Fix thread teardown panic when weakref callback fires during cleanup (RustPython#7965)

JamesClarke7283 · web-flow · commit a5775e0c07b8 · 2026-05-25T13:55:32.000+09:00
diff --git a/.gitignore b/.gitignore
@@ -27,4 +27,4 @@ Lib/site-packages/*
 Lib/test/data/*
 !Lib/test/data/README
 cpython/
-
+.claude/scheduled_tasks.lock
diff --git a/crates/vm/src/stdlib/_thread.rs b/crates/vm/src/stdlib/_thread.rs
@@ -530,10 +530,16 @@ pub(crate) mod _thread {
         // Increment thread count when thread actually starts executing
         vm.state.thread_count.fetch_add(1);
 
-        match func.invoke(args, vm) {
-            Ok(_obj) => {}
-            Err(e) if e.fast_isinstance(vm.ctx.exceptions.system_exit) => {}
-            Err(exc) => {
+        // Inner scope: drop `func` (and its Python refs) before the thread
+        // slot is torn down below. Otherwise the parameter `func` would drop
+        // at end-of-function, after cleanup_current_thread_frames has cleared
+        // CURRENT_THREAD_SLOT, and a weakref callback fired during that drop
+        // would panic in push_thread_frame.
+        {
+            let func = func;
+            if let Err(exc) = func.invoke(args, vm)
+                && !exc.fast_isinstance(vm.ctx.exceptions.system_exit)
+            {
                 vm.run_unraisable(
                     exc,
                     Some("Exception ignored in thread started by".to_owned()),
@@ -1663,11 +1669,18 @@ pub(crate) mod _thread {
                 // Increment thread count when thread actually starts executing
                 vm_state.thread_count.fetch_add(1);
 
-                // Run the function
-                match func.invoke((), vm) {
-                    Ok(_) => {}
-                    Err(e) if e.fast_isinstance(vm.ctx.exceptions.system_exit) => {}
-                    Err(exc) => {
+                // Inner scope: drop `func` (and its Python refs) before the
+                // outer scopeguard::defer tears down the thread slot. As a
+                // `move` closure capture, `func` would otherwise drop after
+                // all locals (including the scopeguard `_guard`), and a
+                // weakref callback fired during that drop would panic in
+                // push_thread_frame.
+                {
+                    let func = func;
+                    // Run the function
+                    if let Err(exc) = func.invoke((), vm)
+                        && !exc.fast_isinstance(vm.ctx.exceptions.system_exit)
+                    {
                         vm.run_unraisable(
                             exc,
                             Some("Exception ignored in thread started by".to_owned()),
diff --git a/extra_tests/snippets/stdlib_threading.py b/extra_tests/snippets/stdlib_threading.py
@@ -1,6 +1,7 @@
 import multiprocessing
 import os
 import threading
+import time
 
 
 def import_in_thread(module_name):
@@ -62,6 +63,48 @@ def start_fork_process_after_thread():
     assert process.exitcode == 0, process.exitcode
 
 
+def thread_join_ordering():
+    output = []
+
+    def thread_function(name):
+        output.append((name, 0))
+        time.sleep(2.0)
+        output.append((name, 1))
+
+    output.append((0, 0))
+    x = threading.Thread(target=thread_function, args=(1,))
+    output.append((0, 1))
+    x.start()
+    output.append((0, 2))
+    x.join()
+    output.append((0, 3))
+
+    assert len(output) == 6, output
+    # CPython has [(1, 0), (0, 2)] for the middle 2, but we have [(0, 2), (1, 0)]
+    # TODO: maybe fix this, if it turns out to be a problem?
+    # assert output == [(0, 0), (0, 1), (1, 0), (0, 2), (1, 1), (0, 3)]
+
+
+def thread_exit_without_join():
+    # Regression for https://github.com/RustPython/RustPython/issues/7813:
+    # a thread started without ``.join()`` must exit cleanly even when the
+    # captured target callable drops during teardown (which can fire
+    # weakref callbacks that re-enter the VM).
+    output = []
+
+    def runner():
+        output.append("runner done")
+
+    threading.Thread(target=runner).start()
+    time.sleep(1)
+    output.append("main done")
+    assert "runner done" in output, output
+    assert "main done" in output, output
+
+
+thread_join_ordering()
+thread_exit_without_join()
+
 import_in_thread("functools")
 import_in_thread("tempfile")
 import_in_thread("multiprocessing.connection")
diff --git a/extra_tests/snippets/test_threading.py b/extra_tests/snippets/test_threading.py
