faulthandler: don't dump all threads when running without the GIL

It's not safe to access other threads stacks when running without the GIL.

Lib/test/support/script_helper.py

@@ -39,7 +39,7 @@ def interpreter_requires_environment():
     global __cached_interp_requires_environment
     if __cached_interp_requires_environment is None:
         # If PYTHONHOME is set, assume that we need it
-        if 'PYTHONHOME' in os.environ:
+        if 'PYTHONHOME' in os.environ or 'PYTHONGIL' in os.environ:
             __cached_interp_requires_environment = True
             return True
         # cannot run subprocess, assume we don't need it

Lib/test/test_capi/test_misc.py

@@ -311,7 +311,7 @@ def test_getitem_with_error(self):
                     r'Python runtime state: initialized\n'
                     r'ValueError: bug\n'
                     r'\n'
-                    r'Current thread .* \(most recent call first\):\n'
+                    r'(Current thread|Stack).* \(most recent call first\):\n'
                     r'  File .*, line 6 in <module>\n'
                     r'\n'
                     r'Extension modules: _testcapi \(total: 1\)\n')

Lib/test/test_faulthandler.py

@@ -95,21 +95,25 @@ def check_error(self, code, lineno, fatal_error, *,
 
         Raise an error if the output doesn't match the expected format.
         """
+        if sys.flags.nogil:
+            all_threads = False
         if all_threads:
             if know_current_thread:
                 header = 'Current thread 0x[0-9a-f]+'
             else:
                 header = 'Thread 0x[0-9a-f]+'
-        else:
+        elif know_current_thread:
             header = 'Stack'
         regex = [f'^{fatal_error}']
         if py_fatal_error:
             regex.append("Python runtime state: initialized")
         regex.append('')
-        regex.append(fr'{header} \(most recent call first\):')
-        if garbage_collecting:
+        if all_threads or know_current_thread:
+            regex.append(fr'{header} \(most recent call first\):')
+        if garbage_collecting and all_threads:
             regex.append('  Garbage-collecting')
-        regex.append(fr'  File "<string>", line {lineno} in {function}')
+        if all_threads or know_current_thread:
+            regex.append(fr'  File "<string>", line {lineno} in {function}')
         regex = '\n'.join(regex)
 
         if other_regex:
@@ -691,6 +695,8 @@ def check_register(self, filename=False, all_threads=False,
 
         Raise an error if the output doesn't match the expected format.
         """
+        if sys.flags.nogil:
+            all_threads = False
         signum = signal.SIGUSR1
         code = """
             import faulthandler

Modules/faulthandler.c

@@ -183,6 +183,11 @@ faulthandler_dump_traceback(int fd, int all_threads,
 
     reentrant = 1;
 
+    if (_PyRuntime.preconfig.disable_gil) {
+        // It's not safe to dump all threads when running without the GIL.
+        all_threads = 0;
+    }
+
     /* SIGSEGV, SIGFPE, SIGABRT, SIGBUS and SIGILL are synchronous signals and
        are thus delivered to the thread that caused the fault. Get the Python
        thread state of the current thread.

Python/pylifecycle.c

@@ -2559,7 +2559,14 @@ _Py_FatalError_DumpTracebacks(int fd, PyInterpreterState *interp,
     PUTS(fd, "\n");
 
     /* display the current Python stack */
-    _Py_DumpTracebackThreads(fd, interp, tstate);
+    if (_PyRuntime.preconfig.disable_gil) {
+        if (tstate != NULL) {
+            _Py_DumpTraceback(fd, tstate);
+        }
+    }
+    else {
+        _Py_DumpTracebackThreads(fd, interp, tstate);
+    }
 }
 
 /* Print the current exception (if an exception is set) with its traceback,