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.

Author: dvyukov

pkg/aflow/action/crash/test.go

@@ -47,6 +47,10 @@ func testPatch(ctx *aflow.Context, args testArgs) (testResult, error) {
 	if err != nil {
 		return res, err
 	}
+	if diff == "" {
+		res.TestError = "No patch to test."
+		return res, nil
+	}
 	res.PatchDiff = diff
 
 	if err := kernel.BuildKernel(args.KernelScratchSrc, args.KernelScratchSrc, args.KernelConfig, false); err != nil {

pkg/aflow/flow/patching/patching.go

@@ -131,14 +131,17 @@ The explanation of the root cause of the bug is:
 {{.BugExplanation}}
 
 {{if .TestError}}
-Another developer tried to fix this bug, and come up with the following patch:
 
-{{.PatchDiff}}
-
-and the following explanation:
+Another developer tried to fix this bug, and come up with the following strategy for fixing:
 
 {{.PatchExplanation}}
 
+{{/* A TestError without PatchDiff means the previous invocation did not generate any patch. */}}
+{{if .PatchDiff}}
+and the following patch:
+
+{{.PatchDiff}}
+
 However, the patch testing failed with the following error:
 
 {{.TestError}}
@@ -150,6 +153,9 @@ then create a new patch from scratch.
 Note: in both cases the source tree does not contain the patch yet
 (so if you want to create a new fixed patch, you need to recreate it
 in its entirety from scratch using the codeeditor tool).
+{{else}}
+If the strategy looks reasonable to you, proceed with patch generation.
+{{end}}
 {{end}}
 `
 

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
 				}

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