fix: address review-bot findings on merged PRs (#593)

Squashed from PR #595. Closes #593.

utils/image_monitor.py

@@ -113,7 +113,10 @@ def _start_daemon_observer(observer) -> None:
     def start() -> None:
         try:
             observer.start()
-        except BaseException as e:  # re-raised on the caller's thread
+        # BaseException on purpose: a KeyboardInterrupt or SystemExit raised
+        # by start() is not lost on this thread; _start_daemon_observer
+        # re-raises it on the caller's thread below.
+        except BaseException as e:
             raised.append(e)
 
     starter = threading.Thread(

utils/native_memory.py

@@ -16,8 +16,9 @@ import sys
 import threading
 
 _lock = threading.Lock()
-_trim = None
-_looked_up = False
+# Not looked up yet; after the lookup, the function or None.
+_NOT_LOOKED_UP = object()
+_trim = _NOT_LOOKED_UP
 
 
 def _glibc() -> bool:
@@ -33,10 +34,9 @@ def _glibc() -> bool:
 
 def _malloc_trim():
     """glibc's ``malloc_trim``, looked up once; None where there is none."""
-    global _trim, _looked_up
+    global _trim
     with _lock:
-        if not _looked_up:
-            _looked_up = True
+        if _trim is _NOT_LOOKED_UP:
             _trim = _lookup()
         return _trim
 

utils/parallel.py

@@ -165,10 +165,12 @@ def _collect(func, items, futures):
     and wait for its running ones, as the old per-call pool's exit did."""
     try:
         return _run_rest(func, items, futures)
-    except BaseException:
-        for future in futures:
-            future.cancel()
-        wait(futures)
+    except BaseException:  # re-raised below, after the running items stop
+        # Wait only for what could not be cancelled (running or done). A
+        # future ``shutdown(cancel_futures=True)`` already cancelled stays
+        # CANCELLED, never notified, and ``wait`` would block on it forever
+        # (#593); ``cancel()`` is True for those as well.
+        wait([future for future in futures if not future.cancel()])
         raise