Access browsing context safely
Current browsing context accessor in `Document` unwraps the browsing context `Option` which prevents document to handle correctly the case when there is no browsing context.
This is the reason servo panics with `session-history.max-length=1` (https://github.com/servo/servo/issues/15877).
As it is my first contribution, I added a `safe` method to retrieve the browsing context rather than change the existing method, but I am happy to change if you think this is the right approach. I did not as well replace all existing method call to the `safe` method, to focus on fixing the issue.
If someone can give me a bit of guidance for the test, I will try to contribute one.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [X] These changes fix#15877.
- [ ] There are tests for these changes
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/15906)
<!-- Reviewable:end -->
Current browsing context accessor in Document unwrap the browsing context option which prevents document to handle correctly the case when there is no browsing context.
In support of this goal, the layout thread collects information about
CSS images that are missing image data and hands it off to the script
thread after layout completes. The script thread stores a list of
nodes that will need to be reflowed after the associated network
request is complete. The script thread ensures that the nodes are
not GCed while a request is ongoing, which the layout thread is
incapable of guaranteeing.
The image cache's API has also been redesigned in support of this
work. No network requests are made by the new image cache, since it
does not possess the document-specific information necessary to
initiate them. Instead, there is now a single, synchronous
query operation that optionally reserves a slot when a cache
entry for a URL cannot be found. This reserved slot is then
the responsibility of the queryer to populate with the contents
of the network response for the URL once it is complete. Any
subsequent queries for the same URL will be informed that the
response is pending until that occurs.
The changes to layout also remove the synchronous image loading
code path, which means that reftests now test the same code
that non-test binaries execute. The decision to take a screenshot
now considers whether there are any outstanding image
requests for layout in order to avoid intermittent failures in
reftests that use CSS images.
It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
added dom obj counting to decide sequential/parallel layout (#10110)
This is a rebased version of #11713
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#10110 (github issue number if applicable).
- [X] There are no tests for these changes because it's an optimization with no visible behavioral changes
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12862)
<!-- Reviewable:end -->
