Gracefully handle diff paths with leading slashessh-fix-diffs-with-leading-slashes

If a user used the GitLab API to generate a discussion in a merge
request diff with a leading slash in a path, Gitaly would pass in the
path and cause Git to die. For example:

```
git diff HEAD^1 -- /filename
fatal: /filename: '/filename' is outside repository
```

We should just strip the leading slash to avoid this error.

Closes https://gitlab.com/gitlab-org/gitlab/issues/207586

4 files changed, 28 insertions, 1 deletions

diff --git a/changelogs/unreleased/sh-fix-diffs-with-leading-slashes.yml b/changelogs/unreleased/sh-fix-diffs-with-leading-slashes.yml
new file mode 100644
index 000000000..4744da003
--- /dev/null
+++ b/changelogs/unreleased/sh-fix-diffs-with-leading-slashes.yml
@@ -0,0 +1,5 @@
+---
+title: Gracefully handle diff paths with leading slashes
+merge_request: 1857
+author:
+type: fixed
diff --git a/internal/service/diff/commit.go b/internal/service/diff/commit.go
index 7f46bdc9c..f62489254 100644
--- a/internal/service/diff/commit.go
+++ b/internal/service/diff/commit.go
@@ -3,6 +3,7 @@ package diff
 import (
 	"context"
 	"fmt"
+	"strings"
 
 	grpc_logrus "github.com/grpc-ecosystem/go-grpc-middleware/logging/logrus"
 	log "github.com/sirupsen/logrus"
@@ -52,7 +53,8 @@ func (s *server) CommitDiff(in *gitalypb.CommitDiffRequest, stream gitalypb.Diff
 	}
 	if len(paths) > 0 {
 		for _, path := range paths {
-			cmd.PostSepArgs = append(cmd.PostSepArgs, string(path))
+			s := strings.TrimPrefix(string(path), "/")
+			cmd.PostSepArgs = append(cmd.PostSepArgs, s)
 		}
 	}
 
diff --git a/internal/service/diff/commit_test.go b/internal/service/diff/commit_test.go
index 89610cc1a..ded9a9e5a 100644
--- a/internal/service/diff/commit_test.go
+++ b/internal/service/diff/commit_test.go
@@ -197,6 +197,7 @@ func TestSuccessfulCommitDiffRequestWithPaths(t *testing.T) {
 		IgnoreWhitespaceChange: false,
 		Paths: [][]byte{
 			[]byte("CONTRIBUTING.md"),
+			[]byte("/MAINTENANCE.md"),
 			[]byte("README.md"),
 			[]byte("gitaly/named-file-with-mods"),
 			[]byte("gitaly/mode-file-with-mods"),
@@ -222,6 +223,16 @@ func TestSuccessfulCommitDiffRequestWithPaths(t *testing.T) {
 			Patch:    testhelper.MustReadFile(t, "testdata/contributing-md-chunks.txt"),
 		},
 		{
+			FromID:   "95d9f0a5e7bb054e9dd3975589b8dfc689e20e88",
+			ToID:     "5d9c7c0470bf368d61d9b6cd076300dc9d061f14",
+			OldMode:  0100644,
+			NewMode:  0100644,
+			FromPath: []byte("MAINTENANCE.md"),
+			ToPath:   []byte("MAINTENANCE.md"),
+			Binary:   false,
+			Patch:    testhelper.MustReadFile(t, "testdata/maintenance-file-with-mods-chunks.txt"),
+		},
+		{
 			FromID:   "faaf198af3a36dbf41961466703cc1d47c61d051",
 			ToID:     "877cee6ab11f9094e1bcdb7f1fd9c0001b572185",
 			OldMode:  0100644,
diff --git a/internal/service/diff/testdata/maintenance-file-with-mods-chunks.txt b/internal/service/diff/testdata/maintenance-file-with-mods-chunks.txt
new file mode 100644
index 000000000..093586fb0
--- /dev/null
+++ b/internal/service/diff/testdata/maintenance-file-with-mods-chunks.txt
@@ -0,0 +1,9 @@
+@@ -16,7 +16,7 @@ GitLab follows the [Semantic Versioning](http://semver.org/) for its releases:
+   incorrect behavior.
+ 
+ The current stable release will receive security patches and bug fixes
+-(eg. `5.0` -> `5.0.1`).  Feature releases will mark the next supported stable
++(eg. `5.0` -> `5.0.1`). Feature releases will mark the next supported stable
+ release where the minor version is increased numerically by increments of one
+ (eg. `5.0 -> 5.1`).
+