gh-114271: Make `_thread.ThreadHandle` thread-safe in free-threaded builds (GH-115190)

Make `_thread.ThreadHandle` thread-safe in free-threaded builds

We protect the mutable state of `ThreadHandle` using a `_PyOnceFlag`.
Concurrent operations (i.e. `join` or `detach`) on `ThreadHandle` block
until it is their turn to execute or an earlier operation succeeds.
Once an operation has been applied successfully all future operations
complete immediately.

The `join()` method is now idempotent. It may be called multiple times
but the underlying OS thread will only be joined once. After `join()`
succeeds, any future calls to `join()` will succeed immediately.

The internal thread handle `detach()` method has been removed.

1 files changed, 7 insertions, 17 deletions

diff --git a/Lib/threading.py b/Lib/threading.py
index b6ff00a..ec89550 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -931,7 +931,6 @@ class Thread:
         if _HAVE_THREAD_NATIVE_ID:
             self._native_id = None
         self._tstate_lock = None
-        self._join_lock = None
         self._handle = None
         self._started = Event()
         self._is_stopped = False
@@ -956,14 +955,11 @@ class Thread:
             if self._tstate_lock is not None:
                 self._tstate_lock._at_fork_reinit()
                 self._tstate_lock.acquire()
-            if self._join_lock is not None:
-                self._join_lock._at_fork_reinit()
         else:
             # This thread isn't alive after fork: it doesn't have a tstate
             # anymore.
             self._is_stopped = True
             self._tstate_lock = None
-            self._join_lock = None
             self._handle = None
 
     def __repr__(self):
@@ -996,8 +992,6 @@ class Thread:
         if self._started.is_set():
             raise RuntimeError("threads can only be started once")
 
-        self._join_lock = _allocate_lock()
-
         with _active_limbo_lock:
             _limbo[self] = self
         try:
@@ -1167,17 +1161,9 @@ class Thread:
             self._join_os_thread()
 
     def _join_os_thread(self):
-        join_lock = self._join_lock
-        if join_lock is None:
-            return
-        with join_lock:
-            # Calling join() multiple times would raise an exception
-            # in one of the callers.
-            if self._handle is not None:
-                self._handle.join()
-                self._handle = None
-                # No need to keep this around
-                self._join_lock = None
+        # self._handle may be cleared post-fork
+        if self._handle is not None:
+            self._handle.join()
 
     def _wait_for_tstate_lock(self, block=True, timeout=-1):
         # Issue #18808: wait for the thread state to be gone.
@@ -1478,6 +1464,10 @@ class _MainThread(Thread):
         with _active_limbo_lock:
             _active[self._ident] = self
 
+    def _join_os_thread(self):
+        # No ThreadHandle for main thread
+        pass
+
 
 # Helper thread-local instance to detect when a _DummyThread
 # is collected. Not a part of the public API.