Start work on a speculative HTML Parser in Swift. This component will
walk ahead of the normal HTML parser looking for fetch() requests to
make while the normal parser is blocked. This work exposed many holes in
the Swift C++ interop component, which have been reported upstream.
There are two changes happening here: a correctness fix, and an
optimization. In theory they are unrelated, but the optimization
actually paves the way for the correctness fix.
Before this commit, the HTML tokenizer would attempt to look for named
character references by checking from after the `&` until the end of
m_decoded_input, which meant that it was unable to recognize things like
named character references that are inserted via `document.write` one
byte at a time. For example, if `∉` was written one-byte-at-a-time
with `document.write`, then the tokenizer would only check against `n`
since that's all that would exist at the time of the check and therefore
erroneously conclude that it was an invalid named character reference.
This commit modifies the approach taken for named character reference
matching by using a trie-like structure (specifically, a deterministic
acyclic finite state automaton or DAFSA), which allows for efficiently
matching one-character-at-a-time and therefore it is able to pick up
matching where it left off after each code point is consumed.
Note: Because it's possible for a partial match to not actually develop
into a full match (e.g. `¬indo` which could lead to `⋵̸`),
some backtracking is performed after-the-fact in order to only consume
the code points within the longest match found (e.g. `¬indo` would
backtrack back to `¬`).
With this new approach, `document.write` being called one-byte-at-a-time
is handled correctly, which allows for passing more WPT tests, with the
most directly relevant tests being
`/html/syntax/parsing/html5lib_entities01.html`
and
`/html/syntax/parsing/html5lib_entities02.html`
when run with `?run_type=write_single`. Additionally, the implementation
now better conforms to the language of the spec (and resolves a FIXME)
because exactly the matched characters are consumed and nothing more, so
SWITCH_TO is able to be used as the spec says instead of RECONSUME_IN.
The new approach is also an optimization:
- Instead of a linear search using `starts_with`, the usage of a DAFSA
means that it is always aware of which characters can lead to a match
at any given point, and will bail out whenever a match is no longer
possible.
- The DAFSA is able to take advantage of the note in the section
`13.5 Named character references` that says "This list is static and
will not be expanded or changed in the future." and tailor its Node
struct accordingly to tightly pack each node's data into 32-bits.
Together with the inherent DAFSA property of redundant node
deduplication, the amount of data stored for named character reference
matching is minimized.
In my testing:
- A benchmark tokenizing an arbitrary set of HTML test files was about
1.23x faster (2070ms to 1682ms).
- A benchmark tokenizing a file with tens of thousands of named
character references mixed in with truncated named character
references and arbitrary ASCII characters/ampersands runs about 8x
faster (758ms to 93ms).
- The size of `liblagom-web.so` was reduced by 94.96KiB.
Some technical details:
A DAFSA (deterministic acyclic finite state automaton) is essentially a
trie flattened into an array, but it also uses techniques to minimize
redundant nodes. This provides fast lookups while minimizing the
required data size, but normally does not allow for associating data
related to each word. However, by adding a count of the number of
possible words from each node, it becomes possible to also use it to
achieve minimal perfect hashing for the set of words (which allows going
from word -> unique index as well as unique index -> word). This allows
us to store a second array of data so that the DAFSA can be used as a
lookup for e.g. the associated code points.
For the Swift implementation, the new NamedCharacterReferenceMatcher
was used to satisfy the previous API and the tokenizer was left alone
otherwise. In the future, the Swift implementation should be updated to
use the same implementation for its NamedCharacterReference state as
the updated C++ implementation.
When setting scroll position during page load we need to consider
whether we actually have a fragment to scroll to. A script may already
have run at that point and may already have set a scroll position.
If there is an actual fragment to scroll to, it is fine to scroll to
that fragment, since it should take precedence. If we don't have a
fragment however, we should not unnecessarily overwrite the scroll
position set by the script back to (0, 0).
Since this problem is caused by a spec bug, I have tested the behavior
in the three major browsers engines. Unfortunately they do not agree
fully with each other. If there is no fragment at all (e.g. `foo.html`),
all browsers will respect the scroll position set by the script. If
there is a fragment (e.g. `foo.html#bar`), all browsers will set the
scroll position to the fragment element and ignore the one set by
script. However, when the fragment is empty (e.g. `foo.html#`), then
Blink and WebKit will set scroll position to the fragment, while Gecko
will set scroll position from script. Since all of this is ad-hoc
behavior anyway, I simply implemented the Blink/WebKit behavior because
of the majority vote for now.
This fixes a regression introduced in 51102254b5.
This commit implements the main "render blocking" behavior for link
elements, drastically reducing the amount of FOUC (flash of unstyled
content) we subject our users to.
The document will now block rendering until linked style sheets
referenced by parser-created link elements have loaded (or failed).
Note that we don't yet extend the blocking period until "critical
subresources" such as imported style sheets have been downloaded
as well.
Previously, the charset of name "UTF-16BE/LE" would be checked against
when following standards to convert the charset to UTF-8, but in
reality, the charsets "UTF-16BE" and "UTF-16LE" should be checked
separately.
