Isolate::run_microtasks() already exists but the microtasks queue
is also flushed on script entry and exit. Some embedders want strict
control over when microtasks run, which is set by the policy.
And deprecate Isolate::run_microtasks(). The corresponding V8 API is
tagged with V8_DEPRECATE_SOON.
Certain callbacks (e.g. `WasmLoadSourceMapCallback`) expect the
embedder to return a local handle (a `Local<String>` in this case), but
do not provide a `Local<Context>` as an argument, nor does it provide
any other argument that we might obtain a context from. This is not
unexpected - WASM execution as such is not tied to a context, and a
`Local<String>` can be created without a context too because it's a
primitive value that has no JavaScript prototype.
It turns out that using `v8::internal::MaybeObject` was, although
harmless, not appropriate here, because this function does not deal
deal with (potentially) weak handles.
* Merged all handle type implementations into one file ('handle.h').
* Made it so that `Global` handles cannot be empty.
* Renamed the `AsHandle` trait to `Handle`, and made it more generally
useful.
* Simplified how `PartialEq` is implemented for V8 heap objects and/or
the `Local`/`Global` handles that reference them.
Local handles never need to be mutable. This patch also rounds up the
last few places where we were still asking the user to pass an `&mut T`
to an API method.
According to v8.h, "the returned handle is valid until this TryCatch
block has been destroyed". This is incorrect, as can be demonstrated
with the test below. In practice the return value lives no longer and
no shorter than the active HandleScope at the time these methods are
called. An issue has been opened about this in the V8 bug tracker:
https://bugs.chromium.org/p/v8/issues/detail?id=10537.
```rust
fn try_catch_bad_lifetimes() {
let _setup_guard = setup();
let mut isolate = v8::Isolate::new(Default::default());
let mut hs = v8::HandleScope::new(&mut isolate);
let scope = hs.enter();
let context = v8::Context::new(scope);
let mut cs = v8::ContextScope::new(scope, context);
let scope = cs.enter();
let caught_msg_2 = {
let mut try_catch = v8::TryCatch::new(scope);
let try_catch = try_catch.enter();
let caught_msg_1 = {
let mut hs = v8::HandleScope::new(scope);
let scope = hs.enter();
// Throw exception #1.
let msg_1 = v8::String::new(scope, "BOOM!").unwrap();
let exc_1 = v8::Exception::type_error(scope, msg_1);
scope.isolate().throw_exception(exc_1);
// Catch exception #1.
let caught_msg_1 = try_catch.message().unwrap();
let caught_str_1 =
caught_msg_1.get(scope).to_rust_string_lossy(scope);
assert!(caught_str_1.contains("BOOM"));
// Move `caught_msg_1` out of the HandleScope it was created in.
// The borrow checker allows this because `caught_msg_1`'s
// lifetime is contrained to not outlive the TryCatch, but it is
// allowed to outlive the HandleScope that was active when the
// exception was caught.
caught_msg_1
};
// Next line crashes.
let caught_str_1 =
caught_msg_1.get(scope).to_rust_string_lossy(scope);
assert!(caught_str_1.contains("BOOM"));
// Throws exception #2.
let msg_2 = v8::String::new(scope, "DANG!").unwrap();
let exc_2 = v8::Exception::type_error(scope, msg_2);
scope.isolate().throw_exception(exc_2);
// Catch exception #2.
let caught_msg_2 = try_catch.message().unwrap();
let caught_str_2 =
caught_msg_2.get(scope).to_rust_string_lossy(scope);
assert!(caught_str_2.contains("DANG"));
// Move `caught_msg_2` out of the extent of the TryCatch, but still
// within the extent of its HandleScope. This is unnecessarily
// rejected at compile time.
caught_msg_2
};
let caught_str_2 =
caught_msg_2.get(scope).to_rust_string_lossy(scope);
assert!(caught_str_2.contains("DANG"));
}
```
* Add `SharedPtr` as a nullable sibling to `SharedRef`.
* Add `Borrow`, `AsRef` and `AsMut` implementations as appropriate.
* `SharedRef<T>` now derefs to `T` rather than to `UnsafeCell<T>`.
* `array_buffer::BackingStore` now derefs to `[Cell<u8>]`.
