Return the main thread's short identifier on certain platforms

CPython's pthread-based thread identifier relies on pthread_t being able
to be represented as an unsigned integer type.

This is true in most Linux libc implementations where it's defined as an
unsigned long, however musl typedefs it as a struct *.

If the pointer has the high bit set and is cast to PyThread_ident_t, the
resultant value can be sign-extended [0]. This can cause issues when
comparing against threading._MainThread's identifier. The main thread's
identifier value is retrieved via _get_main_thread_ident which is backed
by an unsigned long which truncates sign extended bits.

  >>> hex(threading.main_thread().ident)
  '0xb6f33f3c'
  >>> hex(threading.current_thread().ident)
  '0xffffffffb6f33f3c'

Work around this by conditionally compiling in some code for non-glibc
based Linux platforms that are at risk of sign-extension to return a
PyLong based on the main thread's unsigned long thread identifier if the
current thread is the main thread.

[0]: https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/Arrays-and-pointers-implementation.html

Signed-off-by: Vincent Fazio <[email protected]>

Author: vfazio

Modules/_threadmodule.c

@@ -2029,6 +2029,13 @@ An obsolete synonym of allocate_lock().");
 static PyObject *
 thread_get_ident(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
+    /* Work around an issue with the main thread ID to failing comparison checks
+       due to sign extension on some Linux libc implemenations. Can be removed
+       when thread identifiers are reworked. */
+#if SIZEOF_LONG < SIZEOF_LONG_LONG && defined(__linux__) && !defined(__GLIBC__)
+    if (_Py_IsMainThread())
+        return PyLong_FromUnsignedLong(_PyRuntime.main_thread);
+#endif
     PyThread_ident_t ident = PyThread_get_thread_ident_ex();
     if (ident == PYTHREAD_INVALID_THREAD_ID) {
         PyErr_SetString(ThreadError, "no current thread ident");