fsmonitor: on macOS also emit NFC spelling for NFD pathname

Emit NFC or NFC and NFD spellings of pathnames on macOS.

MacOS is Unicode composition insensitive, so NFC and NFD spellings are
treated as aliases and collide.  While the spelling of pathnames in
filesystem events depends upon the underlying filesystem, such as
APFS, HFS+ or FAT32, the OS enforces such collisions regardless of
filesystem.

Teach the daemon to always report the NFC spelling and to report
the NFD spelling when stored in that format on the disk.

This is slightly more general than "core.precomposeUnicode".

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/fsmonitor/fsm-listen-darwin.c

@@ -155,6 +155,35 @@ static int ef_ignore_xattr(const FSEventStreamEventFlags ef)
 	return ((ef & mask) == kFSEventStreamEventFlagItemXattrMod);
 }
 
+/*
+ * On MacOS we have to adjust for Unicode composition insensitivity
+ * (where NFC and NFD spellings are not respected).  The different
+ * spellings are essentially aliases regardless of how the path is
+ * actually stored on the disk.
+ *
+ * This is related to "core.precomposeUnicode" (which wants to try
+ * to hide NFD completely and treat everything as NFC).  Here, we
+ * don't know what the value the client has (or will have) for this
+ * config setting when they make a query, so assume the worst and
+ * emit both when the OS gives us an NFD path.
+ */
+static void my_add_path(struct fsmonitor_batch *batch, const char *path)
+{
+	char *composed;
+
+	/* add the NFC or NFD path as received from the OS */
+	fsmonitor_batch__add_path(batch, path);
+
+	/* if NFD, also add the corresponding NFC spelling */
+	composed = (char *)precompose_string_if_needed(path);
+	if (!composed || composed == path)
+		return;
+
+	fsmonitor_batch__add_path(batch, composed);
+	free(composed);
+}
+
+
 static void fsevent_callback(ConstFSEventStreamRef streamRef,
 			     void *ctx,
 			     size_t num_of_events,
@@ -305,7 +334,7 @@ static void fsevent_callback(ConstFSEventStreamRef streamRef,
 
 				if (!batch)
 					batch = fsmonitor_batch__new();
-				fsmonitor_batch__add_path(batch, rel);
+				my_add_path(batch, rel);
 			}
 
 			if (event_flags[k] & kFSEventStreamEventFlagItemIsDir) {
@@ -318,7 +347,7 @@ static void fsevent_callback(ConstFSEventStreamRef streamRef,
 
 				if (!batch)
 					batch = fsmonitor_batch__new();
-				fsmonitor_batch__add_path(batch, tmp.buf);
+				my_add_path(batch, tmp.buf);
 			}
 
 			break;