Remove lock_held_at_fork struct member.

Use a process global instead.  This avoids changing the interpreter
structure.

Co-authored-by: Nathan Goldbaum <nathan.goldbaum@gmail.com>

Author: nascheme

Include/internal/pycore_interp_structs.h

@@ -661,10 +661,6 @@ struct _warnings_runtime_state {
     PyObject *once_registry;  /* Dict */
     PyObject *default_action; /* String */
     _PyRecursiveMutex lock;
-#ifdef HAVE_FORK
-    // Whether the thread calling fork() owns the warnings lock.
-    bool lock_held_at_fork;
-#endif
     long filters_version;
     PyObject *context;
 };

Lib/test/test_warnings/__init__.py

@@ -1693,6 +1693,11 @@ def test_fork_current_thread_holds_lock(self):
             for _ in range(3):
                 _warnings._acquire_lock()
             pid = os.fork()
+            if pid == 0:
+                # Ownership must also survive a second fork in the child.
+                child_pid = os.fork()
+                if child_pid != 0:
+                    support.wait_process(child_pid, exitcode=0)
             try:
                 # A warning must not change the inherited recursion depth.
                 warnings.warn('after fork')

Python/_warnings.c

@@ -278,19 +278,24 @@ warnings_lock_held(WarningsState *st)
 }
 
 #ifdef HAVE_FORK
+// PyOS_BeforeFork() holds HEAD_LOCK while saving this value and until fork(),
+// so concurrent forks cannot overwrite it before the child inherits it.
+// The child consumes its private copy before running Python callbacks.
+static bool warnings_lock_held_at_fork;
+
 void
 _PyWarnings_BeforeFork(PyInterpreterState *interp)
 {
     WarningsState *st = warnings_get_state(interp);
-    st->lock_held_at_fork = _PyRecursiveMutex_IsLockedByCurrentThread(&st->lock);
+    warnings_lock_held_at_fork = _PyRecursiveMutex_IsLockedByCurrentThread(&st->lock);
 }
 
 void
 _PyWarnings_AfterFork(PyInterpreterState *interp)
 {
     WarningsState *st = warnings_get_state(interp);
-    _PyRecursiveMutex_at_fork_reinit(&st->lock, st->lock_held_at_fork);
-    st->lock_held_at_fork = false;
+    _PyRecursiveMutex_at_fork_reinit(&st->lock, warnings_lock_held_at_fork);
+    warnings_lock_held_at_fork = false;
 }
 #endif
 

Tools/c-analyzer/cpython/ignored.tsv

@@ -362,6 +362,7 @@ Objects/unicodeobject.c	unicode_translate_call_errorhandler	argparse	-
 Parser/parser.c	-	reserved_keywords	-
 Parser/parser.c	-	soft_keywords	-
 Parser/lexer/lexer.c	-	type_comment_prefix	-
+Python/_warnings.c	-	warnings_lock_held_at_fork	-
 Python/ceval.c	-	_PyEval_BinaryOps	-
 Python/ceval.c	-	_Py_INTERPRETER_TRAMPOLINE_INSTRUCTIONS	-
 Python/ceval.c	-	_Py_INTERPRETER_TRAMPOLINE_INSTRUCTIONS_PTR	-