p7519: allow running without watchman prereq

p7519 measures the performance of the fsmonitor code. To do this, it
uses the installed copy of Watchman. If Watchman isn't installed, a noop
integration script is installed in its place.

When in the latter mode, it is expected that the script should not write
a "last update token": in fact, it doesn't write anything at all since
the script is blank.

Commit 33226af (t/perf/fsmonitor: improve error message if typoing
hook name, 2020-10-26) made sure that running 'git update-index
--fsmonitor' did not write anything to stderr, but this is not the case
when using the empty Watchman script, since Git will complain that:

    $ which watchman
    watchman not found
    $ cat .git/hooks/fsmonitor-empty
    $ git -c core.fsmonitor=.git/hooks/fsmonitor-empty update-index --fsmonitor
    warning: Empty last update token.

Prior to 33226af, the output wasn't checked at all, which allowed
this noop mode to work. But, 33226af breaks p7519 when running it
without a 'watchman(1)' on your system.

Handle this by only checking that the stderr is empty only when running
with a real watchman executable. Otherwise, assert that the error
message is the expected one when running in the noop mode.

Signed-off-by: Taylor Blau <[email protected]>
Acked-by: Nipunn Koorapati <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

t/perf/p7519-fsmonitor.sh

@@ -129,7 +129,12 @@ setup_for_fsmonitor() {
 
 	git config core.fsmonitor "$INTEGRATION_SCRIPT" &&
 	git update-index --fsmonitor 2>error &&
-	test_must_be_empty error  # ensure no silent error
+	if test_have_prereq WATCHMAN
+	then
+		test_must_be_empty error  # ensure no silent error
+	else
+		grep "Empty last update token" error
+	fi
 }
 
 test_perf_w_drop_caches () {