fix(fsevents): handle just the final state for coalesced events

src/backend/FSEvents.zig

@@ -229,21 +229,36 @@ fn callback(
         // an error back to the caller.  Stopping the stream from inside the
         // callback would require a separate signal channel and is not worth
         // the complexity; the stream will keep delivering future events.
-        if (flags & kFSEventStreamEventFlagItemCreated != 0) {
-            ctx.handler.change(path, .created, ot) catch {};
-        }
+        // FSEvents coalesces multiple operations into a single callback with
+        // multiple flags set.  Processing all flags independently produces
+        // spurious duplicate events (e.g. ItemRemoved|ItemRenamed -> two
+        // deletes; ItemCreated|ItemRemoved -> spurious create before delete).
+        //
+        // Priority chain: Removed > Renamed > Modified > Created.
+        // Modified beats Created because FSEvents sets ItemCreated on writes to
+        // existing files when O_CREAT is used (e.g. `echo > file`), producing
+        // spurious create events.  When both are set, the file already exists,
+        // so Modified is the accurate description.
+        //
+        // Exception: ItemRenamed|ItemModified on an existing path emits both
+        // created and modified, because a rename-in followed by a write can be
+        // coalesced by FSEvents into a single callback.
         if (flags & kFSEventStreamEventFlagItemRemoved != 0) {
             ctx.handler.change(path, .deleted, ot) catch {};
-        }
-        if (flags & kFSEventStreamEventFlagItemRenamed != 0) {
+        } else if (flags & kFSEventStreamEventFlagItemRenamed != 0) {
             // FSEvents fires ItemRenamed for both sides of a rename unpaired.
             // Normalize to created/deleted based on whether the path still exists,
             // so move-in appears as created and move-out as deleted on all platforms.
             const exists = if (std.fs.accessAbsolute(path, .{})) |_| true else |_| false;
             ctx.handler.change(path, if (exists) .created else .deleted, ot) catch {};
-        }
-        if (flags & kFSEventStreamEventFlagItemModified != 0) {
+            // If a write was coalesced with a move-in, also emit the modify.
+            if (exists and flags & kFSEventStreamEventFlagItemModified != 0) {
+                ctx.handler.change(path, .modified, ot) catch {};
+            }
+        } else if (flags & kFSEventStreamEventFlagItemModified != 0) {
             ctx.handler.change(path, .modified, ot) catch {};
+        } else if (flags & kFSEventStreamEventFlagItemCreated != 0) {
+            ctx.handler.change(path, .created, ot) catch {};
         }
     }
 }

src/nightwatch_test.zig

@@ -234,6 +234,11 @@ fn testModifyFile(comptime Watcher: type, allocator: std.mem.Allocator) !void {
     var watcher = try Watcher.init(allocator, &th.handler);
     defer watcher.deinit();
     try watcher.watch(tmp);
+    // Drain before writing: FSEvents may deliver a coalesced create+modify if the
+    // file was created just before the stream started. A drain here separates any
+    // stale creation event from the upcoming write, so the write arrives in its
+    // own callback with only ItemModified set.
+    try drainEvents(Watcher, &watcher);
 
     {
         const f = try std.fs.openFileAbsolute(file_path, .{ .mode = .write_only });
@@ -794,7 +799,9 @@ test "creating a file emits a 'created' event" {
 
 test "writing to a file emits a 'modified' event" {
     inline for (comptime std.enums.values(nw.Variant)) |variant| {
-        try testModifyFile(nw.Create(variant), std.testing.allocator);
+        testModifyFile(nw.Create(variant), std.testing.allocator) catch |e| {
+            if (e != error.SkipZigTest) return e;
+        };
     }
 }
 
@@ -860,7 +867,9 @@ test "rename: old-name event precedes new-name event" {
 
 test "rename-then-modify: rename event precedes the subsequent modify event" {
     inline for (comptime std.enums.values(nw.Variant)) |variant| {
-        try testRenameThenModify(nw.Create(variant), std.testing.allocator);
+        testRenameThenModify(nw.Create(variant), std.testing.allocator) catch |e| {
+            if (e != error.SkipZigTest) return e;
+        };
     }
 }