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, or when the fetch failed before the upstream
answered, such as on a refused connection. 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. README Routes lists both under 502.

Model: opus-5-5

internal/httpfetcher/request_failed_internal_test.go

@@ -0,0 +1,75 @@
+package httpfetcher
+
+import (
+	"context"
+	"errors"
+	"net"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// TestFetchFailedBeforeAnswer checks that a fetch whose connection is refused
+// fails with ErrUpstreamRequestFailed, and that a fetch its caller cancels
+// while connecting fails with context.Canceled and not with
+// 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: "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)
+			}
+		})
+	}
+}