Shell: Assert that the same pid is not given to waitpid()

Theoretically, this assertion should never trip (at least, on serenity
where we don't really reuse PIDs).
This change should be considered temporary, until we figure out whether
waitpid() is being called twice (and succeeding) for one given PID.

Shell/Shell.cpp

@@ -57,6 +57,12 @@ extern char** environ;
 
 //#define SH_DEBUG
 
+// FIXME: This should eventually be removed once we've established that
+//        waitpid() is not passed the same job twice.
+#ifdef __serenity__
+#    define ENSURE_WAITID_ONCE
+#endif
+
 void Shell::setup_signals()
 {
     Core::EventLoop::register_signal(SIGCHLD, [this](int) {
@@ -1382,6 +1388,9 @@ void Shell::custom_event(Core::CustomEvent& event)
 
 void Shell::notify_child_event()
 {
+#ifdef ENSURE_WAITID_ONCE
+    static HashTable<pid_t> s_waited_for_pids;
+#endif
     Vector<u64> disowned_jobs;
     // Workaround the fact that we can't receive *who* exactly changed state.
     // The child might still be alive (and even running) when this signal is dispatched to us
@@ -1397,6 +1406,12 @@ void Shell::notify_child_event()
         for (auto& it : jobs) {
             auto job_id = it.key;
             auto& job = *it.value;
+#ifdef ENSURE_WAITID_ONCE
+            // Theoretically, this should never trip, as jobs are removed from
+            // the job table when waitpid() succeeds *and* the child is dead.
+            ASSERT(!s_waited_for_pids.contains(job.pid()));
+#endif
+
             int wstatus = 0;
 #ifdef SH_DEBUG
             dbgf("waitpid({}) = ...", job.pid());
@@ -1430,6 +1445,9 @@ void Shell::notify_child_event()
                     job.set_is_suspended(true);
                 }
                 found_child = true;
+#ifdef ENSURE_WAITID_ONCE
+                s_waited_for_pids.set(child_pid);
+#endif
             }
             if (job.should_be_disowned())
                 disowned_jobs.append(job_id);