push_cold_blocks_to_end() does not check the return value of basicblock_addop() #159054

Crash report

Summary

push_cold_blocks_to_end() does not check the return value of basicblock_addop() when adding the explicit jump block:

basicblock_addop() can fail with ERROR if allocation of the instruction array fails. In that case, explicit_jump remains empty, but the code
continues and assumes that the instruction was successfully added:

basicblock_last_instr(explicit_jump) then returns NULL, and last->i_target dereferences it, causing a segmentation fault instead of
propagating the MemoryError.

The return value should be checked, for example with RETURN_IF_ERROR().

Reproduction Code

Requires _testcapi. Reproduced on a release build and on a --with-pydebug --disable-gil build of the main branch.

import _testcapi

src = "async def f():\n    await x\n"
compile(src, "<s>", "exec")  # warm up before injecting failures
for n in range(1, 500):
    _testcapi.set_nomemory(n, n + 1)
    try:
        compile(src, "<s>", "exec")
    except MemoryError:
        pass
    finally:
        _testcapi.remove_mem_hooks()
print("no crash")

The same crash occurs with these sources in place of src:

"def g():\n    yield from h()\n"
"async def a():\n    async for i in x:\n        pass\n"
"async def a():\n    return [i async for i in y]\n"

Actual Behavior

The process is killed by SIGSEGV (exit code 139) and no crash is never printed. With -X faulthandler:

Fatal Python error: Segmentation fault
Current thread 0x000075b5d80ec740 [python] (most recent call first):
  File "repro.py", line 8 in <module>

In gdb (debug build):

Program received signal SIGSEGV, Segmentation fault.
3558	            last->i_target = explicit_jump->b_next;
#0  push_cold_blocks_to_end at Python/flowgraph.c:3558
#1  _PyCfg_OptimizeCodeUnit at Python/flowgraph.c:3834
#2  optimize_and_assemble_code_unit at Python/compile.c:1487
#3  _PyCompile_OptimizeAndAssemble at Python/compile.c:1526
#4  codegen_function_body at Python/codegen.c:1474
#5  codegen_function at Python/codegen.c:1565
#6  codegen_visit_stmt at Python/codegen.c:3217
(gdb) p last
$1 = (cfg_instr *) 0x0
(gdb) p explicit_jump->b_iused
$2 = 0
(gdb) p explicit_jump->b_instr
$3 = (cfg_instr *) 0x0

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

No response

Linked PRs

• gh-159054: Check basicblock_addop() failure in push_cold_blocks_to_end() #159057