limithandler: Wrap errors in ResourceExhaustedwc-limit-exhausted

Currently we return `codes.Unavailable` when a request has been
terminated due to the concurrency/rate limiter. However, this code is
also returned when Gitaly is completely down, so all of our clients
(shell, workhorse, rails) return a hard-coded message to contact site
administrators to users when this code is returned.

Looking at how Google Cloud uses this code[0], it's treated as the
equivalent of an HTTP 503. `codes.ResourceExhausted` is the equivalent
of a 429, which is returned when a request has been rate limited.

Let's switch the concurrency limiter to wrap its errors in
`ResourceExhausted` to bring our code usage in line with common
practices and to save our clients from trying to parse the error message
to determine if Gitaly is up.

[0]: https://cloud.google.com/apis/design/errors

Changelog: changed

3 files changed, 6 insertions, 6 deletions

diff --git a/internal/middleware/limithandler/concurrency_limiter.go b/internal/middleware/limithandler/concurrency_limiter.go
index e2595571b..58396d3e2 100644
--- a/internal/middleware/limithandler/concurrency_limiter.go
+++ b/internal/middleware/limithandler/concurrency_limiter.go
@@ -150,7 +150,7 @@ func (c *ConcurrencyLimiter) Limit(ctx context.Context, lockKey string, f Limite
 	if err := c.queueInc(ctx); err != nil {
 		if errors.Is(err, ErrMaxQueueSize) {
 			detailedErr, errGeneratingDetailedErr := helper.ErrWithDetails(
-				helper.ErrUnavailable(ErrMaxQueueSize),
+				helper.ErrResourceExhausted(ErrMaxQueueSize),
 				&gitalypb.LimitError{
 					ErrorMessage: err.Error(),
 					RetryAfter:   durationpb.New(0),
@@ -161,7 +161,7 @@ func (c *ConcurrencyLimiter) Limit(ctx context.Context, lockKey string, f Limite
 					WithError(errGeneratingDetailedErr).
 					Error("failed to generate detailed error")
 
-				return nil, helper.ErrUnavailable(ErrMaxQueueSize)
+				return nil, helper.ErrResourceExhausted(ErrMaxQueueSize)
 			}
 
 			return nil, detailedErr
@@ -186,7 +186,7 @@ func (c *ConcurrencyLimiter) Limit(ctx context.Context, lockKey string, f Limite
 			c.monitor.Dropped(ctx, "max_time")
 
 			detailedErr, errGeneratingDetailedErr := helper.ErrWithDetails(
-				helper.ErrUnavailable(ErrMaxQueueTime),
+				helper.ErrResourceExhausted(ErrMaxQueueTime),
 				&gitalypb.LimitError{
 					ErrorMessage: err.Error(),
 					RetryAfter:   durationpb.New(0),
@@ -197,7 +197,7 @@ func (c *ConcurrencyLimiter) Limit(ctx context.Context, lockKey string, f Limite
 					WithError(errGeneratingDetailedErr).
 					Error("failed to generate detailed error")
 
-				return nil, helper.ErrUnavailable(ErrMaxQueueTime)
+				return nil, helper.ErrResourceExhausted(ErrMaxQueueTime)
 			}
 
 			return nil, detailedErr
diff --git a/internal/middleware/limithandler/middleware_test.go b/internal/middleware/limithandler/middleware_test.go
index c929d19ea..d9a38682d 100644
--- a/internal/middleware/limithandler/middleware_test.go
+++ b/internal/middleware/limithandler/middleware_test.go
@@ -336,7 +336,7 @@ func TestStreamLimitHandler_error(t *testing.T) {
 	}
 
 	err := <-errChan
-	testhelper.RequireGrpcCode(t, err, codes.Unavailable)
+	testhelper.RequireGrpcCode(t, err, codes.ResourceExhausted)
 	// ensure it is a structured error
 	st, ok := status.FromError(err)
 	require.True(t, ok)
diff --git a/internal/middleware/limithandler/rate_limiter.go b/internal/middleware/limithandler/rate_limiter.go
index 63124b0d7..57db2acaa 100644
--- a/internal/middleware/limithandler/rate_limiter.go
+++ b/internal/middleware/limithandler/rate_limiter.go
@@ -44,7 +44,7 @@ func (r *RateLimiter) Limit(ctx context.Context, lockKey string, f LimitedFunc)
 		// of traffic.
 		r.requestsDroppedMetric.Inc()
 		if featureflag.RateLimit.IsEnabled(ctx) {
-			err := helper.ErrUnavailable(ErrRateLimit)
+			err := helper.ErrResourceExhausted(ErrRateLimit)
 
 			detailedErr, errGeneratingDetailedErr := helper.ErrWithDetails(
 				err,