bpo-45953: Statically allocate the main interpreter (and initial thread state). (gh-29883)

Previously, the main interpreter was allocated on the heap during runtime initialization.  Here we instead embed it into _PyRuntimeState, which means it is statically allocated as part of the _PyRuntime global.  The same goes for the initial thread state (of each interpreter, including the main one).  Consequently there are fewer allocations during runtime/interpreter init, fewer possible failures, and better memory locality.

FYI, this also helps efforts to consolidate globals, which in turns helps work on subinterpreter isolation.

https://bugs.python.org/issue45953

Include/cpython/pystate.h

@@ -2,6 +2,9 @@
 #  error "this header file must not be included directly"
 #endif
 
+#include <stdbool.h>
+
+
 PyAPI_FUNC(int) _PyInterpreterState_RequiresIDRef(PyInterpreterState *);
 PyAPI_FUNC(void) _PyInterpreterState_RequireIDRef(PyInterpreterState *, int);
 
@@ -83,6 +86,9 @@ struct _ts {
        after allocation. */
     int _initialized;
 
+    /* Was this thread state statically allocated? */
+    bool _static;
+
     int recursion_remaining;
     int recursion_limit;
     int recursion_headroom; /* Allow 50 more calls to handle any errors. */
@@ -175,9 +181,11 @@ struct _ts {
     PyObject **datastack_top;
     PyObject **datastack_limit;
     /* XXX signal handlers should also be here */
-
 };
 
+
+/* other API */
+
 // Alias for backward compatibility with Python 3.8
 #define _PyInterpreterState_Get PyInterpreterState_Get
 

Include/internal/pycore_global_objects.h

@@ -606,10 +606,6 @@ struct _Py_global_objects {
     }, \
 }
 
-static inline void
-_Py_global_objects_reset(struct _Py_global_objects *objects)
-{
-}
 
 #ifdef __cplusplus
 }

Include/internal/pycore_interp.h

@@ -8,6 +8,8 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
+#include <stdbool.h>
+
 #include "pycore_atomic.h"        // _Py_atomic_address
 #include "pycore_ast_state.h"     // struct ast_state
 #include "pycore_context.h"       // struct _Py_context_state
@@ -70,13 +72,18 @@ struct atexit_state {
 
 /* interpreter state */
 
-// The PyInterpreterState typedef is in Include/pystate.h.
+/* PyInterpreterState holds the global state for one of the runtime's
+   interpreters.  Typically the initial (main) interpreter is the only one.
+
+   The PyInterpreterState typedef is in Include/pystate.h.
+   */
 struct _is {
 
     struct _is *next;
 
     struct pythreads {
         uint64_t next_unique_id;
+        /* The linked list of threads, newest first. */
         struct _ts *head;
         /* Used in Modules/_threadmodule.c. */
         long count;
@@ -104,6 +111,9 @@ struct _is {
     int _initialized;
     int finalizing;
 
+    /* Was this interpreter statically allocated? */
+    bool _static;
+
     struct _ceval_state ceval;
     struct _gc_runtime_state gc;
 
@@ -166,8 +176,26 @@ struct _is {
 
     struct ast_state ast;
     struct type_cache type_cache;
+
+    /* The following fields are here to avoid allocation during init.
+       The data is exposed through PyInterpreterState pointer fields.
+       These fields should not be accessed directly outside of init.
+
+       All other PyInterpreterState pointer fields are populated when
+       needed and default to NULL.
+
+       For now there are some exceptions to that rule, which require
+       allocation during init.  These will be addressed on a case-by-case
+       basis.  Also see _PyRuntimeState regarding the various mutex fields.
+       */
+
+    /* the initial PyInterpreterState.threads.head */
+    struct _ts _initial_thread;
 };
 
+
+/* other API */
+
 extern void _PyInterpreterState_ClearModules(PyInterpreterState *interp);
 extern void _PyInterpreterState_Clear(PyThreadState *tstate);
 

Include/internal/pycore_runtime.h

@@ -11,8 +11,10 @@ extern "C" {
 #include "pycore_atomic.h"          /* _Py_atomic_address */
 #include "pycore_gil.h"             // struct _gil_runtime_state
 #include "pycore_global_objects.h"  // struct _Py_global_objects
+#include "pycore_interp.h"          // struct _is
 #include "pycore_unicodeobject.h"   // struct _Py_unicode_runtime_ids
 
+
 /* ceval state */
 
 struct _ceval_runtime_state {
@@ -53,6 +55,9 @@ typedef struct _Py_AuditHookEntry {
 
 /* Full Python runtime state */
 
+/* _PyRuntimeState holds the global state for the CPython runtime.
+   That data is exposed in the internal API as a static variable (_PyRuntime).
+   */
 typedef struct pyruntimestate {
     /* Has been initialized to a safe state.
 
@@ -81,7 +86,11 @@ typedef struct pyruntimestate {
 
     struct pyinterpreters {
         PyThread_type_lock mutex;
+        /* The linked list of interpreters, newest first. */
         PyInterpreterState *head;
+        /* The runtime's initial interpreter, which has a special role
+           in the operation of the runtime.  It is also often the only
+           interpreter. */
         PyInterpreterState *main;
         /* _next_interp_id is an auto-numbered sequence of small
            integers.  It gets initialized in _PyInterpreterState_Init(),
@@ -118,25 +127,44 @@ typedef struct pyruntimestate {
 
     struct _Py_unicode_runtime_ids unicode_ids;
 
+    /* All the objects that are shared by the runtime's interpreters. */
     struct _Py_global_objects global_objects;
-    // If anything gets added after global_objects then
-    // _PyRuntimeState_reset() needs to get updated to clear it.
+
+    /* The following fields are here to avoid allocation during init.
+       The data is exposed through _PyRuntimeState pointer fields.
+       These fields should not be accessed directly outside of init.
+
+       All other _PyRuntimeState pointer fields are populated when
+       needed and default to NULL.
+
+       For now there are some exceptions to that rule, which require
+       allocation during init.  These will be addressed on a case-by-case
+       basis.  Most notably, we don't pre-allocated the several mutex
+       (PyThread_type_lock) fields, because on Windows we only ever get
+       a pointer type.
+       */
+
+    /* PyInterpreterState.interpreters.main */
+    PyInterpreterState _main_interpreter;
 } _PyRuntimeState;
 
+#define _PyThreadState_INIT \
+    { \
+        ._static = 1, \
+    }
+#define _PyInterpreterState_INIT \
+    { \
+        ._static = 1, \
+        ._initial_thread = _PyThreadState_INIT, \
+    }
 #define _PyRuntimeState_INIT \
     { \
         .global_objects = _Py_global_objects_INIT, \
+        ._main_interpreter = _PyInterpreterState_INIT, \
     }
-/* Note: _PyRuntimeState_INIT sets other fields to 0/NULL */
 
-static inline void
-_PyRuntimeState_reset(_PyRuntimeState *runtime)
-{
-    /* Make it match _PyRuntimeState_INIT. */
-    memset(runtime, 0, (size_t)&runtime->global_objects - (size_t)runtime);
-    _Py_global_objects_reset(&runtime->global_objects);
-}
 
+/* other API */
 
 PyAPI_DATA(_PyRuntimeState) _PyRuntime;