Ensure that all unmerged files are merged when conflict checking (#20528)

There is a subtle bug in the code relating to collating the results of
`git ls-files -u -z` in `unmergedFiles()`. The code here makes the
mistake of assuming that every unmerged file will always have a stage 1
conflict, and this results in conflicts that occur in stage 3 only being
dropped.

This PR simply adjusts this code to ensure that any empty unmergedFile
will always be passed down the channel.

The PR also adds a lot of Trace commands to attempt to help find future
bugs in this code.

Fix #19527

Signed-off-by: Andrew Thornton <art27@cantab.net>

2 files changed, 27 insertions, 2 deletions

diff --git a/services/pull/patch.go b/services/pull/patch.go
index bb09acc89f..32895b2e78 100644
--- a/services/pull/patch.go
+++ b/services/pull/patch.go
@@ -124,6 +124,7 @@ func (e *errMergeConflict) Error() string {
 }
 
 func attemptMerge(ctx context.Context, file *unmergedFile, tmpBasePath string, gitRepo *git.Repository) error {
+	log.Trace("Attempt to merge:\n%v", file)
 	switch {
 	case file.stage1 != nil && (file.stage2 == nil || file.stage3 == nil):
 		// 1. Deleted in one or both:
@@ -295,7 +296,8 @@ func checkConflicts(ctx context.Context, pr *issues_model.PullRequest, gitRepo *
 		var treeHash string
 		treeHash, _, err = git.NewCommand(ctx, "write-tree").RunStdString(&git.RunOpts{Dir: tmpBasePath})
 		if err != nil {
-			return false, err
+			lsfiles, _, _ := git.NewCommand(ctx, "ls-files", "-u").RunStdString(&git.RunOpts{Dir: tmpBasePath})
+			return false, fmt.Errorf("unable to write unconflicted tree: %w\n`git ls-files -u`:\n%s", err, lsfiles)
 		}
 		treeHash = strings.TrimSpace(treeHash)
 		baseTree, err := gitRepo.GetTree("base")
diff --git a/services/pull/patch_unmerged.go b/services/pull/patch_unmerged.go
index 3839419142..465465d0da 100644
--- a/services/pull/patch_unmerged.go
+++ b/services/pull/patch_unmerged.go
@@ -42,6 +42,17 @@ func (line *lsFileLine) SameAs(other *lsFileLine) bool {
 		line.path == other.path
 }
 
+// String provides a string representation for logging
+func (line *lsFileLine) String() string {
+	if line == nil {
+		return "<nil>"
+	}
+	if line.err != nil {
+		return fmt.Sprintf("%d %s %s %s %v", line.stage, line.mode, line.path, line.sha, line.err)
+	}
+	return fmt.Sprintf("%d %s %s %s", line.stage, line.mode, line.path, line.sha)
+}
+
 // readUnmergedLsFileLines calls git ls-files -u -z and parses the lines into mode-sha-stage-path quadruplets
 // it will push these to the provided channel closing it at the end
 func readUnmergedLsFileLines(ctx context.Context, tmpBasePath string, outputChan chan *lsFileLine) {
@@ -118,6 +129,17 @@ type unmergedFile struct {
 	err    error
 }
 
+// String provides a string representation of the an unmerged file for logging
+func (u *unmergedFile) String() string {
+	if u == nil {
+		return "<nil>"
+	}
+	if u.err != nil {
+		return fmt.Sprintf("error: %v\n%v\n%v\n%v", u.err, u.stage1, u.stage2, u.stage3)
+	}
+	return fmt.Sprintf("%v\n%v\n%v", u.stage1, u.stage2, u.stage3)
+}
+
 // unmergedFiles will collate the output from readUnstagedLsFileLines in to file triplets and send them
 // to the provided channel, closing at the end.
 func unmergedFiles(ctx context.Context, tmpBasePath string, unmerged chan *unmergedFile) {
@@ -138,6 +160,7 @@ func unmergedFiles(ctx context.Context, tmpBasePath string, unmerged chan *unmer
 
 	next := &unmergedFile{}
 	for line := range lsFileLineChan {
+		log.Trace("Got line: %v Current State:\n%v", line, next)
 		if line.err != nil {
 			log.Error("Unable to run ls-files -u -z! Error: %v", line.err)
 			unmerged <- &unmergedFile{err: fmt.Errorf("unable to run ls-files -u -z! Error: %v", line.err)}
@@ -149,7 +172,7 @@ func unmergedFiles(ctx context.Context, tmpBasePath string, unmerged chan *unmer
 		case 0:
 			// Should not happen as this represents successfully merged file - we will tolerate and ignore though
 		case 1:
-			if next.stage1 != nil {
+			if next.stage1 != nil || next.stage2 != nil || next.stage3 != nil {
 				// We need to handle the unstaged file stage1,stage2,stage3
 				unmerged <- next
 			}