row 248: row 235 follow-up, the grant-executor hold-add and hold-remove stubs stop when their test dir is gone, so no stub outlives its test (#1750)

The row 197 r8 D5 proof stopped on a leftover hold-add stub. G23 and G13 write release, finish and
remove the test dir before the stub polls again, so the stub looped forever on sshd's side. 4 runs
of the file left 7 stubs. Both hold loops now also stop once $D is gone.

Candidate 4d99c178b, approved by Webby in round 1 (rev 2883, webby-0080). Gate on Sage's slot
13:05-13:07Z: the whole file 3 runs at 37/0 with no stub left, bus 596/0, and a mutant that drops
the hold-add check leaves 2 stubs. The late remote add the leak exposed is row 249 (#1751).

packages/bus/tests/grant-executor.test.mjs

@@ -51,6 +51,7 @@ async function until(fn, ms = 10000) {
 
 // The stubs read flag files in the test dir: fail-<verb> exits 1, noop-add exits 0 and writes nothing,
 // hold-add waits for `release` (then exits 1 if `skip` is there), and hold-remove waits for `release`.
+// Both also stop when the test dir is gone, since a held stub on the sshd side outlives its test (#1750).
 // wrap.sh logs every request. The sudo stub logs its arguments to sudo.log and runs the command as the
 // test's user.
 async function sshd(t, dir) {
@@ -63,12 +64,12 @@ async function sshd(t, dir) {
   stub('list', '[ -e "$D/fail-list" ] && exit 1\n[ -e "$D/granted" ] && grep " mosaic-[0-9a-f]*$" "$D/granted"\nexit 0');
   stub(
     'add',
-    'if [ -e "$D/hold-add" ]; then : > "$D/holding"; while [ ! -e "$D/release" ]; do sleep 0.05; done; [ -e "$D/skip" ] && exit 1; fi\n' +
+    'if [ -e "$D/hold-add" ]; then : > "$D/holding"; while [ ! -e "$D/release" ] && [ -d "$D" ]; do sleep 0.05; done; [ -e "$D/skip" ] && exit 1; fi\n' +
       '[ -e "$D/fail-add" ] && exit 1\n[ -e "$D/noop-add" ] && exit 0\n' +
       'grep -q " $4\\$" "$D/granted" 2>/dev/null && exit 0\necho "restrict $2 $3 $4" >> "$D/granted"',
   );
   stub('sudo', 'printf "%s\\n" "$*" >> "$D/sudo.log"\nshift 2\nexec "$@"');
-  stub('remove', 'if [ -e "$D/hold-remove" ]; then : > "$D/holding-remove"; while [ ! -e "$D/release" ]; do sleep 0.05; done; fi\n' +
+  stub('remove', 'if [ -e "$D/hold-remove" ]; then : > "$D/holding-remove"; while [ ! -e "$D/release" ] && [ -d "$D" ]; do sleep 0.05; done; fi\n' +
     '[ -e "$D/fail-remove" ] && exit 1\n[ -e "$D/granted" ] || exit 0\ngrep -v " $2\\$" "$D/granted" > "$D/granted.tmp"\nmv "$D/granted.tmp" "$D/granted"');
   writeFileSync(
     join(dir, 'wrap.sh'),