Fix synctl and duplicate worker spawning (#8798)

Synctl did not check if a worker thread was already running when using
`synctl start` and would naively start a fresh copy. This would
sometimes lead to cases where many duplicate copies of a single worker
would run.

This fix adds a pid check when starting worker threads and synctl will
now refuse to start individual workers if they're already running.

changelog.d/8798.bugfix

@@ -0,0 +1 @@
+Fix a bug where synctl could spawn duplicate copies of a worker. Contributed by Waylon Cude.

synctl

@@ -358,6 +358,13 @@ def main():
         for worker in workers:
             env = os.environ.copy()
 
+            # Skip starting a worker if its already running
+            if os.path.exists(worker.pidfile) and pid_running(
+                int(open(worker.pidfile).read())
+            ):
+                print(worker.app + " already running")
+                continue
+
             if worker.cache_factor:
                 os.environ["SYNAPSE_CACHE_FACTOR"] = str(worker.cache_factor)