retry only on real network errors

Now retries only happen when actual network errors occur and not only
anymore based on the HTTP status code. This is because the backend could
also send this status codes as their normal interface and in that case
we don't want to retry.

server/errorhandler_test.go

@@ -0,0 +1,88 @@
+package server
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+type timeoutError struct{}
+
+func (e *timeoutError) Error() string   { return "i/o timeout" }
+func (e *timeoutError) Timeout() bool   { return true }
+func (e *timeoutError) Temporary() bool { return true }
+
+func TestServeHTTP(t *testing.T) {
+	tests := []struct {
+		name               string
+		err                error
+		wantHTTPStatus     int
+		wantNetErrRecorded bool
+	}{
+		{
+			name:               "net.Error",
+			err:                net.UnknownNetworkError("any network error"),
+			wantHTTPStatus:     http.StatusBadGateway,
+			wantNetErrRecorded: true,
+		},
+		{
+			name:               "net.Error with Timeout",
+			err:                &timeoutError{},
+			wantHTTPStatus:     http.StatusGatewayTimeout,
+			wantNetErrRecorded: true,
+		},
+		{
+			name:               "io.EOF",
+			err:                io.EOF,
+			wantHTTPStatus:     http.StatusBadGateway,
+			wantNetErrRecorded: true,
+		},
+		{
+			name:               "custom error",
+			err:                errors.New("any error"),
+			wantHTTPStatus:     http.StatusInternalServerError,
+			wantNetErrRecorded: false,
+		},
+		{
+			name:               "nil error",
+			err:                nil,
+			wantHTTPStatus:     http.StatusInternalServerError,
+			wantNetErrRecorded: false,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			recorder := httptest.NewRecorder()
+
+			errorRecorder := &netErrorRecorder{}
+			req := httptest.NewRequest(http.MethodGet, "http://localhost:3000/any", nil)
+
+			recordingErrorHandler := NewRecordingErrorHandler(errorRecorder)
+			recordingErrorHandler.ServeHTTP(recorder, req, test.err)
+
+			if recorder.Code != test.wantHTTPStatus {
+				t.Errorf("got HTTP status code %v, wanted %v", recorder.Code, test.wantHTTPStatus)
+			}
+			if errorRecorder.netErrorWasRecorded != test.wantNetErrRecorded {
+				t.Errorf("net error recording wrong, got %v wanted %v", errorRecorder.netErrorWasRecorded, test.wantNetErrRecorded)
+			}
+		})
+	}
+}
+
+type netErrorRecorder struct {
+	netErrorWasRecorded bool
+}
+
+func (recorder *netErrorRecorder) Record(ctx context.Context) {
+	recorder.netErrorWasRecorded = true
+}