fsmonitor: use IPC to query the builtin FSMonitor daemon

Use simple IPC to directly communicate with the new builtin file
system monitor daemon when `core.fsmonitor` is set to true.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Jeff Hostetler <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jeffhostetler

fsmonitor.c

@@ -241,8 +241,41 @@ void refresh_fsmonitor(struct index_state *istate)
 	trace_printf_key(&trace_fsmonitor, "refresh fsmonitor");
 
 	if (fsm_mode == FSMONITOR_MODE_IPC) {
-		/* TODO */
-		return;
+		query_success = !fsmonitor_ipc__send_query(
+			istate->fsmonitor_last_update ?
+			istate->fsmonitor_last_update : "builtin:fake",
+			&query_result);
+		if (query_success) {
+			/*
+			 * The response contains a series of nul terminated
+			 * strings.  The first is the new token.
+			 *
+			 * Use `char *buf` as an interlude to trick the CI
+			 * static analysis to let us use `strbuf_addstr()`
+			 * here (and only copy the token) rather than
+			 * `strbuf_addbuf()`.
+			 */
+			buf = query_result.buf;
+			strbuf_addstr(&last_update_token, buf);
+			bol = last_update_token.len + 1;
+			is_trivial = query_result.buf[bol] == '/';
+			if (is_trivial)
+				trace2_data_intmax("fsm_client", NULL,
+						   "query/trivial-response", 1);
+		} else {
+			/*
+			 * The builtin daemon is not available on this
+			 * platform -OR- we failed to get a response.
+			 *
+			 * Generate a fake token (rather than a V1
+			 * timestamp) for the index extension.  (If
+			 * they switch back to the hook API, we don't
+			 * want ambiguous state.)
+			 */
+			strbuf_addstr(&last_update_token, "builtin:fake");
+		}
+
+		goto apply_results;
 	}
 
 	assert(fsm_mode == FSMONITOR_MODE_HOOK);
@@ -315,6 +348,7 @@ void refresh_fsmonitor(struct index_state *istate)
 				 query_success ? "success" : "failure");
 	}
 
+apply_results:
 	/*
 	 * The response from FSMonitor (excluding the header token) is
 	 * either: