[WIP]  Implement parallel CSS parsing

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Opening for discussion.

The basic idea is to spawn a parser thread on a global thread-pool (like in Gecko) with an `AsyncStylesheetLoader` (instead of the previous `StylesheetLoader`), used when encountering an `@import` rule to send a task to script thread from the parser thread, which simply instantiates an async fetch on the corresponding document. After the parsing is done, another task is sent to script thread to do the post-parsing steps, like setting the parsed stylesheet on its link element (if applicable), firing the load event, etc.

A problem was that verifying the request generation id before parsing (still on the script thread) introduced a race condition, so I moved that to the post-parse task run on the script thread. This (seems to have) solved the race condition, but it differs in semantics from the sync parsing in that it's now no longer checked before doing the parsing, but after, which incurs an unnecessary parse overhead if the sheet is not applicable (I think).

Also, there seems to be an occasional time-out in one of the CSS tests, and the code needs some cleaning up.

---
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- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #20721 (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because existing CSS tests should cover them.

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[WIP]  Implement parallel CSS parsing

<!-- Please describe your changes on the following line: -->
Opening for discussion.

The basic idea is to spawn a parser thread on a global thread-pool (like in Gecko) with an `ThreadSafeStylesheetLoader` (instead of the previous `StylesheetLoader`), used when encountering an `@import` rule to send a task to script thread from the parser thread, which simply instantiates an async fetch on the corresponding document. After the parsing is done, another task is sent to script thread to do the post-parsing steps, like setting the parsed stylesheet on its link element (if applicable), firing the load event, etc.

A problem was that verifying the request generation id before parsing (still on the script thread) introduced a race condition, so I moved that to the post-parse task run on the script thread. This (seems to have) solved the race condition, but it differs in semantics from the sync parsing in that it's now no longer checked before doing the parsing, but after, which incurs an unnecessary parse overhead if the sheet is not applicable (I think).

Also, there seems to be an occasional time-out in one of the CSS tests, and the code needs some cleaning up.

---
<!-- 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
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #20721 (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because existing CSS tests should cover them.

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…r-process.

Instead of per-document. This also allows to reuse this thread-pool if needed
for other stuff, like parallel CSS parsing (servo#22478), and to share more code with
Gecko, which is always nice.

…er-process.

Instead of per-document. This also allows to reuse this thread-pool if needed
for other stuff, like parallel CSS parsing (servo#22478), and to share more code with
Gecko, which is always nice.

style: Make Servo use a single thread-pool for layout-related tasks per process.

Instead of per-document. This also allows to reuse this thread-pool if needed
for other stuff, like parallel CSS parsing (#22478), and to share more code with
Gecko, which is always nice.

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[WIP]  Implement parallel CSS parsing

<!-- Please describe your changes on the following line: -->
Opening for discussion.

The basic idea is to spawn a parser thread on a global thread-pool (like in Gecko) with an `ThreadSafeStylesheetLoader` (instead of the previous `StylesheetLoader`), used when encountering an `@import` rule to send a task to script thread from the parser thread, which simply instantiates an async fetch on the corresponding document. After the parsing is done, another task is sent to script thread to do the post-parsing steps, like setting the parsed stylesheet on its link element (if applicable), firing the load event, etc.

A problem was that verifying the request generation id before parsing (still on the script thread) introduced a race condition, so I moved that to the post-parse task run on the script thread. This (seems to have) solved the race condition, but it differs in semantics from the sync parsing in that it's now no longer checked before doing the parsing, but after, which incurs an unnecessary parse overhead if the sheet is not applicable (I think).

Also, there seems to be an occasional time-out in one of the CSS tests, and the code needs some cleaning up.

---
<!-- 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
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #20721 (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because existing CSS tests should cover them.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

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…e listener

The goal of this change is to prevent having to copy so much data out of
listeners when a fetch completes, which will be particularly important
for off-the-main thread parsing of CSS (see servo#22478). This change has
pros and cons:

Pros:
 - This makes the design of the `FetchResponseListener` a great deal simpler.
   They no longer individually store a dummy `ResourceFetchTiming` that is
   only replaced right before `process_response_eof`.
 - The creation of the `Arc<Mutex<FetchResponseListener>>` in the
   `NetworkListener` is abstracted away from clients and now they just
   pass the `FetchResponseListener` to the fetch methods in the global.

Cons:
 - Now each `FetchResponseListener` must explicitly call `submit_timing`
   instead of having the `NetworkListener` do it. This is arguably a bit
   easier to follow in the code.
 - Since the internal data of the `NetworkListener` is now an
   `Arc<Mutex<Option<FetchResponseListener>>>`, when the fetching code
   needs to share state with the `NetworkListener` it either needs to
   share an `Option` or some sort of internal state. In one case I've
   stored the `Option` and in another case, I've stored a new inner
   shared value.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

ste#

…e listener (#40556)

The goal of this change is to prevent having to copy so much data out of
listeners when a fetch completes, which will be particularly important
for off-the-main thread parsing of CSS (see #22478). This change has
pros and cons:

Pros:
- This makes the design of the `FetchResponseListener` a great deal
simpler.
They no longer individually store a dummy `ResourceFetchTiming` that is
   only replaced right before `process_response_eof`.
 - The creation of the `Arc<Mutex<FetchResponseListener>>` in the
   `NetworkListener` is abstracted away from clients and now they just
   pass the `FetchResponseListener` to the fetch methods in the global.

Cons:
 - Now each `FetchResponseListener` must explicitly call `submit_timing`
   instead of having the `NetworkListener` do it. This is arguably a bit
   easier to follow in the code.
 - Since the internal data of the `NetworkListener` is now an
   `Arc<Mutex<Option<FetchResponseListener>>>`, when the fetching code
   needs to share state with the `NetworkListener` it either needs to
   share an `Option` or some sort of internal state. In one case I've
   stored the `Option` and in another case, I've stored a new inner
   shared value.

Testing: This should not change observable behavior and is thus covered
by existing tests.
Fixes: #22550

---------

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

This change is a rework of servo#22478, originally authored by @vimpunk.

It adds parsing of CSS in parallel with the main script thread. The
big idea here is that when the transfer of stylesheet bytes is
finished, the actual parsing is pushed to a worker thread from the Stylo
thread pool. This also applies for subsequent loads triggered by
`@import` statements.

The design is quite similar to the previous PR with a few significant
changes:

 - Error handling works properly. The `CSSErrorReporter` is a crossbeam
   `Sender` and a `PipelineId` so it can be trivially cloned and sent to
   the worker thread.
 - Generation checking is done both before and after parsing, in order
   to both remove the race condition and avoid extra work when the
   generations do not match.
 - The design is reworked a bit to avoid code duplication, dropping added
   lines from 345 to 160.
 - Now that `process_response_eof` gives up ownership to the
   `FetchResponseListener`, this change avoids all extra copies.

Co-authored-by: mandreyel <mandreyel@protonmail.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>

This change is a rework of servo#22478, originally authored by @vimpunk.

It adds parsing of CSS in parallel with the main script thread. The
big idea here is that when the transfer of stylesheet bytes is
finished, the actual parsing is pushed to a worker thread from the Stylo
thread pool. This also applies for subsequent loads triggered by
`@import` statements.

The design is quite similar to the previous PR with a few significant
changes:

 - Error handling works properly. The `CSSErrorReporter` is a crossbeam
   `Sender` and a `PipelineId` so it can be trivially cloned and sent to
   the worker thread.
 - Generation checking is done both before and after parsing, in order
   to both remove the race condition and avoid extra work when the
   generations do not match.
 - The design is reworked a bit to avoid code duplication, dropping added
   lines from 345 to 160.
 - Now that `process_response_eof` gives up ownership to the
   `FetchResponseListener`, this change avoids all extra copies.

Co-authored-by: mandreyel <mandreyel@protonmail.com>
Signed-off-by: Martin Robinson <mrobinson@igalia.com>

This change is a rework of #22478, originally authored by @vimpunk.

It adds parsing of CSS in parallel with the main script thread. The
big idea here is that when the transfer of stylesheet bytes is
finished, the actual parsing is pushed to a worker thread from the Stylo
thread pool. This also applies for subsequent loads triggered by
`@import` statements.

The design is quite similar to the previous PR with a few significant
changes:

 - Error handling works properly. The `CSSErrorReporter` is a crossbeam
   `Sender` and a `PipelineId` so it can be trivially cloned and sent to
   the worker thread.
 - Generation checking is done both before and after parsing, in order
   to both remove the race condition and avoid extra work when the
   generations do not match.
- The design is reworked a bit to avoid code duplication, dropping added
   lines from 345 to 160.
 - Now that `process_response_eof` gives up ownership to the
   `FetchResponseListener`, this change avoids all extra copies.

Testing: This shouldn't change observable behavior, so is covered
by existing tests.
Fixes: #20721
Closes: #22478

---------

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: mandreyel <mandreyel@protonmail.com>