fsmonitor: fix race seen in t7527

Fix racy tests in t7527 by forcing the use of cookie files during all
types of queries.  There were originaly observed on M1 macs with file
system encryption enabled.

There were a series of simple tests, such as "edit some files" and
"create some files", that started the daemon with GIT_TRACE_FSMONITOR
enabled so that the daemon would emit "event: <path>" messages to the
trace log.  The test would make worktree modifications and then grep
the log file to confirm it contained the expected trace messages.
The greps would occasionally racily-fail.  The expected messages
were always present in the log file, just not yet always present
when the greps ran.

NEEDSWORK: One could argue that the tests should use the `test-tool
fsmonitor-client query` and search for the expected pathnames in the
output rather than grepping the trace log, but I'll leave that for a
later exercise.

The racy tests called `test-tool fsmonitor-client query --token 0`
before grepping the log file.  (Presumably to introduce a small delay
and/or to let the daemon sync with the file system following the last
modification, but that was not always sufficient and hence the race.)

When the query arg is just "0", the daemon treated it as a V1
(aka timestamp-relative request) and responded with a "trivial
response" and a new token, but without trying to catch up to the
the file system event stream.  So the "event: <path>" messages
may or may not yet be in the log file when the grep commands
started.

FWIW, if the tests had sent `--token builtin:0:0` instead, it would
have forced a slightly different code path in the daemon that would
cause the daemon to use a cookie file and let it catch up with the
file system event stream.  I did not see any test failures with this
change.

Instead of modifying the test, I updated the fsmonitor--daemon to
always use a cookie file and catch up to the file system on any
query operation, regardless of the format of the request token.
This is safer.

FWIW, I think the effect of the race was limited to the test.
Commands like `git status` would always do a full scan when getting a
trivial response.  The fact that the daemon was slighly behind the
file system when it generated the response token would cause a second
`git status` to get a few extra paths that the client would have to
examine, but it would not be missing paths.

FWIW, I also think that an earlier version of the code always did
the cookie file for all types of queries, but it was optimized out
during a round of reviews or rework and we didn't notice the race.

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

Author: jeffhostetler

builtin/fsmonitor--daemon.c

@@ -708,6 +708,7 @@ static int do_handle_client(struct fsmonitor_daemon_state *state,
 				  "fsmonitor: unsupported V1 protocol '%s'"),
 				 command);
 		do_trivial = 1;
+		do_cookie = 1;
 
 	} else {
 		/* We have "builtin:*" */
@@ -717,6 +718,7 @@ static int do_handle_client(struct fsmonitor_daemon_state *state,
 					 "fsmonitor: invalid V2 protocol token '%s'",
 					 command);
 			do_trivial = 1;
+			do_cookie = 1;
 
 		} else {
 			/*