Avoid waiting for zombie processes in synctl stop (#11490)

changelog.d/11490.feature

@@ -0,0 +1 @@
+`synctl stop` will now wait for Synapse to exit before returning.

synctl

@@ -41,12 +41,25 @@ NORMAL = "\x1b[m"
 def pid_running(pid):
     try:
         os.kill(pid, 0)
-        return True
     except OSError as err:
         if err.errno == errno.EPERM:
-            return True
+            pass  # process exists
+        else:
             return False
 
+    # When running in a container, orphan processes may not get reaped and their
+    # PIDs may remain valid. Try to work around the issue.
+    try:
+        with open(f"/proc/{pid}/status") as status_file:
+            if "zombie" in status_file.read():
+                return False
+    except Exception:
+        # This isn't Linux or `/proc/` is unavailable.
+        # Assume that the process is still running.
+        pass
+
+    return True
+
 
 def write(message, colour=NORMAL, stream=sys.stdout):
     # Lets check if we're writing to a TTY before colouring