fix(diff): load both blobs of renamed files for diff rendering (#39969)

When opening a pull request that renamed a CSV file:
the compare page fails with a 500.

Fix: load the left blob for every file that was not added, and the right blob
(from `Name`) for every file that was not deleted. That matches what the
templates did before #38517.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/gitdiff/gitdiff.go

@@ -522,13 +522,13 @@ func (diffFile *DiffFile) prepareDiffRenderDetail(ctx context.Context, gitRepo *
 	// * for "text" type: need to read up to "highlight limit size" to do full-file-highlighting
 	contentLimit := util.Iif(diffFile.IsBin, typesniffer.SniffContentSize, MaxFullFileHighlightSizeLimit)
 	var leftBlobType, rightBlobType typesniffer.SniffedType
-	if (diffFile.Type == DiffFileDel || diffFile.Type == DiffFileChange) && leftCommit != nil {
+	if diffFile.Type != DiffFileAdd && leftCommit != nil {
 		c := getCommitFileBlobAndLimitedContent(ctx, gitRepo, leftCommit, diffFile.OldName, contentLimit)
 		diffFile.LeftBlob, diffFile.LeftBlobSize, ret.leftLineCount, ret.leftContent = c.gitBlob, c.blobSize, c.lineCount, c.limitedContent
 		leftBlobType = typesniffer.DetectContentType(ret.leftContent.buf.Bytes())
 	}
-	if (diffFile.Type == DiffFileAdd || diffFile.Type == DiffFileChange) && rightCommit != nil {
-		c := getCommitFileBlobAndLimitedContent(ctx, gitRepo, rightCommit, diffFile.OldName, contentLimit)
+	if diffFile.Type != DiffFileDel && rightCommit != nil {
+		c := getCommitFileBlobAndLimitedContent(ctx, gitRepo, rightCommit, diffFile.Name, contentLimit)
 		diffFile.RightBlob, diffFile.RightBlobSize, ret.rightLineCount, ret.rightContent = c.gitBlob, c.blobSize, c.lineCount, c.limitedContent
 		rightBlobType = typesniffer.DetectContentType(ret.rightContent.buf.Bytes())
 	}

services/gitdiff/gitdiff_render_test.go

@@ -116,3 +116,40 @@ func TestGetDiffForRender(t *testing.T) {
 		assert.Equal(t, exp, actual, "idx=%d", idx)
 	}
 }
+
+func TestGetDiffForRenderRenamedFile(t *testing.T) {
+	repoDir := filepath.Join(t.TempDir(), "temp-repo")
+	require.NoError(t, git.InitRepositoryLocal(t.Context(), repoDir, false, git.Sha1ObjectFormat.Name(), ""))
+
+	gitRepo, err := git.OpenRepositoryLocal(t.Context(), repoDir)
+	require.NoError(t, err)
+	defer gitRepo.Close()
+
+	content := "id,name\n1,Alice\n2,Bob\n"
+	require.NoError(t, git.ForceFastImport(t.Context(), gitRepo, []git.FastImportCommit{
+		{Ref: "refs/heads/b1", Files: []git.FastImportFile{{Path: "original/sample.csv", Content: content}}},
+		{Ref: "refs/heads/b2", Files: []git.FastImportFile{{Path: "moved/sample.csv", Content: content}}},
+	}))
+
+	beforeCommit, err := gitRepo.GetBranchCommit(t.Context(), "b1")
+	require.NoError(t, err)
+	afterCommit, err := gitRepo.GetBranchCommit(t.Context(), "b2")
+	require.NoError(t, err)
+
+	diff, err := GetDiffForRender(t.Context(), "/any/repo-link", gitRepo, &DiffOptions{
+		BeforeCommitID:    beforeCommit.ID.String(),
+		AfterCommitID:     afterCommit.ID.String(),
+		MaxLines:          setting.Git.MaxGitDiffLines,
+		MaxLineCharacters: setting.Git.MaxGitDiffLineCharacters,
+		MaxFiles:          setting.Git.MaxGitDiffFiles,
+	})
+	require.NoError(t, err)
+	require.Len(t, diff.Files, 1)
+
+	// the CSV diff of a renamed file needs both blobs, otherwise it panics
+	diffFile := diff.Files[0]
+	assert.Equal(t, DiffFileRename, diffFile.Type)
+	assert.True(t, diffFile.IsBlobTypeCsv)
+	assert.NotNil(t, diffFile.LeftBlob)
+	assert.NotNil(t, diffFile.RightBlob)
+}