Co-authored-by: Jelle Raaijmakers <jelle@ladybird.org>
There's a quirk in HTML where the parser should ignore any line feed
character immediately following a `pre` or `textarea` start tag.
This was working fine when we could peek ahead in the input stream and
see the next token, but didn't work in character-at-a-time parsing with
document.write().
This commit adds the "can ignore next line feed character" as a parser
flag that is maintained across invocations, making it work in this
parsing mode as well.
20 new passes in WPT/html/syntax/parsing/ :^)
Instead of always inserting a new text node, we now continue appending
to an extisting text node if the parser's character insertion point is
a suitable text node.
This fixes an issue where multiple invocations of document.write() would
create unnecessary sequences of text nodes. Such sequences are now
merged automatically.
19 new passes in WPT/html/syntax/parsing/ :^)
We were neglecting to return after handling the `frameset` start tag,
which caused us to process it twice, once properly and once generically.
54 new passes in WPT/html/syntax/parsing/ :^)
Before this change, the explicit EOF inserted by document.close() would
instantly abort the parser. This meant that parsing algorithms that ran
as part of the parser unwinding on EOF would never actually run.
591 new passes in WPT/html/syntax/parsing/ :^)
This exposed a problem where the parser would try to insert a root
<html> element on EOF in a document where someone already inserted such
an element via direct DOM manipulation. The parser now gracefully
handles this scenario. It's covered by existing tests (which would
crash without this change.)
This fixes a crash in the included test that regressed in 0adf261,
and is hit by the following HTML:
```html
<body></body>
<script>
const frame = document.body.appendChild(document.createElement("iframe"));
frame.contentDocument.open();
const child = frame.contentDocument.createElement("html")
const html = frame.contentDocument.appendChild(child);
frame.contentDocument.close();
</script>
```
I am not 100% sure this is fully the correct fix and there are other
cases which would not work properly. But it's definitely an improvement
to make the confuisingly named 'insert_an_eof' function of the tokenizer
actually do something.
We've historically asserted that no "saturated" size values end up as
final metrics for boxes in layout. This always had a chance of producing
false positives, since you can trivially create extremely large boxes
with CSS.
The reason we had those assertions was to catch bugs in our own engine
code where we'd incorrectly end up with non-finite values in layout
algorithms. At this point, we've found and fixed all known bugs of that
nature, and what remains are a bunch of false positives on pages that
create very large scrollable areas, iframes etc.
So, let's change it! We now clamp content width and height of boxes to
17895700 pixels, apparently the same cap as Firefox uses.
There's also the issue of calc() being able to produce non-finite
values. Note that we don't clamp the result of calc() directly, but
instead just clamp values when assigning them to content sizes.
Fixes#645.
Fixes#1236.
Fixes#1249.
Fixes#1908.
Fixes#3057.
This makes it more convenient to use the 'relvant agent' concept,
instead of the awkward dynamic casts we needed to do for every call
site.
mutation_observers is also changed to hold a GC::Root instead of raw
GC::Ptr. Somehow this was not causing problems before, but trips up CI
after these changes.
Previously, if the NumericCharacterReferenceEnd state was reached when
current_input_character was None, then the
DONT_CONSUME_NEXT_INPUT_CHARACTER macro would restore back before the
EOF, and allow the next state (after the SWITCH_TO_RETURN_STATE) to
proceed with the last digit of the numeric character reference.
For example, with something like `ї`, before this commit the
output would incorrectly be `<code point with the value 1111>1` instead
of just `<code point with the value 1111>`.
Instead of putting the `if (current_input_character.has_value())` check
inside NumericCharacterReferenceEnd directly, it was instead added to
DONT_CONSUME_NEXT_INPUT_CHARACTER, because all usages of the macro
benefit from this check, even if the other existing usage sites don't
exhibit any bugs without it:
- In MarkupDeclarationOpen, if the current_input_character is EOF, then
the previous character is always `!`, so restoring and then checking
forward for strings like `--`, `DOCTYPE`, etc won't match and the
BogusComment state will run one extra time (once for `!` and once
for EOF) with no practical consequences. With the `has_value()` check,
BogusComment will only run once with EOF.
- In AfterDOCTYPEName, ConsumeNextResult::RanOutOfCharacters can only
occur when stopping at the insertion point, and because of how
the code is structured, it is guaranteed that current_input_character
is either `P` or `S`, so the `has_value()` check is irrelevant.
Using a default reference capture for these kinds of tasks is dangerous
and prone to error. Some of the variables should for sure be captured
by value so that we can keep a GC object alive rather than trying to
refer to stack objects.
In particular, input character lookahead now knows how to stop at the
insertion point marker if needed.
This makes it possible to do amazing things like having document.write()
insert doctypes one character at a time.
If we reach the insertion point at the same time as we switch to another
tokenizer state, we have to bail immediately without proceeding with the
next code point. Otherwise we'd fetch the next token, get an EOF marker,
and then proceed as if we're at the end of the input stream, even though
more data may be coming (with more calls to document.write()..)
