[3.15] gh-159739: Handle overlapping buffers in struct.pack_into (GH-159985) (#160101)

`struct.pack_into()` can use the same bytearray as both the destination and an `s` or `p` value. The copy then passes overlapping regions to `memcpy`, which is undefined in C and can produce platform-dependent results or a sanitizer abort. `memmove` makes the overlapping copy defined.
(cherry picked from commit 31a7d99)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>

Author: miss-islington

Misc/NEWS.d/next/Library/2026-10-20-00-00-00.gh-issue-159739.eb8987.rst

@@ -0,0 +1,2 @@
+Avoid undefined behavior when :func:`struct.pack_into` copies a bytearray into
+overlapping output.

Modules/_struct.c

@@ -2366,7 +2366,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const *args,
                 if (n > code->size)
                     n = code->size;
                 if (n > 0)
-                    memcpy(res, p, n);
+                    memmove(res, p, n);
             } else if (strcmp(e->format, "p") == 0) {
                 Py_ssize_t n;
                 int isstring;
@@ -2392,7 +2392,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const *args,
                     n = code->size - 1;
                 }
                 if (n > 0)
-                    memcpy(res + 1, p, n);
+                    memmove(res + 1, p, n);
                 if (n > 255)
                     n = 255;
                 if (n > 0) {