Also found with the same reproducer, just setting nomemory at a different time. In _PyCfg_FromInstructionSequence if _PyCfgBuilder_Addop fails on the first instruction, it jumps to error and calls _PyCfgBuilder_Free(g). The entry block is still empty, but cfg_builder_check asserts g_entryblock->b_iused > 0.

If you want to reproduce, you'll have to play around a little with the start/stop for nomemory, but for me it's:

./python -c 'import _testcapi

compile("pass", "<warmup>", "single")
_testcapi.set_nomemory(40, 41)
try:
    compile("pass", "<trigger>", "single")
except MemoryError:
    pass
first = bytes(128)
second = bytes(128)'
python: Python/flowgraph.c:407: cfg_builder_check: Assertion `g->g_entryblock->b_iused > 0' failed.
Aborted                    (core dumped) ./python -c 'import _testcapi

Debug only so skipping news.

• Issue: Double free corruption when allocation failures occur in assemble_init #151112