fix: off-by-one errors in code comment patches (#39887)

Code comments next to a diff hunk could show one line too high in the
conversation view vs. in the code view that showed them on the correct
line.

- Treat the hunk end in `CutDiffAroundLine` as exclusive, so the line
after a hunk falls back to the unchanged-line context
- Don't count `\ No newline at end of file` markers as lines when
cutting the hunk
- Share the unchanged-line fallback with migrated review comments

services/gitdiff/gitdiff.go

@@ -1596,6 +1596,22 @@ func CommentAsDiff(ctx context.Context, c *issues_model.Comment) (*Diff, error)
 	return diff, nil
 }
 
+// GetCodeCommentPatch returns the code snippet stored with a code comment
+func GetCodeCommentPatch(ctx context.Context, gitRepo *git.Repository, mergeBase, headCommitID, treePath string, line int64) (string, error) {
+	patch, err := git.GetFileDiffCutAroundLine(ctx, gitRepo, mergeBase, headCommitID, treePath,
+		int64((&issues_model.Comment{Line: line}).UnsignedLine()), line < 0, setting.UI.CodeCommentLines,
+	)
+	contextCommitID := util.Iif(line < 0, mergeBase, headCommitID)
+	if err != nil || patch != "" || contextCommitID == "" {
+		return patch, err
+	}
+	patch, err = GeneratePatchForUnchangedLine(ctx, gitRepo, contextCommitID, treePath, line, setting.UI.CodeCommentLines)
+	if err != nil {
+		log.Debug("Unable to generate patch for unchanged line (file=%s, line=%d, commit=%s): %v", treePath, line, contextCommitID, err)
+	}
+	return patch, nil
+}
+
 // GeneratePatchForUnchangedLine creates a patch showing code context for an unchanged line
 func GeneratePatchForUnchangedLine(ctx context.Context, gitRepo *git.Repository, commitID, treePath string, line int64, contextLines int) (string, error) {
 	commit, err := gitRepo.GetCommit(ctx, commitID)

services/migrations/gitea_uploader.go

@@ -31,6 +31,7 @@ import (
 	"gitea.dev/modules/timeutil"
 	"gitea.dev/modules/uri"
 	"gitea.dev/modules/util"
+	"gitea.dev/services/gitdiff"
 	"gitea.dev/services/pull"
 	repo_service "gitea.dev/services/repository"
 )
@@ -905,10 +906,7 @@ func (g *GiteaLocalUploader) CreateReviews(ctx context.Context, reviews ...*base
 			// SECURITY: The TreePath must be cleaned! use relative path
 			comment.TreePath = util.PathJoinRel(comment.TreePath)
 
-			patch, _ := git.GetFileDiffCutAroundLine(ctx,
-				g.gitRepo, pr.MergeBase, headCommitID, comment.TreePath,
-				int64((&issues_model.Comment{Line: int64(line + comment.Position - 1)}).UnsignedLine()), line < 0, setting.UI.CodeCommentLines,
-			)
+			patch, _ := gitdiff.GetCodeCommentPatch(ctx, g.gitRepo, pr.MergeBase, headCommitID, comment.TreePath, int64(line+comment.Position-1))
 
 			if comment.CreatedAt.IsZero() {
 				comment.CreatedAt = review.CreatedAt

services/pull/review.go

@@ -17,7 +17,6 @@ import (
 	"gitea.dev/modules/git/gitcmd"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/optional"
-	"gitea.dev/modules/setting"
 	"gitea.dev/modules/util"
 	"gitea.dev/services/gitdiff"
 	notify_service "gitea.dev/services/notify"
@@ -258,23 +257,10 @@ func createCodeComment(ctx context.Context, doer *user_model.User, repo *repo_mo
 			commitID = headCommitID
 		}
 
-		patch, err = git.GetFileDiffCutAroundLine(ctx,
-			gitRepo, pr.MergeBase, headCommitID, treePath,
-			int64((&issues_model.Comment{Line: line}).UnsignedLine()), line < 0, setting.UI.CodeCommentLines,
-		)
+		patch, err = gitdiff.GetCodeCommentPatch(ctx, gitRepo, pr.MergeBase, headCommitID, treePath, line)
 		if err != nil {
 			return nil, err
 		}
-
-		// If patch is still empty (unchanged line), generate code context
-		contextCommitID := util.Iif(line < 0, pr.MergeBase, headCommitID) // the line number is relative to the diff sides, not to the blamed commitID
-		if patch == "" && contextCommitID != "" {
-			patch, err = gitdiff.GeneratePatchForUnchangedLine(ctx, gitRepo, contextCommitID, treePath, line, setting.UI.CodeCommentLines)
-			if err != nil {
-				// Log the error but don't fail comment creation
-				log.Debug("Unable to generate patch for unchanged line (file=%s, line=%d, commit=%s): %v", treePath, line, contextCommitID, err)
-			}
-		}
 	}
 	return issues_model.CreateComment(ctx, &issues_model.CreateCommentOptions{
 		Type:        issues_model.CommentTypeCode,