timeout: Warn if the thread did not exit.

As noted in the docstring for this function, the timeout is
best-effort only -- if the thread is blocked in a syscall, it will not
service the exception until it returns.  It can also choose to catch
and ignore the TimeoutExpired; in either case it will still be running
even after the `timeout()` function returns.

Raising a vare TimeoutExpired it still somewhat accurate, but obscures
that the backend thread may still be running along merrily.  Notice
such cases, and log a warning about them.

(cherry picked from commit 3af2c8d9a3)

zerver/lib/timeout.py

@@ -1,4 +1,5 @@
 import ctypes
+import logging
 import sys
 import threading
 import time
@@ -82,6 +83,12 @@ def timeout(timeout: float, func: Callable[[], ResultT]) -> ResultT:
             # Re-raise the exception we sent, if possible, so the
             # stacktrace originates in the slow code
             raise thread.exc_info[1].with_traceback(thread.exc_info[2])
+        # If we don't have that for some reason (e.g. we failed to
+        # kill it), just raise from here; the thread _may still be
+        # running_ because it failed to see any of our exceptions, and
+        # we just ignore it.
+        if thread.is_alive():
+            logging.warning("Failed to time out backend thread")
         raise TimeoutExpired
 
     if thread.exc_info[1] is not None: