Close #16742: Fix misuse of memory allocations in PyOS_Readline()

The GIL must be held to call PyMem_Malloc(), whereas PyOS_Readline() releases
the GIL to read input.

The result of the C callback PyOS_ReadlineFunctionPointer must now be a string
allocated by PyMem_RawMalloc() or PyMem_RawRealloc() (or NULL if an error
occurred), instead of a string allocated by PyMem_Malloc() or PyMem_Realloc().

Fixing this issue was required to setup a hook on PyMem_Malloc(), for example
using the tracemalloc module.

PyOS_Readline() copies the result of PyOS_ReadlineFunctionPointer() into a new
buffer allocated by PyMem_Malloc(). So the public API of PyOS_Readline() does
not change.

1 files changed, 8 insertions, 0 deletions

diff --git a/Doc/c-api/veryhigh.rst b/Doc/c-api/veryhigh.rst
index 19af7bf..a129963 100644
--- a/Doc/c-api/veryhigh.rst
+++ b/Doc/c-api/veryhigh.rst
@@ -166,6 +166,14 @@ the same library that the Python runtime is using.
    resulting string.  For example, The :mod:`readline` module sets
    this hook to provide line-editing and tab-completion features.
 
+   The result must be a string allocated by :c:func:`PyMem_RawMalloc` or
+   :c:func:`PyMem_RawRealloc`, or *NULL* if an error occurred.
+
+   .. versionchanged:: 3.4
+      The result must be allocated by :c:func:`PyMem_RawMalloc` or
+      :c:func:`PyMem_RawRealloc`, instead of being allocated by
+      :c:func:`PyMem_Malloc` or :c:func:`PyMem_Realloc`.
+
 
 .. c:function:: struct _node* PyParser_SimpleParseString(const char *str, int start)