("%s: status = %d, want %d",
-				name, rec.Code, http.StatusServiceUnavailable)
-		}
-
-		if !strings.Contains(rec.Body.String(), "server busy, try again later") {
-			t.Errorf("%s: body = %q, want the server busy error",
-				name, rec.Body.String())
-		}
+	if !strings.Contains(rec.Body.String(), "server busy, try again later") {
+		t.Errorf("body = %q, want the server busy error", rec.Body.String())
 	}
 }