Test that a variant is made from a cached source of a negative-cached URL

The negative cache refuses only a fetch from the upstream: a variant not
in the cache is made from the cached source, and refused once that
source is evicted.

Model: opus-5-5

internal/imgcache/negative_cache_internal_test.go

@@ -97,8 +97,9 @@ 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.
+// evicted. The cached variant is still served, and a variant not in the cache
+// is made from the cached source. Once the source is evicted, a variant not in
+// the cache needs a fetch and is refused.
 func TestService_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
 	t.Parallel()
 
@@ -129,11 +130,31 @@ func TestService_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
 			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)
+	resp, err = svc.Get(t.Context(), photoVariant(fixtures, 40, FitCover))
+	t.Logf("Get() of a variant made from the cached source: error = %v", err)
+
+	if err != nil {
+		t.Fatalf("Get() of a variant made from the cached source error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	contentHash, _, err := svc.cache.LookupSource(t.Context(),
+		photoVariant(fixtures, 85, FitCover))
+	if err != nil || contentHash == "" {
+		t.Fatalf("LookupSource() = %q, %v, want the cached source", contentHash, err)
+	}
+
+	err = svc.cache.evictSourceBlob(t.Context(), contentHash)
+	if err != nil {
+		t.Fatalf("evictSourceBlob() error = %v", err)
+	}
+
+	_, err = svc.Get(t.Context(), photoVariant(fixtures, 60, FitCover))
+	t.Logf("Get() of a variant whose source was evicted: error = %v", err)
 
 	if !errors.Is(err, ErrNegativeCached) {
-		t.Errorf("Get() of a variant not in the cache error = %v, want %v",
+		t.Errorf("Get() of a variant whose source was evicted error = %v, want %v",
 			err, ErrNegativeCached)
 	}
 }