fsmonitor: enhance existing comments, clarify trivial response handling

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

fsmonitor.c

@@ -168,29 +168,15 @@ static int query_fsmonitor(int version, const char *last_update, struct strbuf *
 
 	if (result)
 		trace2_data_intmax("fsm_hook", NULL, "query/failed", result);
-	else {
+	else
 		trace2_data_intmax("fsm_hook", NULL, "query/response-length",
 				   query_result->len);
 
-		if (fsmonitor_is_trivial_response(query_result))
-			trace2_data_intmax("fsm_hook", NULL,
-					   "query/trivial-response", 1);
-	}
-
 	trace2_region_leave("fsm_hook", "query", NULL);
 
 	return result;
 }
 
-int fsmonitor_is_trivial_response(const struct strbuf *query_result)
-{
-	static char trivial_response[3] = { '\0', '/', '\0' };
-
-	return query_result->len >= 3 &&
-		!memcmp(trivial_response,
-			&query_result->buf[query_result->len - 3], 3);
-}
-
 static void fsmonitor_refresh_callback(struct index_state *istate, char *name)
 {
 	int i, len = strlen(name);
@@ -238,6 +224,7 @@ void refresh_fsmonitor(struct index_state *istate)
 	struct strbuf last_update_token = STRBUF_INIT;
 	char *buf;
 	unsigned int i;
+	int is_trivial = 0;
 
 	if (!core_fsmonitor || istate->fsmonitor_has_run_once)
 		return;
@@ -283,6 +270,7 @@ void refresh_fsmonitor(struct index_state *istate)
 					query_success = 0;
 				} else {
 					bol = last_update_token.len + 1;
+					is_trivial = query_result.buf[bol] == '/';
 				}
 			} else if (hook_version < 0) {
 				hook_version = HOOK_INTERFACE_VERSION1;
@@ -294,16 +282,38 @@ void refresh_fsmonitor(struct index_state *istate)
 		if (hook_version == HOOK_INTERFACE_VERSION1) {
 			query_success = !query_fsmonitor(HOOK_INTERFACE_VERSION1,
 				istate->fsmonitor_last_update, &query_result);
+			if (query_success)
+				is_trivial = query_result.buf[0] == '/';
 		}
 
+		if (is_trivial)
+			trace2_data_intmax("fsm_hook", NULL,
+					   "query/trivial-response", 1);
+
 		trace_performance_since(last_update, "fsmonitor process '%s'", core_fsmonitor);
 		trace_printf_key(&trace_fsmonitor, "fsmonitor process '%s' returned %s",
 			core_fsmonitor, query_success ? "success" : "failure");
 	}
 
-	/* a fsmonitor process can return '/' to indicate all entries are invalid */
-	if (query_success && query_result.buf[bol] != '/') {
-		/* Mark all entries returned by the monitor as dirty */
+	/*
+	 * The response from FSMonitor (excluding the header token) is
+	 * either:
+	 *
+	 * [a] a (possibly empty) list of NUL delimited relative
+	 *     pathnames of changed paths.  This list can contain
+	 *     files and directories.  Directories have a trailing
+	 *     slash.
+	 *
+	 * [b] a single '/' to indicate the provider had no
+	 *     information and that we should consider everything
+	 *     invalid.  We call this a trivial response.
+	 */
+	if (query_success && !is_trivial) {
+		/*
+		 * Mark all pathnames returned by the monitor as dirty.
+		 *
+		 * This updates both the cache-entries and the untracked-cache.
+		 */
 		buf = query_result.buf;
 		for (i = bol; i < query_result.len; i++) {
 			if (buf[i] != '\0')
@@ -318,11 +328,16 @@ void refresh_fsmonitor(struct index_state *istate)
 		if (istate->untracked)
 			istate->untracked->use_fsmonitor = 1;
 	} else {
-
-		/* We only want to run the post index changed hook if we've actually changed entries, so keep track
-		 * if we actually changed entries or not */
+		/*
+		 * We failed to get a response or received a trivial response,
+		 * so invalidate everything.
+		 *
+		 * We only want to run the post index changed hook if
+		 * we've actually changed entries, so keep track if we
+		 * actually changed entries or not.
+		 */
 		int is_cache_changed = 0;
-		/* Mark all entries invalid */
+
 		for (i = 0; i < istate->cache_nr; i++) {
 			if (istate->cache[i]->ce_flags & CE_FSMONITOR_VALID) {
 				is_cache_changed = 1;
@@ -330,7 +345,10 @@ void refresh_fsmonitor(struct index_state *istate)
 			}
 		}
 
-		/* If we're going to check every file, ensure we save the results */
+		/*
+		 * If we're going to check every file, ensure we save
+		 * the results.
+		 */
 		if (is_cache_changed)
 			istate->cache_changed |= FSMONITOR_CHANGED;