compat/fsmonitor/fsm-listen-darwin: ignore FSEvents caused by xattr changes on MacOS

Ignore FSEvents resulting from `xattr` changes.  Git does not care about
xattr's or changes to xattr's, so don't waste time collecting these
events in the daemon nor transmitting them to clients.

Various security tools add xattrs to files and/or directories, such as
to mark them as having been downloaded.  We should ignore these events
since it doesn't affect the content of the file/directory or the normal
meta-data that Git cares about.

Signed-off-by: Jeff Hostetler <[email protected]>

Author: jeffhostetler

compat/fsmonitor/fsm-listen-darwin.c

@@ -172,7 +172,7 @@ static void log_flags_set(const char *path, const FSEventStreamEventFlags flag)
 	if (flag & kFSEventStreamEventFlagItemCloned)
 		strbuf_addstr(&msg, "ItemCloned|");
 
-	trace_printf_key(&trace_fsmonitor, "fsevent: '%s', flags=%u %s",
+	trace_printf_key(&trace_fsmonitor, "fsevent: '%s', flags=0x%x %s",
 			 path, flag, msg.buf);
 
 	strbuf_release(&msg);
@@ -197,6 +197,31 @@ static int ef_is_dropped(const FSEventStreamEventFlags ef)
 		ef & kFSEventStreamEventFlagUserDropped);
 }
 
+/*
+ * If an `xattr` change is the only reason we received this event,
+ * then silently ignore it.  Git doesn't care about xattr's.  We
+ * have to be careful here because the kernel can combine multiple
+ * events for a single path.  And because events always have certain
+ * bits set, such as `ItemIsFile` or `ItemIsDir`.
+ *
+ * Return 1 if we should ignore it.
+ */
+static int ef_ignore_xattr(const FSEventStreamEventFlags ef)
+{
+	static const FSEventStreamEventFlags mask =
+		kFSEventStreamEventFlagItemChangeOwner |
+		kFSEventStreamEventFlagItemCreated |
+		kFSEventStreamEventFlagItemFinderInfoMod |
+		kFSEventStreamEventFlagItemInodeMetaMod |
+		kFSEventStreamEventFlagItemModified |
+		kFSEventStreamEventFlagItemRemoved |
+		kFSEventStreamEventFlagItemRenamed |
+		kFSEventStreamEventFlagItemXattrMod |
+		kFSEventStreamEventFlagItemCloned;
+
+	return ((ef & mask) == kFSEventStreamEventFlagItemXattrMod);
+}
+
 static void fsevent_callback(ConstFSEventStreamRef streamRef,
 			     void *ctx,
 			     size_t num_of_events,
@@ -262,6 +287,13 @@ static void fsevent_callback(ConstFSEventStreamRef streamRef,
 			continue;
 		}
 
+		if (ef_ignore_xattr(event_flags[k])) {
+			trace_printf_key(&trace_fsmonitor,
+					 "ignore-xattr: '%s', flags=0x%x",
+					 path_k, event_flags[k]);
+			continue;
+		}
+
 		switch (fsmonitor_classify_path_absolute(state, path_k)) {
 
 		case IS_INSIDE_DOT_GIT_WITH_COOKIE_PREFIX: