fix(compute): ignore attribute-only (Chmod) events in serve --watch

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 ### Bug Fixes:
 - fix(docs): corrected stale and missing API reference links in usage.json metadata([#1803](https://github.com/fastly/cli/pull/1803))
+- fix(compute): `serve --watch` no longer rebuilds on attribute-only (Chmod) filesystem events, preventing an endless rebuild loop when another process changes a watched file's metadata such as its access time ([#1808](https://github.com/fastly/cli/pull/1808))
 - fix(docs): expand and correct API reference links for `fastly service` subcommands in usage.json metadata ([#1810](https://github.com/fastly/cli/pull/1810))
 
 ### Enhancements:

pkg/commands/compute/serve.go

@@ -1295,6 +1295,10 @@ func watchFiles(root string, gi *ignore.GitIgnore, verbose bool, s *fstexec.Stre
 				if !ok {
 					return
 				}
+				// Ignore attribute-only (Chmod) events; on macOS an atime bump alone would loop.
+				if !isContentChange(event.Op) {
+					continue
+				}
 				debounced(func() {
 					eventHandler(event.Name, event.Op)
 				})
@@ -1393,6 +1397,15 @@ func readIgnoreFile(path string) (lines []string) {
 	return strings.Split(string(bs), "\n")
 }
 
+// isContentChange reports whether op is a content or existence change
+// (Create/Write/Remove/Rename) and not an attribute-only Chmod. fsnotify
+// recommends ignoring Chmod; see its "Why do I get many Chmod events?" FAQ:
+// https://github.com/fsnotify/fsnotify#why-do-i-get-many-chmod-events
+func isContentChange(op fsnotify.Op) bool {
+	return op.Has(fsnotify.Create) || op.Has(fsnotify.Write) ||
+		op.Has(fsnotify.Remove) || op.Has(fsnotify.Rename)
+}
+
 func watchFile(path string, watcher *fsnotify.Watcher, verbose bool, out io.Writer) {
 	absolute, err := filepath.Abs(path)
 	if err != nil && verbose {

pkg/commands/compute/serve_internal_test.go

@@ -0,0 +1,32 @@
+package compute
+
+import (
+	"testing"
+
+	"github.com/fsnotify/fsnotify"
+)
+
+// TestIsContentChange checks that content/existence events trigger a rebuild and
+// attribute-only (Chmod) events do not.
+func TestIsContentChange(t *testing.T) {
+	scenarios := []struct {
+		name string
+		op   fsnotify.Op
+		want bool
+	}{
+		{name: "create", op: fsnotify.Create, want: true},
+		{name: "write", op: fsnotify.Write, want: true},
+		{name: "remove", op: fsnotify.Remove, want: true},
+		{name: "rename", op: fsnotify.Rename, want: true},
+		{name: "chmod only", op: fsnotify.Chmod, want: false},
+		{name: "write combined with chmod", op: fsnotify.Write | fsnotify.Chmod, want: true},
+		{name: "no op", op: 0, want: false},
+	}
+	for _, testcase := range scenarios {
+		t.Run(testcase.name, func(t *testing.T) {
+			if got := isContentChange(testcase.op); got != testcase.want {
+				t.Errorf("isContentChange(%s) = %v, want %v", testcase.op, got, testcase.want)
+			}
+		})
+	}
+}