VM crash when inspecting a parameter in VSCode debugger #59653
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Summary: VS Code debugger causes VM crash when inspecting a local variable's value during debugging. The crash occurs in |
dart-github-bot
added
area-vm
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//cc @derekxu16 |
a-siva
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vm-service
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A smaller repro, that I arrived at by reducing the given repro, is to hover over I haven't found the root cause of the crash yet, but it's possible to avoid the crash by removing the extension type, i.e. changing the code to: So, I should be able to figure out the root cause by looking into what the VM is doing differently when the extension type is present versus when it isn't. |
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I’ve now created two repros that are even smaller. Example 1 includes an extension type and crashes: import 'dart:developer' show debugger;
abstract interface class SharedTypeStructure {}
extension type SharedTypeSchemaView<TypeStructure extends SharedTypeStructure>(
TypeStructure _typeStructure
)
implements Object {}
class C extends SharedTypeStructure {}
class ClassWithTypeAnalyzer<TypeStructure extends SharedTypeStructure> {
SharedTypeSchemaView<TypeStructure> dispatchExpression(
SharedTypeSchemaView<TypeStructure> schema,
) {
return schema;
}
void analyzeExpression(SharedTypeSchemaView<TypeStructure> schema) {
debugger(); // Evaluate 'dispatchExpression(schema)';
}
}
void main() {
final classWithTypeAnalyzer = ClassWithTypeAnalyzer<C>();
classWithTypeAnalyzer.analyzeExpression(SharedTypeSchemaView(C()));
}Example 2 does not include an extension type and does not crash: import 'dart:developer' show debugger;
abstract interface class SharedTypeStructure {}
class C extends SharedTypeStructure {}
class ClassWithTypeAnalyzer<TypeStructure> {
int dispatchExpression(TypeStructure schema) {
return 123;
}
void analyzeExpression(TypeStructure schema) {
debugger(); // Evaluate 'dispatchExpression(schema as TypeStructure)';
}
}
void main() {
final classWithTypeAnalyzer = ClassWithTypeAnalyzer<C>();
classWithTypeAnalyzer.analyzeExpression(C());
}The crash in example 1 happens when the VM tries to finalize the signature of the expression evaluation function. The sdk/runtime/vm/class_finalizer.cc Line 388 in 751fe13 The problem is that the @johnniwinther or @jensjoha, is it possible that references to the Here is the human-readable dump of the expression evaluation function kernel from example 1: Here is the human-readable dump of the expression evaluation function kernel from example 2: |
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I might have changed the examples a little bit, but nevertheless. What the normal compile creates for a similar thing: What the expression compiler creates: I'd say that looks fair. Then the whole thing is wrapped in a component though: Notice how we now reference the type parameter on the Let's compare that to a version without an extension type. From the normal compiler: What the expression compiler creates: And wrapped in a component. Notice how we don't reference any type parameter at all (this is probably because we use the runtime type --- which we don't on extension types because they don't 'exist' at runtime --- but still). In the VM source I found this: So - at least in some places in the VM - when we see, say, class From other debug-printing I noticed this: Notice how it seemingly knows X0, but also doesn't know X0? And from debug-printing the "normal load": Lots of other prints etc I got to this: https://dart-review.googlesource.com/c/sdk/+/405381 It at least fixes this reproduction. |
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Thanks, @jensjoha!
I think setting |
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Maybe we should eagerly set However, substituting |
(Note: I've been seeing crashes like this for at least a month; this is the first time I've figured out a repro)
Build the latest Dart SDK as of abb17bc. Then check out this code.
Using Visual Studio Code, with the SDK set to the SDK that was just built:
pkg/_fe_analyzer_shared/lib/src/type_inference/null_shorting.dartand place a breakpoint on line 17.pkg/analyzer/test/src/dart/resolution/extension_method_test.dart. Click on the "Debug" button that appears above line 12.pkg/_fe_analyzer_shared/lib/src/type_inference/type_analyzer.dartto line 583.nodeon line 568.Expected behavior: the value of the local variable
nodeis shown in a pop-up.Observed behavior: the VM crashes with the following stacktrace:
Stacktrace
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