fix: skip rewriting files with special mode bits

file_handling_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"os"
 	"path/filepath"
 	"testing"
 )
@@ -77,3 +78,18 @@ func TestNewFile(t *testing.T) {
 		})
 	}
 }
+
+func TestHasSpecialFileModeBits(t *testing.T) {
+	if !hasSpecialFileModeBits(os.ModeSetuid | 0o755) {
+		t.Error("expected setuid bit to be detected")
+	}
+	if !hasSpecialFileModeBits(os.ModeSetgid | 0o755) {
+		t.Error("expected setgid bit to be detected")
+	}
+	if !hasSpecialFileModeBits(os.ModeSticky | 0o755) {
+		t.Error("expected sticky bit to be detected")
+	}
+	if hasSpecialFileModeBits(0o644) {
+		t.Error("expected plain mode to pass through")
+	}
+}

find_replace.go

@@ -200,6 +200,15 @@ func (fr *findReplace) RenameFile(f *File) error {
 // ReplaceContents rewrites the file at f if its contents contain the find
 // string. Binary-looking files (where Read returns "") are skipped silently.
 func (fr *findReplace) ReplaceContents(f *File) error {
+	mode, err := f.Mode()
+	if err != nil {
+		return err
+	}
+	if hasSpecialFileModeBits(mode) {
+		log.Printf("Skipping rewrite of %v: setuid, setgid, or sticky bit set", f.Path)
+		return nil
+	}
+
 	content, err := f.Read()
 	if err != nil {
 		return err

find_replace_test.go

@@ -660,6 +660,47 @@ func withWorkingDir(t *testing.T, dir string) {
 	t.Cleanup(func() { _ = os.Chdir(prev) })
 }
 
+
+func TestReplaceContentsSkipsSetuidFile(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("special mode bits not applicable")
+	}
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, "setuid.txt")
+	if err := os.WriteFile(path, []byte("needle"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chmod(path, 0o4755); err != nil {
+		t.Fatal(err)
+	}
+
+	f, err := NewFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	mode, err := f.Mode()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !hasSpecialFileModeBits(mode) {
+		t.Skip("setuid bit not supported in this environment")
+	}
+
+	fr := findReplace{find: "needle", replace: "hay"}
+	if err := fr.ReplaceContents(f); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "needle" {
+		t.Fatalf("content = %q; want unchanged %q", got, "needle")
+	}
+}
+
 func CloneRepoToTestDir(b *testing.B, repoUrl string) *File {
 	b.Helper()
 	d := newTestDir(b, "", "*")

mode_bits.go

@@ -0,0 +1,7 @@
+package main
+
+import "os"
+
+func hasSpecialFileModeBits(mode os.FileMode) bool {
+	return mode&os.ModeSetuid != 0 || mode&os.ModeSetgid != 0 || mode&os.ModeSticky != 0
+}