gh-98627: Add an Optional Check for Extension Module Subinterpreter Compatibility (gh-99040)

Enforcing (optionally) the restriction set by PEP 489 makes sense. Furthermore, this sets the stage for a potential restriction related to a per-interpreter GIL.

This change includes the following:

* add tests for extension module subinterpreter compatibility
* add _PyInterpreterConfig.check_multi_interp_extensions
* add Py_RTFLAGS_MULTI_INTERP_EXTENSIONS
* add _PyImport_CheckSubinterpIncompatibleExtensionAllowed()
* fail iff the module does not implement multi-phase init and the current interpreter is configured to check

https://github.com/python/cpython/issues/98627

2 files changed, 6 insertions, 0 deletions

diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index 6ce42b4..a070fa9 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -248,6 +248,7 @@ typedef struct {
     int allow_exec;
     int allow_threads;
     int allow_daemon_threads;
+    int check_multi_interp_extensions;
 } _PyInterpreterConfig;
 
 #define _PyInterpreterConfig_INIT \
@@ -256,6 +257,7 @@ typedef struct {
         .allow_exec = 0, \
         .allow_threads = 1, \
         .allow_daemon_threads = 0, \
+        .check_multi_interp_extensions = 1, \
     }
 
 #define _PyInterpreterConfig_LEGACY_INIT \
@@ -264,6 +266,7 @@ typedef struct {
         .allow_exec = 1, \
         .allow_threads = 1, \
         .allow_daemon_threads = 1, \
+        .check_multi_interp_extensions = 0, \
     }
 
 /* --- Helper functions --------------------------------------- */
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index be1fcb6..3efb241 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -11,6 +11,9 @@ is available in a given context.  For example, forking the process
 might not be allowed in the current interpreter (i.e. os.fork() would fail).
 */
 
+/* Set if import should check a module for subinterpreter support. */
+#define Py_RTFLAGS_MULTI_INTERP_EXTENSIONS (1UL << 8)
+
 /* Set if threads are allowed. */
 #define Py_RTFLAGS_THREADS (1UL << 10)