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[mypyc] Fix AttributeError in async try/finally with mixed return paths #19361

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Merged
merged 3 commits into from
Jul 2, 2025
Merged

[mypyc] Fix AttributeError in async try/finally with mixed return paths #19361

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7b0abac
[mypyc] Fix AttributeError in async try/finally with mixed return paths
Chainfire Jun 30, 2025
db7d454
Make GetAttrNullable a subclass of GetAttr
Chainfire Jun 30, 2025
7601eeb
Remove GetAttrNullable, use allow_null flag on GetAttr instead
Chainfire Jul 1, 2025
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4 changes: 4 additions & 0 deletions mypyc/analysis/dataflow.py
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Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,7 @@
FloatNeg,
FloatOp,
GetAttr,
GetAttrNullable,
GetElementPtr,
Goto,
IncRef,
Expand Down Expand Up @@ -209,6 +210,9 @@ def visit_load_literal(self, op: LoadLiteral) -> GenAndKill[T]:
def visit_get_attr(self, op: GetAttr) -> GenAndKill[T]:
return self.visit_register_op(op)

def visit_get_attr_nullable(self, op: GetAttrNullable) -> GenAndKill[T]:
return self.visit_register_op(op)

def visit_set_attr(self, op: SetAttr) -> GenAndKill[T]:
return self.visit_register_op(op)

Expand Down
5 changes: 5 additions & 0 deletions mypyc/analysis/ircheck.py
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,7 @@
FloatNeg,
FloatOp,
GetAttr,
GetAttrNullable,
GetElementPtr,
Goto,
IncRef,
Expand Down Expand Up @@ -319,6 +320,10 @@ def visit_get_attr(self, op: GetAttr) -> None:
# Nothing to do.
pass

def visit_get_attr_nullable(self, op: GetAttrNullable) -> None:
# Nothing to do.
pass

def visit_set_attr(self, op: SetAttr) -> None:
# Nothing to do.
pass
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8 changes: 8 additions & 0 deletions mypyc/analysis/selfleaks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@
FloatNeg,
FloatOp,
GetAttr,
GetAttrNullable,
GetElementPtr,
Goto,
InitStatic,
Expand Down Expand Up @@ -114,6 +115,13 @@ def visit_get_attr(self, op: GetAttr) -> GenAndKill:
return self.check_register_op(op)
return CLEAN

def visit_get_attr_nullable(self, op: GetAttrNullable) -> GenAndKill:
cl = op.class_type.class_ir
if cl.get_method(op.attr):
# Property -- calls a function
return self.check_register_op(op)
return CLEAN

def visit_set_attr(self, op: SetAttr) -> GenAndKill:
cl = op.class_type.class_ir
if cl.get_method(op.attr):
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19 changes: 19 additions & 0 deletions mypyc/codegen/emitfunc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,7 @@
FloatNeg,
FloatOp,
GetAttr,
GetAttrNullable,
GetElementPtr,
Goto,
IncRef,
Expand Down Expand Up @@ -426,6 +427,24 @@ def visit_get_attr(self, op: GetAttr) -> None:
elif not always_defined:
self.emitter.emit_line("}")

def visit_get_attr_nullable(self, op: GetAttrNullable) -> None:
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If you implement the suggestion to not use a subclass, it still makes sense to keep this code in a separate method that is called from the visit_get_attr handler if the flag is set.

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Not sure I understand what you mean here

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Even if the functionality is merged from GetAttrNullable into the GetAttr class, and there is no longer a visit_get_attr_nullable method in OpVisitor, the code here doesn't need to be merged into the visit_get_attr method, but this method can be renamed and called from visit_get_attr method to handle the nullable case. I.e. visit_get_attr could look like this (some details omitted):

    def visit_get_attr(self, op: GetAttr) -> None:
        if op.allow_null:
            self.get_attr_with_allow_null(op)
            return
        ... <existing code of visit_get_attr>

    def get_attr_with_allow_null(self, op: GetAttr) -> None:
        ... <similar to visit_get_attr_nullable in original version of this PR>

"""Handle GetAttrNullable which allows NULL without raising AttributeError."""
dest = self.reg(op)
obj = self.reg(op.obj)
rtype = op.class_type
cl = rtype.class_ir
attr_rtype, decl_cl = cl.attr_details(op.attr)

# Direct struct access without NULL check
attr_expr = self.get_attr_expr(obj, op, decl_cl)
self.emitter.emit_line(f"{dest} = {attr_expr};")

# Only emit inc_ref if not NULL
if attr_rtype.is_refcounted and not op.is_borrowed:
self.emitter.emit_line(f"if ({dest} != NULL) {{")
self.emitter.emit_inc_ref(dest, attr_rtype)
self.emitter.emit_line("}")

def next_branch(self) -> Branch | None:
if self.op_index + 1 < len(self.ops):
next_op = self.ops[self.op_index + 1]
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22 changes: 22 additions & 0 deletions mypyc/ir/ops.py
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Original file line number Diff line number Diff line change
Expand Up @@ -799,6 +799,24 @@ def accept(self, visitor: OpVisitor[T]) -> T:
return visitor.visit_get_attr(self)


class GetAttrNullable(GetAttr):
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It would be more consistent with other ops to add a new flag to GetAttr instead of adding a subclass. For example, add allow_null: bool = False as a keyword-only argument to GetAttr.__init__, and set the error kind accordingly in __init__ when the flag is true, and a boolean attribute to GetAttr. I expect that this change would also reduce the the size of the PR. Can you make this change?

