gh-116167: Allow disabling the GIL with `PYTHON_GIL=0` or `-X gil=0` (#116338)

In free-threaded builds, running with `PYTHON_GIL=0` will now disable the
GIL. Follow-up issues track work to re-enable the GIL when loading an
incompatible extension, and to disable the GIL by default.

In order to support re-enabling the GIL at runtime, all GIL-related data
structures are initialized as usual, and disabling the GIL simply sets a flag
that causes `take_gil()` and `drop_gil()` to return early.

2 files changed, 17 insertions, 0 deletions

diff --git a/Include/internal/pycore_gil.h b/Include/internal/pycore_gil.h
index 19b0d23..d36b4c0 100644
--- a/Include/internal/pycore_gil.h
+++ b/Include/internal/pycore_gil.h
@@ -20,6 +20,11 @@ extern "C" {
 #define FORCE_SWITCHING
 
 struct _gil_runtime_state {
+#ifdef Py_GIL_DISABLED
+    /* Whether or not this GIL is being used. Can change from 0 to 1 at runtime
+       if, for example, a module that requires the GIL is loaded. */
+    int enabled;
+#endif
     /* microseconds (the Python API uses seconds, though) */
     unsigned long interval;
     /* Last PyThreadState holding / having held the GIL. This helps us
diff --git a/Include/internal/pycore_initconfig.h b/Include/internal/pycore_initconfig.h
index c869882..1c68161 100644
--- a/Include/internal/pycore_initconfig.h
+++ b/Include/internal/pycore_initconfig.h
@@ -153,6 +153,18 @@ typedef enum {
     _PyConfig_INIT_ISOLATED = 3
 } _PyConfigInitEnum;
 
+typedef enum {
+    /* For now, this means the GIL is enabled.
+
+       gh-116329: This will eventually change to "the GIL is disabled but can
+       be reenabled by loading an incompatible extension module." */
+    _PyConfig_GIL_DEFAULT = -1,
+
+    /* The GIL has been forced off or on, and will not be affected by module loading. */
+    _PyConfig_GIL_DISABLE = 0,
+    _PyConfig_GIL_ENABLE = 1,
+} _PyConfigGILEnum;
+
 // Export for '_testembed' program
 PyAPI_FUNC(void) _PyConfig_InitCompatConfig(PyConfig *config);