gh-123880: Allow recursive import of single-phase-init modules (GH-123950)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Co-authored-by: Brett Cannon <brett@python.org>

1 files changed, 13 insertions, 5 deletions

diff --git a/Python/import.c b/Python/import.c
index 26ad843..460b1fe 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -815,6 +815,8 @@ static int clear_singlephase_extension(PyInterpreterState *interp,
 
 // Currently, this is only used for testing.
 // (See _testinternalcapi.clear_extension().)
+// If adding another use, be careful about modules that import themselves
+// recursively (see gh-123880).
 int
 _PyImport_ClearExtension(PyObject *name, PyObject *filename)
 {
@@ -1322,12 +1324,16 @@ _extensions_cache_set(PyObject *path, PyObject *name,
     value = entry == NULL
         ? NULL
         : (struct extensions_cache_value *)entry->value;
-    /* We should never be updating an existing cache value. */
-    assert(value == NULL);
     if (value != NULL) {
-        PyErr_Format(PyExc_SystemError,
-                     "extension module %R is already cached", name);
-        goto finally;
+        /* gh-123880: If there's an existing cache value, it means a module is
+         * being imported recursively from its PyInit_* or Py_mod_* function.
+         * (That function presumably handles returning a partially
+         *  constructed module in such a case.)
+         * We can reuse the existing cache value; it is owned by the cache.
+         * (Entries get removed from it in exceptional circumstances,
+         *  after interpreter shutdown, and in runtime shutdown.)
+         */
+        goto finally_oldvalue;
     }
     newvalue = alloc_extensions_cache_value();
     if (newvalue == NULL) {
@@ -1392,6 +1398,7 @@ finally:
         cleanup_old_cached_def(&olddefbase);
     }
 
+finally_oldvalue:
     extensions_lock_release();
     if (key != NULL) {
         hashtable_destroy_str(key);
@@ -2128,6 +2135,7 @@ error:
 }
 
 
+// Used in _PyImport_ClearExtension; see notes there.
 static int
 clear_singlephase_extension(PyInterpreterState *interp,
                             PyObject *name, PyObject *path)