From 6049834af3051b41f9509d18306b59f2a7bd3e0b Mon Sep 17 00:00:00 2001
From: Billy Keyes <commits@billykeyes.dev>
Date: Wed, 3 Sep 2025 20:16:31 -0400
Subject: [PATCH] Report short sources as conflicts during apply

Previously, this returned an io.ErrUnexpectedEOF. This is not wrong, in
that we did unexpectedly hit the end of the input file, but it is vague
and implies a possible library bug rather than a problem with the patch
or the input. This condition is really a conflict, as the changes
described by the patch are not compatible with the state of the input.
---
 gitdiff/apply_test.go | 4 ++--
 gitdiff/apply_text.go | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/gitdiff/apply_test.go b/gitdiff/apply_test.go
index f19153b..05915ba 100644
--- a/gitdiff/apply_test.go
+++ b/gitdiff/apply_test.go
@@ -31,14 +31,14 @@ func TestApplyTextFragment(t *testing.T) {
 				Src:   "text_fragment_error.src",
 				Patch: "text_fragment_error_short_src_before.patch",
 			},
-			Err: io.ErrUnexpectedEOF,
+			Err: &Conflict{},
 		},
 		"errorShortSrc": {
 			Files: applyFiles{
 				Src:   "text_fragment_error.src",
 				Patch: "text_fragment_error_short_src.patch",
 			},
-			Err: io.ErrUnexpectedEOF,
+			Err: &Conflict{},
 		},
 		"errorContextConflict": {
 			Files: applyFiles{
diff --git a/gitdiff/apply_text.go b/gitdiff/apply_text.go
index 43af83a..8d0accb 100644
--- a/gitdiff/apply_text.go
+++ b/gitdiff/apply_text.go
@@ -1,6 +1,7 @@
 package gitdiff
 
 import (
+	"errors"
 	"io"
 )
 
@@ -85,6 +86,11 @@ func (a *TextApplier) ApplyFragment(f *TextFragment) error {
 	preimage := make([][]byte, fragEnd-start)
 	n, err := a.lineSrc.ReadLinesAt(preimage, start)
 	if err != nil {
+		// an EOF indicates that source file is shorter than the patch expects,
+		// which should be reported as a conflict rather than a generic error
+		if errors.Is(err, io.EOF) {
+			err = &Conflict{"src has fewer lines than required by fragment"}
+		}
 		return applyError(err, lineNum(start+int64(n)))
 	}