Answer 502 for an upstream host with no address or a failed fetch (closes #367)

The image routes answered 500 and logged at error level when the upstream
host had no address, when the fetch failed before the upstream answered,
such as on a refused connection, or when the fetcher refused a redirect.
The fetcher now marks such a failure with ErrUpstreamRequestFailed; a
cancelled fetch is not marked. respondImageError answers it, and
ErrInvalidHost, 502 "upstream error" and logs them at warn level. A
timeout that does not match context.DeadlineExceeded, such as the TLS
handshake timeout, is now ErrUpstreamTimeout and answers 504. README
Routes lists these answers.

Model: opus-5-5

internal/handlers/image.go

@@ -254,7 +254,9 @@ func (s *Handlers) respondImageError(
 	switch {
 	case errors.Is(err, httpfetcher.ErrSSRFBlocked):
 		status, message = http.StatusForbidden, "forbidden"
-	case errors.Is(err, httpfetcher.ErrUpstreamError):
+	case errors.Is(err, httpfetcher.ErrUpstreamError),
+		errors.Is(err, httpfetcher.ErrInvalidHost),
+		errors.Is(err, httpfetcher.ErrUpstreamRequestFailed):
 		status, message = http.StatusBadGateway, "upstream error"
 	case errors.Is(err, httpfetcher.ErrUpstreamTimeout):
 		status, message = http.StatusGatewayTimeout, "upstream timeout"

internal/handlers/image_errors_internal_test.go

@@ -12,6 +12,7 @@ import (
 	"net/http/httptest"
 	"slices"
 	"strings"
+	"syscall"
 	"testing"
 	"testing/fstest"
 	"time"
@@ -461,6 +462,13 @@ func TestImageRoutes_SameErrorAnswers(t *testing.T) {
 		{"upstream error status",
 			fmt.Errorf("%w: status 404", httpfetcher.ErrUpstreamError),
 			http.StatusBadGateway, slog.LevelWarn},
+		{"upstream host with no address",
+			fmt.Errorf("%w: cdn.example.com", httpfetcher.ErrInvalidHost),
+			http.StatusBadGateway, slog.LevelWarn},
+		{"connection refused",
+			fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamRequestFailed,
+				syscall.ECONNREFUSED),
+			http.StatusBadGateway, slog.LevelWarn},
 		{"no response headers within the fetch timeout",
 			httpfetcher.ErrUpstreamTimeout, http.StatusGatewayTimeout,
 			slog.LevelWarn},

internal/httpfetcher/httpfetcher.go

@@ -69,7 +69,11 @@ var (
 	ErrInvalidContentType = errors.New("invalid or unsupported content type")
 	ErrUpstreamError      = errors.New("upstream server error")
 	ErrUpstreamTimeout    = errors.New("upstream request timeout")
-	ErrTooManyConnections = errors.New("too many concurrent upstream connections")
+	// ErrUpstreamRequestFailed is a fetch that failed before the upstream
+	// answered, other than by timeout or cancellation: a failed connection
+	// or TLS handshake, or a redirect the fetcher refuses.
+	ErrUpstreamRequestFailed = errors.New("upstream request failed")
+	ErrTooManyConnections    = errors.New("too many concurrent upstream connections")
 )
 
 // Internal fetcher errors.
@@ -283,11 +287,7 @@ func (f *HTTPFetcher) Fetch(
 	fetchDuration := time.Since(startTime)
 
 	if err != nil {
-		if errors.Is(err, context.DeadlineExceeded) {
-			return nil, ErrUpstreamTimeout
-		}
-
-		return nil, fmt.Errorf("upstream request failed: %w", err)
+		return nil, upstreamRequestError(err)
 	}
 
 	// The deferred release gives the connection back, as no body follows
@@ -316,6 +316,29 @@ func (f *HTTPFetcher) Fetch(
 	return result, nil
 }
 
+// upstreamRequestError returns the error Fetch reports when its upstream
+// request fails with err:
+//   - ErrUpstreamTimeout for a timeout.
+//   - err, still matching context.Canceled, when the caller cancelled the
+//     fetch.
+//   - err wrapped in ErrUpstreamRequestFailed otherwise.
+func upstreamRequestError(err error) error {
+	var netErr net.Error
+
+	switch {
+	case errors.Is(err, context.DeadlineExceeded),
+		errors.As(err, &netErr) && netErr.Timeout():
+		// A timeout other than the fetch timeout, such as the TLS
+		// handshake timeout, does not match context.DeadlineExceeded
+		return ErrUpstreamTimeout
+	case errors.Is(err, context.Canceled):
+		// The caller cancelled the fetch; the upstream did not fail
+		return fmt.Errorf("upstream request cancelled: %w", err)
+	default:
+		return fmt.Errorf("%w: %w", ErrUpstreamRequestFailed, err)
+	}
+}
+
 // requestHeaders returns the headers of a fetch: User-Agent, Accept, the
 // ID of the request the fetch serves, so the fetch can be found in the
 // upstream host's logs, and a non-empty ifNoneMatch or ifModifiedSince.

internal/httpfetcher/request_failed_internal_test.go

@@ -0,0 +1,86 @@
+package httpfetcher
+
+import (
+	"context"
+	"errors"
+	"net"
+	"os"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// TestFetchFailedBeforeAnswer checks the error of a fetch whose connection
+// fails:
+//   - refused: ErrUpstreamRequestFailed.
+//   - timed out: ErrUpstreamTimeout, even for a timeout error that does not
+//     match context.DeadlineExceeded, such as the TLS handshake timeout.
+//   - cancelled by its caller: context.Canceled, not ErrUpstreamRequestFailed.
+func TestFetchFailedBeforeAnswer(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		dial       func(cancel context.CancelFunc) error
+		wantErr    error
+		wantNotErr error
+	}{
+		{
+			name: "connection refused",
+			dial: func(context.CancelFunc) error {
+				return syscall.ECONNREFUSED
+			},
+			wantErr:    ErrUpstreamRequestFailed,
+			wantNotErr: context.Canceled,
+		},
+		{
+			name: "timed out while connecting",
+			dial: func(context.CancelFunc) error {
+				return os.ErrDeadlineExceeded
+			},
+			wantErr:    ErrUpstreamTimeout,
+			wantNotErr: ErrUpstreamRequestFailed,
+		},
+		{
+			name: "cancelled while connecting",
+			dial: func(cancel context.CancelFunc) error {
+				cancel()
+
+				return context.Canceled
+			},
+			wantErr:    context.Canceled,
+			wantNotErr: ErrUpstreamRequestFailed,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			ctx, cancel := context.WithCancel(testContext(t))
+			defer cancel()
+
+			f := New(&Config{
+				Timeout:               30 * time.Second,
+				MaxResponseSize:       50 << 20,
+				AllowHTTP:             true,
+				MaxConnectionsPerHost: 20,
+				MaxConnections:        64,
+				DialContext: func(context.Context, string, string) (net.Conn, error) {
+					return nil, tt.dial(cancel)
+				},
+			})
+
+			_, err := f.Fetch(ctx, upstreamSource("/image"), "", "")
+			t.Logf("Fetch() error: %v", err)
+
+			if !errors.Is(err, tt.wantErr) {
+				t.Errorf("Fetch() error = %v, want %v", err, tt.wantErr)
+			}
+
+			if errors.Is(err, tt.wantNotErr) {
+				t.Errorf("Fetch() error = %v, must not be %v", err, tt.wantNotErr)
+			}
+		})
+	}
+}