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gh-156466: ensure in codegen that a pushed COMPILE_FBLOCK_ASYNC_COMPREHENSION_GENERATOR is always popped #157338

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gh-156466: ensure in codegen that a pushed COMPILE_FBLOCK_ASYNC_COMPREHENSION_GENERATOR is always popped #157338

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74 changes: 43 additions & 31 deletions Python/codegen.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4765,40 +4765,16 @@ codegen_sync_comprehension_generator(compiler *c, location loc,
}

static int
codegen_async_comprehension_generator(compiler *c, location loc,
asdl_comprehension_seq *generators,
int gen_index, int depth,
expr_ty elt, expr_ty val, int type,
IterStackPosition iter_pos, bool avoid_creation)
codegen_async_comprehension_generator_body(
compiler *c, location loc,
asdl_comprehension_seq *generators, int gen_index, int depth,
expr_ty elt, expr_ty val, int type, bool avoid_creation,
comprehension_ty gen, jump_target_label start)
{
NEW_JUMP_TARGET_LABEL(c, start);
NEW_JUMP_TARGET_LABEL(c, send);
NEW_JUMP_TARGET_LABEL(c, except);
NEW_JUMP_TARGET_LABEL(c, if_cleanup);

comprehension_ty gen = (comprehension_ty)asdl_seq_GET(generators,
gen_index);

if (iter_pos == ITERABLE_IN_LOCAL) {
if (gen_index == 0) {
assert(METADATA(c)->u_argcount == 1);
ADDOP_I(c, loc, LOAD_FAST, 0);
}
else {
/* Sub-iter - calculate on the fly */
VISIT(c, expr, gen->iter);
}
}
if (iter_pos != ITERATOR_ON_STACK) {
ADDOP(c, LOC(gen->iter), GET_AITER);
}

USE_LABEL(c, start);
/* Runtime will push a block here, so we need to account for that */
RETURN_IF_ERROR(
_PyCompile_PushFBlock(c, loc, COMPILE_FBLOCK_ASYNC_COMPREHENSION_GENERATOR,
start, NO_LABEL, NULL));

ADDOP_JUMP(c, loc, SETUP_FINALLY, except);
ADDOP(c, loc, GET_ANEXT);
ADDOP(c, loc, PUSH_NULL);
Expand Down Expand Up @@ -4907,15 +4883,51 @@ codegen_async_comprehension_generator(compiler *c, location loc,
USE_LABEL(c, if_cleanup);
ADDOP_JUMP(c, elt_loc, JUMP, start);

_PyCompile_PopFBlock(c, COMPILE_FBLOCK_ASYNC_COMPREHENSION_GENERATOR, start);

USE_LABEL(c, except);

ADDOP_JUMP(c, loc, END_ASYNC_FOR, send);

return SUCCESS;
}

static int
codegen_async_comprehension_generator(compiler *c, location loc,
asdl_comprehension_seq *generators,
int gen_index, int depth,
expr_ty elt, expr_ty val, int type,
IterStackPosition iter_pos, bool avoid_creation)
{
NEW_JUMP_TARGET_LABEL(c, start);

comprehension_ty gen = (comprehension_ty)asdl_seq_GET(generators,
gen_index);

if (iter_pos == ITERABLE_IN_LOCAL) {
if (gen_index == 0) {
assert(METADATA(c)->u_argcount == 1);
ADDOP_I(c, loc, LOAD_FAST, 0);
}
else {
/* Sub-iter - calculate on the fly */
VISIT(c, expr, gen->iter);
}
}
if (iter_pos != ITERATOR_ON_STACK) {
ADDOP(c, LOC(gen->iter), GET_AITER);
}

/* Runtime will push a block here, so we need to account for that */
RETURN_IF_ERROR(
_PyCompile_PushFBlock(c, loc, COMPILE_FBLOCK_ASYNC_COMPREHENSION_GENERATOR,
start, NO_LABEL, NULL));

int ret = codegen_async_comprehension_generator_body(
c, loc, generators, gen_index, depth, elt, val, type, avoid_creation,
gen, start);
_PyCompile_PopFBlock(c, COMPILE_FBLOCK_ASYNC_COMPREHENSION_GENERATOR, start);

@carljm carljm Sep 11, 2026

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This can pop the wrong block after a codegen error in a nested inlined comprehension. For example, compiling this source aborts a debug build:

async def f(it):
    return [[f(a=1, a=2) for y in z] async for x in it]

I verified that the parent commit raises SyntaxError: keyword argument repeated: a, while this PR aborts on the block-type assertion in _PyCompile_PopFBlock.

The inner comprehension leaves its COMPILE_FBLOCK_INLINED_COMPREHENSION pushed when codegen_comprehension takes its error path, so it is still above the async block when we get here. Could we make sure nested inlined-comprehension blocks are cleaned up on error before making this pop unconditional, and add a regression test for this case?

@iritkatriel iritkatriel Sep 12, 2026

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This seems to be all or nothing: either we always pop any block on error, or we allow them to remain on the stack until they are bulk removed on unrolling. I prefer the former, but we probably need to switch to that in one go. I'll try to stay in the current scheme on the subscope branch for now.

return ret;
}

static int
codegen_push_inlined_comprehension_locals(compiler *c, location loc,
PySTEntryObject *comp,
Expand Down
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