Without this check we would do an unnecessary partial second round
trip because of the call chain:
LayerListWidget::set_selected_layer() ->
LayerListWidget::on_layer_select() ->
ImageEditor::set_active_layer() ->
ImageEditor::on_active_layer_change() ->
LayerListWidget::set_selected_layer()
Prior this change, custom title metrics seen in the Basalt theme
and custom icon paths in Redmond themes were dropped when saving a file.
Now any entry, even empty, will be saved. This may end up with slightly
larger files, but on other hand, users will be able to see every option
in a text editor, without a need to look at the code/docs.
Because we were holding a strong ref to the OpenFileDescription in
LocalSocket and a strong ref to the LocalSocket in Inode, we were
creating a reference cycle in the event of the socket being cleaned up
after the file description did (i.e. unlinking the file before closing
the socket), because the file description never got destructed.
The TimeWait state is intended to prevent another socket from taking the
address tuple in case any packets are still in transit after the final
close. Since this state never delivers packets to userspace, it doesn't
make sense to keep the receive buffer around.
POSIX just says that mbstate_t should be an "object type other than an
array type" that can hold the conversion state for converting between
(multi-byte) characters and wide characters.
Since no other information regarding the contents is given, this
apparently means that we should add whatever we need once we start
implementing that conversion.
Clang was failing because because it rightfully saw we were attempting
to call a deleted constructor of `MarkedValueList`. If you explicitly
called move(list) then GCC would complain that the move was unnecessary.
For what ever reason both tool chains accept when we construct the
ThrowCompletionOr explicitly that we move the list into and return that.
We decided that we want to move away from throwing exceptions in AOs
and regular functions implicitly and then returning some
default-constructed value (usually an empty JS::Value) - this requires
remembering to check for an exception at the call site, which is
error-prone. It's also awkward for return values that cannot be
default-constructed, e.g. MarkedValueList.
Instead, the thrown value should be part of the function return value.
The solution to this is moving closer to the spec and using something
they call "completion records":
https://tc39.es/ecma262/#sec-completion-record-specification-type
This has various advantages:
- It becomes crystal clear whether some AO can fail or not, and errors
need to be handled and values unwrapped explicitly (for that reason
compatibility with the TRY() macro is already built-in, and a similar
TRY_OR_DISCARD() macro has been added specifically for use in LibJS,
while the majority of functions doesn't return ThrowCompletionOr yet)
- We no longer need to mix "valid" and "invalid" values of various types
for the success and exception outcomes (e.g. null/non-null AK::String,
empty/non-empty JS::Value)
- Subsequently it's no longer possible to accidentally use an exception
outcome return value as a success outcome return value (e.g. any AO
that returns a numeric type would return 0 even after throwing an
exception, at least before we started making use of Optional for that)
- Eventually the VM will no longer need to store an exception, and
temporarily clearing an exception (e.g. to call a function) becomes
obsolete - instead, completions will simply propagate up to the caller
outside of LibJS, which then can deal with it in any way
- Similar to throw we'll be able to implement the functionality of
break, continue, and return using completions, which will lead to
easier to understand code and fewer workarounds - the current
unwinding mechanism is not even remotely similar to the spec's
approach
The spec's NormalCompletion and ThrowCompletion AOs have been
implemented as simple wrappers around the JS::Completion constructor.
UpdateEmpty has been implemented as a JS::Completion method.
There's also a new VM::throw_completion<T>() helper, which basically
works like VM::throw_exception<T>() - it creates a T object (usually a
JS::Error), and returns it wrapped in a JS::Completion of Type::Throw.
Two temporary usage patterns have emerged:
1. Callee already returns ThrowCompletionOr, but caller doesn't:
auto foo = TRY_OR_DISCARD(bar());
2. Caller already returns ThrowCompletionOr, but callee doesn't:
auto foo = bar();
if (auto* exception = vm.exception())
return throw_completion(exception->value());
Eventually all error handling and unwrapping can be done with just TRY()
or possibly even operator? in the future :^)
Co-authored-by: Andreas Kling <kling@serenityos.org>
"result" is a tad bit too generic to provide a clash-free experience -
we found instances in LibJS where this breaks already. Essentially this
doesn't work:
auto foo = TRY(bar(result));
Because it expands to the following within the TRY() scope:
{
auto result = bar(result);
...
}
And that of course fails:
error: use of ‘result’ before deduction of ‘auto’
The simple solution here is to use a name that is much less likely to
clash with anything used in the expression ("_temporary_result"). :^)
This is much more common across the whole codebase and even these two
files. The same is used to return an empty JS::Value in an exception
check, for example.
