Don't hold a reference to the last job in the thread cache

newsfragments/1638.fix.rst

@@ -0,0 +1 @@
+The thread cache didn't release its reference to the previous job.

trio/_core/_thread_cache.py

@@ -69,6 +69,8 @@ def _work(self):
                 # instead of spawning a new thread.
                 self._thread_cache._idle_workers[self] = None
                 deliver(result)
+                del fn
+                del deliver
             else:
                 # Timeout acquiring lock, so we can probably exit. But,
                 # there's a race condition: we might be assigned a job *just*

trio/_core/tests/test_thread_cache.py

@@ -4,7 +4,7 @@
 import time
 import sys
 
-from .tutil import slow
+from .tutil import slow, gc_collect_harder
 from .. import _thread_cache
 from .._thread_cache import start_thread_soon, ThreadCache
 
@@ -25,6 +25,29 @@ def deliver(outcome):
         outcome.unwrap()
 
 
+def test_thread_cache_deref():
+    res = [False]
+
+    class del_me:
+        def __call__(self):
+            return 42
+
+        def __del__(self):
+            res[0] = True
+
+    q = Queue()
+
+    def deliver(outcome):
+        q.put(outcome)
+
+    start_thread_soon(del_me(), deliver)
+    outcome = q.get()
+    assert outcome.unwrap() == 42
+
+    gc_collect_harder()
+    assert res[0]
+
+
 @slow
 def test_spawning_new_thread_from_deliver_reuses_starting_thread():
     # We know that no-one else is using the thread cache, so if we keep