gh-140544: Always assume that thread locals are available (GH-140690)

Python has required thread local support since 3.12 (see GH-103324). By assuming that thread locals are always supported, we can improve the performance of third-party extensions by allowing them to access the attached thread and interpreter states directly.

Include/pyport.h

@@ -509,23 +509,18 @@ extern "C" {
 #endif
 
 #ifdef WITH_THREAD
-#  ifdef Py_BUILD_CORE
-#    ifdef HAVE_THREAD_LOCAL
-#      error "HAVE_THREAD_LOCAL is already defined"
-#    endif
-#    define HAVE_THREAD_LOCAL 1
-#    ifdef thread_local
-#      define _Py_thread_local thread_local
-#    elif __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
-#      define _Py_thread_local _Thread_local
-#    elif defined(_MSC_VER)  /* AKA NT_THREADS */
-#      define _Py_thread_local __declspec(thread)
-#    elif defined(__GNUC__)  /* includes clang */
-#      define _Py_thread_local __thread
-#    else
-       // fall back to the PyThread_tss_*() API, or ignore.
-#      undef HAVE_THREAD_LOCAL
-#    endif
+// HAVE_THREAD_LOCAL is just defined here for compatibility's sake
+#  define HAVE_THREAD_LOCAL 1
+#  ifdef thread_local
+#    define _Py_thread_local thread_local
+#  elif __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
+#    define _Py_thread_local _Thread_local
+#  elif defined(_MSC_VER)  /* AKA NT_THREADS */
+#    define _Py_thread_local __declspec(thread)
+#  elif defined(__GNUC__)  /* includes clang */
+#    define _Py_thread_local __thread
+#  else
+#    error "no supported thread-local variable storage classifier"
 #  endif
 #endif
 

Python/import.c

@@ -782,18 +782,13 @@ _PyImport_ClearModulesByIndex(PyInterpreterState *interp)
    substitute this (if the name actually matches).
 */
 
-#ifdef HAVE_THREAD_LOCAL
 _Py_thread_local const char *pkgcontext = NULL;
 # undef PKGCONTEXT
 # define PKGCONTEXT pkgcontext
-#endif
 
 const char *
 _PyImport_ResolveNameWithPackageContext(const char *name)
 {
-#ifndef HAVE_THREAD_LOCAL
-    PyMutex_Lock(&EXTENSIONS.mutex);
-#endif
     if (PKGCONTEXT != NULL) {
         const char *p = strrchr(PKGCONTEXT, '.');
         if (p != NULL && strcmp(name, p+1) == 0) {
@@ -801,23 +796,14 @@ _PyImport_ResolveNameWithPackageContext(const char *name)
             PKGCONTEXT = NULL;
         }
     }
-#ifndef HAVE_THREAD_LOCAL
-    PyMutex_Unlock(&EXTENSIONS.mutex);
-#endif
     return name;
 }
 
 const char *
 _PyImport_SwapPackageContext(const char *newcontext)
 {
-#ifndef HAVE_THREAD_LOCAL
-    PyMutex_Lock(&EXTENSIONS.mutex);
-#endif
     const char *oldcontext = PKGCONTEXT;
     PKGCONTEXT = newcontext;
-#ifndef HAVE_THREAD_LOCAL
-    PyMutex_Unlock(&EXTENSIONS.mutex);
-#endif
     return oldcontext;
 }
 

Python/pystate.c

@@ -67,9 +67,6 @@ to avoid the expense of doing their own locking).
    For each of these functions, the GIL must be held by the current thread.
  */
 
-#ifndef HAVE_THREAD_LOCAL
-#  error "no supported thread-local variable storage classifier"
-#endif
 
 /* The attached thread state for the current thread. */
 _Py_thread_local PyThreadState *_Py_tss_tstate = NULL;