test: proxy HTTP error branches (403/502) + revalidation upstream error

internal/proxy/engine_test.go

@@ -361,6 +361,34 @@ func TestWaitForStoreCoalesces(t *testing.T) {
 	}
 }
 
+func TestRevalidationUpstreamError(t *testing.T) {
+	requireStack(t)
+	ctx := context.Background()
+	r := seed(t, models.Remote{Name: "eng-reval-err", PackageType: models.PackageNPM, RepoType: models.RepoTypeRemote, BaseURL: upstream.URL, CheckMutable: true, MutableTTL: 3600, StaleOnError: true})
+	p := prov(t, models.PackageNPM)
+
+	res, err := testEngine.Fetch(ctx, r, "pkg", p)
+	if err != nil {
+		t.Fatalf("initial fetch: %v", err)
+	}
+	res.Reader.Close()
+
+	// Expire freshness but keep the ETag, then break the upstream: the
+	// conditional HEAD (checkUpstream) errors, and stale-on-error serves the
+	// stored index.
+	testCache.SetTTL(ctx, "eng-reval-err", "pkg", time.Millisecond)
+	time.Sleep(10 * time.Millisecond)
+	r.BaseURL = "http://127.0.0.1:1"
+	res, err = testEngine.Fetch(ctx, r, "pkg", p)
+	if err != nil {
+		t.Fatalf("expected stale serve on revalidation error, got %v", err)
+	}
+	if res.Source != "cache" {
+		t.Errorf("expected stale cache source, got %s", res.Source)
+	}
+	res.Reader.Close()
+}
+
 func TestUpstreamErrorUnwrap(t *testing.T) {
 	base := context.DeadlineExceeded
 	ue := &UpstreamError{Err: base}