sdnotify tests: try real hard to kill socat processes

podman gating tests are hanging in the new Fedora CI setup;
long and tedious investigation suggests that 'socat' processes
are being left unkilled, which then causes BATS to hang when
it (presumably) runs a final 'wait' in its end cleanup.

The two principal changes are to exec socat in a subshell
with fd3 closed, and to pkill its child processes before
killing the process itself. I don't know if both are needed.
The pkill definitely is; the exec may just be superstition.
Since I've wasted more than a day of PTO time on this, I'm
okay with a little superstition. What I do know is that with
these two changes, my reproducer fails to reproduce in over
one hour of trying (normally it fails within 5 minutes).

AND, update: only rawhide (f35) leaves stray socat processes
behind. f33 and ubuntu do not, so 'pkill -P' fails.

I really have no idea what's going on.

Signed-off-by: Ed Santiago <[email protected]>

test/system/260-sdnotify.bats

@@ -42,21 +42,30 @@ function _start_socat() {
     _SOCAT_LOG="$PODMAN_TMPDIR/socat.log"
 
     rm -f $_SOCAT_LOG
-    socat unix-recvfrom:"$NOTIFY_SOCKET",fork \
-          system:"(cat;echo) >> $_SOCAT_LOG" &
+    # Execute in subshell so we can close fd3 (which BATS uses).
+    # This is a superstitious ritual to try to avoid leaving processes behind,
+    # and thus prevent CI hangs.
+    (exec socat unix-recvfrom:"$NOTIFY_SOCKET",fork \
+          system:"(cat;echo) >> $_SOCAT_LOG" 3>&-) &
     _SOCAT_PID=$!
 }
 
 # Stop the socat background process and clean up logs
 function _stop_socat() {
     if [[ -n "$_SOCAT_PID" ]]; then
+        # Kill all child processes, then the process itself.
+        # This is a superstitious incantation to avoid leaving processes behind.
+        # The '|| true' is because only f35 leaves behind socat processes;
+        # f33 (and perhaps others?) behave nicely. ARGH!
+        pkill -P $_SOCAT_PID || true
         kill $_SOCAT_PID
     fi
     _SOCAT_PID=
 
     if [[ -n "$_SOCAT_LOG" ]]; then
         rm -f $_SOCAT_LOG
     fi
+    _SOCAT_LOG=
 }
 
 # Check that MAINPID=xxxxx points to a running conmon process