(Your approach clearly has some benefits over adding a flag, as it's arguably easy to forget to handle the flag in a visitor, but since we already have a bunch of flags that impact the semantics in Op subclasses, this is something that already needs to be remembered when writing a visitor.)

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@Chainfire Chainfire Jul 1, 2025 •
edited
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I used the flag approach somewhere in my journey, my concern was that this might reduce performance as every GetAttr would be a tiny bit slower. That being said, if you prefer to have it that way, I guess I can make the change.

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I expect the performance impact to be quite small, since checking a boolean flag is quick (here branch prediction success rate should be high). I try to avoid subclasses of concrete classes such as GetAttr as a style convention, especially in new code, so I'd prefer to have this as a flag.

"""obj.attr (for a native object) - allows NULL without raising AttributeError

This is used for spill targets where NULL indicates the non-return path was taken.
Unlike GetAttr, this won't raise AttributeError when the attribute is NULL.
"""

error_kind = ERR_NEVER

def __init__(self, obj: Value, attr: str, line: int, *, borrow: bool = False) -> None:
super().__init__(obj, attr, line, borrow=borrow)
# Override error_kind since GetAttr sets it based on attr_type.error_overlap
self.error_kind = ERR_NEVER

def accept(self, visitor: OpVisitor[T]) -> T:
return visitor.visit_get_attr_nullable(self)


class SetAttr(RegisterOp):
"""obj.attr = src (for a native object)

Expand Down Expand Up @@ -1728,6 +1746,10 @@ def visit_load_literal(self, op: LoadLiteral) -> T:
def visit_get_attr(self, op: GetAttr) -> T:
raise NotImplementedError

@abstractmethod
def visit_get_attr_nullable(self, op: GetAttrNullable) -> T:
raise NotImplementedError

@abstractmethod
def visit_set_attr(self, op: SetAttr) -> T:
raise NotImplementedError
Expand Down
4 changes: 4 additions & 0 deletions mypyc/ir/pprint.py
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,7 @@
FloatNeg,
FloatOp,
GetAttr,
GetAttrNullable,
GetElementPtr,
Goto,
IncRef,
Expand Down Expand Up @@ -128,6 +129,9 @@ def visit_load_literal(self, op: LoadLiteral) -> str:
def visit_get_attr(self, op: GetAttr) -> str:
return self.format("%r = %s%r.%s", op, self.borrow_prefix(op), op.obj, op.attr)

def visit_get_attr_nullable(self, op: GetAttrNullable) -> str:
return self.format("%r = %s%r.%s?", op, self.borrow_prefix(op), op.obj, op.attr)

def borrow_prefix(self, op: Op) -> str:
if op.is_borrowed:
return "borrow "
Expand Down
10 changes: 10 additions & 0 deletions mypyc/irbuild/builder.py
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Original file line number Diff line number Diff line change
Expand Up @@ -72,6 +72,7 @@
Branch,
ComparisonOp,
GetAttr,
GetAttrNullable,
InitStatic,
Integer,
IntOp,
Expand Down Expand Up @@ -708,6 +709,15 @@ def read(

assert False, "Unsupported lvalue: %r" % target

def read_nullable_attr(self, obj: Value, attr: str, line: int = -1) -> Value:
"""Read an attribute that might be NULL without raising AttributeError.

This is used for reading spill targets in try/finally blocks where NULL
indicates the non-return path was taken.
"""
assert isinstance(obj.type, RInstance) and obj.type.class_ir.is_ext_class
return self.add(GetAttrNullable(obj, attr, line))

def assign(self, target: Register | AssignmentTarget, rvalue_reg: Value, line: int) -> None:
if isinstance(target, Register):
self.add(Assign(target, self.coerce_rvalue(rvalue_reg, target.type, line)))
Expand Down
10 changes: 8 additions & 2 deletions mypyc/irbuild/statement.py
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Original file line number Diff line number Diff line change
Expand Up @@ -46,6 +46,7 @@
YieldExpr,
YieldFromExpr,
)
from mypyc.common import TEMP_ATTR_NAME
from mypyc.ir.ops import (
NAMESPACE_MODULE,
NO_TRACEBACK_LINE_NO,
Expand Down Expand Up @@ -653,10 +654,15 @@ def try_finally_resolve_control(
if ret_reg:
builder.activate_block(rest)
return_block, rest = BasicBlock(), BasicBlock()
builder.add(Branch(builder.read(ret_reg), rest, return_block, Branch.IS_ERROR))
# For spill targets in try/finally, use nullable read to avoid AttributeError
if isinstance(ret_reg, AssignmentTargetAttr) and ret_reg.attr.startswith(TEMP_ATTR_NAME):
ret_val = builder.read_nullable_attr(ret_reg.obj, ret_reg.attr, -1)
else:
ret_val = builder.read(ret_reg)
builder.add(Branch(ret_val, rest, return_block, Branch.IS_ERROR))

builder.activate_block(return_block)
builder.nonlocal_control[-1].gen_return(builder, builder.read(ret_reg), -1)
builder.nonlocal_control[-1].gen_return(builder, ret_val, -1)

# TODO: handle break/continue
builder.activate_block(rest)
Expand Down
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