Provide access to the import lock, fixing SF bug #580952. This is

mostly from SF patch #683257, but I had to change unlock_import() to
return an error value to avoid fatal error.

Should this be backported?  The patch requested this, but it's a new
feature.

Misc/NEWS

@@ -10,7 +10,7 @@ What's New in Python 2.3 alpha 2?
 *Release date: XX-XXX-2003*
 
 Core and builtins
-----------------
+-----------------
 
 - Through a bytecode optimizer bug (and I bet you didn't even know
   Python *had* a bytecode optimizer :-), "unsigned" hex/oct constants
@@ -75,6 +75,12 @@ Core and builtins
 Extension modules
 -----------------
 
+- The imp module now has ways to acquire and release the "import
+  lock": imp.acquire_lock() and imp.release_lock().  Note: this is a
+  reentrant lock, so releasing the lock only truly releases it when
+  this is the last release_lock() call.  You can check with
+  imp.lock_held().  (SF bug #580952 and patch #683257.)
+
 - Fix some bugs in the parser module.  SF bug #678518.
 
 - Thanks to Scott David Daniels, a subtle bug in how the zlib

Python/import.c

@@ -252,7 +252,8 @@ lock_import(void)
 		import_lock_level++;
 		return;
 	}
-	if (import_lock_thread != -1 || !PyThread_acquire_lock(import_lock, 0)) {
+	if (import_lock_thread != -1 || !PyThread_acquire_lock(import_lock, 0))
+	{
 		PyThreadState *tstate = PyEval_SaveThread();
 		PyThread_acquire_lock(import_lock, 1);
 		PyEval_RestoreThread(tstate);
@@ -261,25 +262,26 @@ lock_import(void)
 	import_lock_level = 1;
 }
 
-static void
+static int
 unlock_import(void)
 {
 	long me = PyThread_get_thread_ident();
 	if (me == -1)
-		return; /* Too bad */
+		return 0; /* Too bad */
 	if (import_lock_thread != me)
-		Py_FatalError("unlock_import: not holding the import lock");
+		return -1;
 	import_lock_level--;
 	if (import_lock_level == 0) {
 		import_lock_thread = -1;
 		PyThread_release_lock(import_lock);
 	}
+	return 1;
 }
 
 #else
 
 #define lock_import()
-#define unlock_import()
+#define unlock_import() 0
 
 #endif
 
@@ -295,6 +297,32 @@ imp_lock_held(PyObject *self, PyObject *args)
 #endif
 }
 
+static PyObject *
+imp_acquire_lock(PyObject *self, PyObject *args)
+{
+	if (!PyArg_ParseTuple(args, ":acquire_lock"))
+		return NULL;
+#ifdef WITH_THREAD
+	lock_import();
+#endif
+    return Py_None;
+}
+
+static PyObject *
+imp_release_lock(PyObject *self, PyObject *args)
+{
+	if (!PyArg_ParseTuple(args, ":release_lock"))
+		return NULL;
+#ifdef WITH_THREAD
+	if (unlock_import() < 0) {
+		PyErr_SetString(PyExc_RuntimeError,
+				"not holding the import lock");
+		return NULL;
+	}
+#endif
+    return Py_None;
+}
+
 /* Helper for sys */
 
 PyObject *
@@ -1970,7 +1998,12 @@ PyImport_ImportModuleEx(char *name, PyObject *globals, PyObject *locals,
 	PyObject *result;
 	lock_import();
 	result = import_module_ex(name, globals, locals, fromlist);
-	unlock_import();
+	if (unlock_import() < 0) {
+		Py_XDECREF(result);
+		PyErr_SetString(PyExc_RuntimeError,
+				"not holding the import lock");
+		return NULL;
+	}
 	return result;
 }
 
@@ -2743,6 +2776,17 @@ PyDoc_STRVAR(doc_lock_held,
 Return 1 if the import lock is currently held.\n\
 On platforms without threads, return 0.");
 
+PyDoc_STRVAR(doc_acquire_lock,
+"acquire_lock() -> None\n\
+Acquires the interpreter's import lock for the current thread.  This lock
+should be used by import hooks to ensure thread-safety when importing modules.
+On platforms without threads, this function does nothing.");
+
+PyDoc_STRVAR(doc_release_lock,
+"release_lock() -> None\n\
+Release the interpreter's import lock.\n\
+On platforms without threads, this function does nothing.");
+
 static PyMethodDef imp_methods[] = {
 	{"find_module",		imp_find_module, METH_VARARGS, doc_find_module},
 	{"get_magic",		imp_get_magic,	 METH_VARARGS, doc_get_magic},
@@ -2750,6 +2794,8 @@ static PyMethodDef imp_methods[] = {
 	{"load_module",		imp_load_module, METH_VARARGS, doc_load_module},
 	{"new_module",		imp_new_module,	 METH_VARARGS, doc_new_module},
 	{"lock_held",		imp_lock_held,	 METH_VARARGS, doc_lock_held},
+	{"acquire_lock",    imp_acquire_lock, METH_VARARGS, doc_acquire_lock},
+	{"release_lock",    imp_release_lock, METH_VARARGS, doc_release_lock},
 	/* The rest are obsolete */
 	{"get_frozen_object",	imp_get_frozen_object,	METH_VARARGS},
 	{"init_builtin",	imp_init_builtin,	METH_VARARGS},