Fix ResolveConflicts file limit error

There has been a high occurrence of errors in the Go implementation of
ResolveConflicts in production. After close examination, it was found
that the maximum conflict file size was being inaccurately calculated.
This fix ensures that the file scanner only counts bytes that have
already been advanced, rather than the size of the remaining bytes in
the buffer. This will prevent the same bytes from being counted multuple
times.

3 files changed, 17 insertions, 1 deletions

diff --git a/changelogs/unreleased/po-fix-resolve-conflicts-file-limit.yml b/changelogs/unreleased/po-fix-resolve-conflicts-file-limit.yml
new file mode 100644
index 000000000..1f70d8ae5
--- /dev/null
+++ b/changelogs/unreleased/po-fix-resolve-conflicts-file-limit.yml
@@ -0,0 +1,5 @@
+---
+title: Fix ResolveConflicts file limit error
+merge_request: 3004
+author:
+type: fixed
diff --git a/internal/git/conflict/parser.go b/internal/git/conflict/parser.go
index aba1385be..f752921d0 100644
--- a/internal/git/conflict/parser.go
+++ b/internal/git/conflict/parser.go
@@ -144,7 +144,8 @@ func Parse(src io.Reader, ourPath, theirPath, parentPath string) (File, error) {
 	s.Buffer(make([]byte, 4096), fileLimit) // allow for line scanning up to the file limit
 
 	s.Split(func(data []byte, atEOF bool) (advance int, token []byte, err error) {
-		bytesRead += len(data)
+		defer func() { bytesRead += advance }()
+
 		if bytesRead >= fileLimit {
 			return 0, nil, ErrUnmergeableFile
 		}
@@ -212,6 +213,9 @@ func Parse(src io.Reader, ourPath, theirPath, parentPath string) (File, error) {
 	}
 
 	if err := s.Err(); err != nil {
+		if errors.Is(err, bufio.ErrTooLong) {
+			return File{}, ErrUnmergeableFile
+		}
 		return File{}, err
 	}
 
diff --git a/internal/git/conflict/parser_test.go b/internal/git/conflict/parser_test.go
index b15cc84cc..10780f0ed 100644
--- a/internal/git/conflict/parser_test.go
+++ b/internal/git/conflict/parser_test.go
@@ -99,6 +99,13 @@ we can both agree on this line though
 			parseErr: ErrMissingEndDelimiter,
 		},
 		{
+			name:         "Conflict file under file limit",
+			ourPath:      "conflict.txt",
+			theirPath:    "conflict.txt",
+			parentPath:   "conflict.txt",
+			conflictFile: strings.NewReader(strings.Repeat("x", fileLimit-2) + "\n"),
+		},
+		{
 			name:         "ErrUnmergeableFile over file limit",
 			ourPath:      "conflict.txt",
 			theirPath:    "conflict.txt",