gh-133079: Remove Py_C_RECURSION_LIMIT & PyThreadState.c_recursion_remaining (GH-133080)

Both were added in 3.13, are undocumented, and don't make sense in 3.14 due to
changes in the stack overflow detection machinery (gh-112282).

PEP 387 exception for skipping a deprecation period: python/steering-council#288

Author: encukou

Doc/whatsnew/3.14.rst

@@ -2280,3 +2280,10 @@ Removed
 * Remove the private ``_Py_InitializeMain()`` function. It was a
   :term:`provisional API` added to Python 3.8 by :pep:`587`.
   (Contributed by Victor Stinner in :gh:`129033`.)
+
+* The undocumented APIs :c:macro:`!Py_C_RECURSION_LIMIT` and
+  :c:member:`!PyThreadState.c_recursion_remaining`, added in 3.13, are removed
+  without a deprecation period.
+  Please use :c:func:`Py_EnterRecursiveCall` to guard against runaway recursion
+  in C code.
+  (Removed in :gh:`133079`, see also :gh:`130396`.)

Include/cpython/pystate.h

@@ -118,8 +118,6 @@ struct _ts {
 
     int py_recursion_remaining;
     int py_recursion_limit;
-
-    int c_recursion_remaining; /* Retained for backwards compatibility. Do not use */
     int recursion_headroom; /* Allow 50 more calls to handle any errors. */
 
     /* 'tracing' keeps track of the execution depth when tracing/profiling.
@@ -210,8 +208,6 @@ struct _ts {
     _PyRemoteDebuggerSupport remote_debugger_support;
 };
 
-# define Py_C_RECURSION_LIMIT 5000
-
 /* other API */
 
 /* Similar to PyThreadState_Get(), but don't issue a fatal error

Misc/NEWS.d/next/C_API/2025-04-28-13-27-48.gh-issue-133079.DJL2sK.rst

@@ -0,0 +1,3 @@
+The undocumented APIs :c:macro:`!Py_C_RECURSION_LIMIT` and
+:c:member:`!PyThreadState.c_recursion_remaining`, added in 3.13, are removed
+without a deprecation period.

Python/pystate.c

@@ -1559,7 +1559,6 @@ init_threadstate(_PyThreadStateImpl *_tstate,
 
     tstate->py_recursion_limit = interp->ceval.recursion_limit;
     tstate->py_recursion_remaining = interp->ceval.recursion_limit;
-    tstate->c_recursion_remaining = 2;
     tstate->exc_info = &tstate->exc_state;
 
     // PyGILState_Release must not try to delete this thread state.

Python/vm-state.md

@@ -73,7 +73,8 @@ It will be more complex in the JIT.
 
 Another important piece of VM state is the **thread state**, held in `tstate`.
 The current frame pointer, `frame`, is always equal to `tstate->current_frame`.
-The thread state also holds the exception state (`tstate->exc_info`) and the recursion counters (`tstate->c_recursion_remaining` and `tstate->py_recursion_remaining`).
+The thread state also holds the exception state (`tstate->exc_info`) and
+recursion tracking data (`tstate->py_recursion_remaining`, `tstate->c_stack*`).
 
 The thread state is also used to access the **interpreter state** (`tstate->interp`), which is important since the "eval breaker" flags are stored there (`tstate->interp->ceval.eval_breaker`, an "atomic" variable), as well as the "PEP 523 function" (`tstate->interp->eval_frame`).
 The interpreter state also holds the optimizer state (`optimizer` and some counters).