Test that a cached variant is served while its URL is negative-cached

Get refuses a cached variant with the negative-cache error while its
source URL has a negative-cache row. The new test caches a variant,
stores a row for its URL, and expects the variant to be served from the
cache, and a variant not in the cache to be refused as before. It fails
until Get checks the negative cache only on a miss
(#245).

Model: opus-5-5

internal/imgcache/negative_cache_internal_test.go

@@ -94,6 +94,50 @@ func TestNegativeCache_Expired(t *testing.T) {
 	}
 }
 
+// TestService_Get_ServesCachedVariantOfNegativeCachedURL caches a variant of
+// the test photo, then keeps a failed fetch of the photo's URL in the negative
+// cache, as a failed fetch for another variant does once the source has been
+// evicted. The cached variant is still served; a variant not in the cache is
+// still refused.
+func TestService_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := SetupTestService(t)
+
+	resp, err := svc.Get(t.Context(), photoVariant(fixtures, 85, FitCover))
+	if err != nil {
+		t.Fatalf("Get() first request error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	err = svc.cache.StoreNegative(t.Context(), photoVariant(fixtures, 85, FitCover),
+		httpStatusBadGateway, "upstream answered 503")
+	if err != nil {
+		t.Fatalf("StoreNegative() error = %v", err)
+	}
+
+	resp, err = svc.Get(t.Context(), photoVariant(fixtures, 85, FitCover))
+	if err != nil {
+		t.Fatalf("Get() of the cached variant error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	if resp.CacheStatus != CacheHit {
+		t.Errorf("cached variant CacheStatus = %q, want %q",
+			resp.CacheStatus, CacheHit)
+	}
+
+	_, err = svc.Get(t.Context(), photoVariant(fixtures, 40, FitCover))
+	t.Logf("Get() of a variant not in the cache: error = %v", err)
+
+	if !errors.Is(err, ErrNegativeCached) {
+		t.Errorf("Get() of a variant not in the cache error = %v, want %v",
+			err, ErrNegativeCached)
+	}
+}
+
 func TestService_Get_ReturnsErrorForNegativeCachedURL(t *testing.T) {
 	t.Parallel()