Change FindTag err when tag not found to NotFoundsteveazz/tag-not-found

What
---
- Update the error message to be `Not Found` rather then `Internal` when
  a tag is not found behind a feature flag.

Why
---
We burn through our error budget when a single user sends a bunch of
requests to tags that don't exist because we classify the error as
`Internal`, instead it should be `NotFound`.

We need to set this behind a feature flag to prevent any deployment
issues because the [rails application](https://bit.ly/3onH0x6) expects
`Internal` err. In
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/93009
we are adding support for `NotFound`.

Changelog: changed
Reference: https://gitlab.com/gitlab-org/gitaly/-/issues/4366
Signed-off-by: Steve Azzopardi <sazzopardi@gitlab.com>

3 files changed, 34 insertions, 0 deletions

diff --git a/internal/gitaly/service/ref/refs.go b/internal/gitaly/service/ref/refs.go
index 19e532dc0..2fe6cf4a8 100644
--- a/internal/gitaly/service/ref/refs.go
+++ b/internal/gitaly/service/ref/refs.go
@@ -257,6 +257,10 @@ func (s *server) findTag(ctx context.Context, repo git.RepositoryExecutor, tagNa
 			return nil, err
 		}
 	} else {
+		if featureflag.FindTagNotFound.IsEnabled(ctx) {
+			return nil, helper.ErrNotFoundf("no tag found")
+		}
+
 		return nil, errors.New("no tag found")
 	}
 
diff --git a/internal/gitaly/service/ref/refs_test.go b/internal/gitaly/service/ref/refs_test.go
index aa4e0faff..5f3ec6b38 100644
--- a/internal/gitaly/service/ref/refs_test.go
+++ b/internal/gitaly/service/ref/refs_test.go
@@ -1212,3 +1212,23 @@ func TestInvalidFindTagRequest(t *testing.T) {
 		})
 	}
 }
+
+func TestNotFoundFindTagRequest(t *testing.T) {
+	t.Parallel()
+	testhelper.NewFeatureSets(featureflag.FindTagNotFound).Run(t, testNotFoundFindTagRequest)
+}
+
+func testNotFoundFindTagRequest(t *testing.T, ctx context.Context) {
+	t.Parallel()
+
+	_, repoProto, _, client := setupRefService(ctx, t)
+
+	rpcRequest := &gitalypb.FindTagRequest{Repository: repoProto, TagName: []byte("not-found")}
+
+	_, err := client.FindTag(ctx, rpcRequest)
+	if featureflag.FindTagNotFound.IsEnabled(ctx) {
+		testhelper.RequireGrpcError(t, err, helper.ErrNotFoundf("no tag found"))
+		return
+	}
+	testhelper.RequireGrpcError(t, err, helper.ErrInternalf("no tag found"))
+}
diff --git a/internal/metadata/featureflag/ff_find_tag_not_found.go b/internal/metadata/featureflag/ff_find_tag_not_found.go
new file mode 100644
index 000000000..c00653118
--- /dev/null
+++ b/internal/metadata/featureflag/ff_find_tag_not_found.go
@@ -0,0 +1,10 @@
+package featureflag
+
+// FindTagNotFound will return NotFound instead of Internal when the Git Tag
+// doesn't exist.
+var FindTagNotFound = NewFeatureFlag(
+	"find_tag_not_found",
+	"v15.2.0",
+	"https://gitlab.com/gitlab-org/gitaly/-/issues/4366",
+	false,
+)