From 71ad32b1e95a1c1dfca8e6ca4a530b77afcc4974 Mon Sep 17 00:00:00 2001
From: Dmitry Vyukov <dvyukov@google.com>
Date: Tue, 27 Jan 2026 08:57:52 +0100
Subject: pkg/aflow/flow/patching: harden against empty generated patches

Make codeeditor error on nop changes that don't actually change the code.
Make patch testing error on empty patch.

Perhaps we need a notion of "mandatory" tools that must be called
successfully at least once... not sure yet.
---
 pkg/aflow/tool/codeeditor/codeeditor.go      |  8 ++++++--
 pkg/aflow/tool/codeeditor/codeeditor_test.go | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)

(limited to 'pkg/aflow/tool/codeeditor')

diff --git a/pkg/aflow/tool/codeeditor/codeeditor.go b/pkg/aflow/tool/codeeditor/codeeditor.go
index 750dba5d8..4ee3fe0d2 100644
--- a/pkg/aflow/tool/codeeditor/codeeditor.go
+++ b/pkg/aflow/tool/codeeditor/codeeditor.go
@@ -79,7 +79,11 @@ func codeeditor(ctx *aflow.Context, state state, args args) (struct{}, error) {
 		return struct{}{}, aflow.BadCallError("CurrentCode snippet matched %v places,"+
 			" increase context in CurrentCode to avoid ambiguity", matches)
 	}
-	err = osutil.WriteFile(file, slices.Concat(newLines...))
+	newFileData := slices.Concat(newLines...)
+	if bytes.Equal(fileData, newFileData) {
+		return struct{}{}, aflow.BadCallError("The edit does not change the code.")
+	}
+	err = osutil.WriteFile(file, newFileData)
 	return struct{}{}, err
 }
 
@@ -97,7 +101,7 @@ func replace(lines, src, dst [][]byte, fuzzy bool) (newLines [][]byte, matches i
 					si++
 					continue
 				}
-				if len(l) == 0 {
+				if len(l) == 0 && li != i {
 					li++
 					continue
 				}
diff --git a/pkg/aflow/tool/codeeditor/codeeditor_test.go b/pkg/aflow/tool/codeeditor/codeeditor_test.go
index 06a97d7af..0b066e3b5 100644
--- a/pkg/aflow/tool/codeeditor/codeeditor_test.go
+++ b/pkg/aflow/tool/codeeditor/codeeditor_test.go
@@ -105,6 +105,25 @@ foo`)
 	)
 }
 
+func TestCodeeditorNopEdit(t *testing.T) {
+	dir := writeTestFile(t, "src.c", `
+line0
+line1
+`)
+	aflow.TestTool(t, Tool,
+		state{
+			KernelScratchSrc: dir,
+		},
+		args{
+			SourceFile:  "src.c",
+			CurrentCode: "  line0",
+			NewCode:     "line0",
+		},
+		struct{}{},
+		`The edit does not change the code.`,
+	)
+}
+
 func TestCodeeditorReplacement(t *testing.T) {
 	type Test struct {
 		curFile string
-- 
cgit mrf-deployment