unicodeobject.c has a static pointer to a unicode name CAPI capsule:

static _PyUnicode_Name_CAPI *ucnhash_capi = NULL;

The capsule is initialized on demand when the parser encounters a named unicode representation like "\N{digit nine}". Once the capsule pointer ucnhash_capi has been initialized, it is never reset. Not even a full interpreter shutdown invalidates the pointer.

A shutdown of the main interpreter with Py_Finalize() renders the pointer invalid. If the interpreter is re-initialized again, the invalid pointer causes a segfault. The problem was first discovered by Trey Hunner in ethanhs/python-wasm#69

python.js:219 Uncaught RuntimeError: null function or function signature mismatch
at _PyUnicode_DecodeUnicodeEscapeInternal (unicodeobject.c:6493:25)
at decode_unicode_with_escapes (string_parser.c:121:13)
at _PyPegen_parsestr (string_parser.c:273:1)
at strings_rule (action_helpers.c:901:20)
at atom_rule (parser.c:14293:27)
at primary_rule (parser.c:13916:17)
at await_primary_rule (parser.c:13666:17)
at factor_rule (parser.c:13542:29)
at term_rule (parser.c:13330:17)
at sum_rule (parser.c:13044:17)

I can reproduce the issue with pure C code:

$ gcc -Xlinker -export-dynamic -g -IInclude/ -I. -o ucnbug ucnbug.c libpython3.11.a -lm -ldl
$ gdb ucnbug
(gdb) run
0
9
Done

1

Program received signal SIGSEGV, Segmentation fault.
0x0000000000000000 in ?? ()
(gdb) bt
#0 0x0000000000000000 in ?? ()
#1 0x00000000005729a8 in _PyUnicode_DecodeUnicodeEscapeInternal (s=<optimized out>, s@entry=0x7fffea53b6d0 "\\N{digit nine}", size=<optimized out>, errors=errors@entry=0x0,
consumed=consumed@entry=0x0, first_invalid_escape=first_invalid_escape@entry=0x7fffffffc748) at Objects/unicodeobject.c:6490
#2 0x0000000000644fe3 in decode_unicode_with_escapes (parser=parser@entry=0x7fffea5e45d0, s=0x7fffea53b6d0 "\\N{digit nine}", s@entry=0x7fffea6af1d1 "\\N{digit nine}'", len=<optimized out>,
len@entry=14, t=t@entry=0x7fffea606910) at Parser/string_parser.c:118
#3 0x0000000000645675 in _PyPegen_parsestr (p=p@entry=0x7fffea5e45d0, bytesmode=bytesmode@entry=0x7fffffffc838, rawmode=rawmode@entry=0x7fffffffc83c, result=result@entry=0x7fffffffc848,
fstr=fstr@entry=0x7fffffffc850, fstrlen=fstrlen@entry=0x7fffffffc858, t=0x7fffea606910) at Parser/string_parser.c:269
#4 0x0000000000644163 in _PyPegen_concatenate_strings (p=p@entry=0x7fffea5e45d0, strings=strings@entry=0x94e310) at Parser/action_helpers.c:896
#5 0x00000000004791e6 in strings_rule (p=p@entry=0x7fffea5e45d0) at Parser/parser.c:15463
#6 0x000000000047c498 in atom_rule (p=p@entry=0x7fffea5e45d0) at Parser/parser.c:14274
#7 0x000000000047e159 in primary_raw (p=0x7fffea5e45d0) at Parser/parser.c:13908
#8 primary_rule (p=p@entry=0x7fffea5e45d0) at Parser/parser.c:13706

New changeset 44e9150 by Christian Heimes in branch 'main':
bpo-47182: Fix crash by named unicode characters after interpreter reinitialization (GH-32212)
44e9150

New changeset 55d5c96 by Christian Heimes in branch '3.10':
[3.10] bpo-47182: Fix crash by named unicode characters after interpreter reinitialization (GH-32212) (GH-32216)
55d5c96

Fixed by #32212