HTML API: Ensure bookmark exaustion does not error

[sirreal]

Bookmark exhaustion, typically from deep nesting, can cause the HTML Processor to throw an Exception.

The Exception is thrown by a private method, handle the exception and return false to indicate a failure to process.

Trac ticket: https://core.trac.wordpress.org/ticket/64394

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[sirreal]

bookmark_token() actually throws, but it's not handled here. The annotation may not be appropriate.

[sirreal]

Exhausted bookmarks throw a generic Exception.

This block catches the exceptions thrown by insert_virtual_token().

[github-actions]

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[westonruter]

Will this not perhaps lead a developer to think that they reached the end of the document, when in reality the nesting is too large (or the max bookmarks exceeded)? In that way, I think an exception is more helpful. Otherwise, wouldn't every loop over tokens in a doc need to do something like:

while ( $p->next_tag() ) {
    // ...
}
if ( WP_HTML_Processor::ERROR_EXCEEDED_MAX_BOOKMARKS === $p->get_last_error() ) {
     // Handle max bookmark error.
}

This would put the exception case in the regular code that always runs. Since exceeding the max bookmarks should be exceptional, I would think an exception is preferred:

try {
    while ( $p->next_tag() ) {
        // ...
    }
} catch ( Exception $e ) {
     if ( WP_HTML_Processor::ERROR_EXCEEDED_MAX_BOOKMARKS === $p->get_last_error() ) {
          // Handle max bookmark error.
     }
}

But since this is the only exception that WP_HTML_Tag_Processor throws (currently), then it could be just:

try {
    while ( $p->next_tag() ) {
        // ...
    }
} catch ( Exception $e ) {
    // Handle max bookmark error.
}

[sirreal]

But since this is the only exception that WP_HTML_Tag_Processor throws

This is only in the WP_HTML_Processor. WP_HTML_Tag_Processor should not throw any errors.

Will this not perhaps lead a developer to think that they reached the end of the document

That's already the case, the HTML processor has avoided throwing errors and exposes problems through some getters. Primarily, ::get_last_error() should be used:

<?php
require '/wordpress/wp-load.php';
echo '<plaintext>';
echo "WordPress " . wp_get_wp_version() . "\n";

$p = WP_HTML_Processor::create_fragment('<table><tbody>unsupported');
while( $p->next_token() ) {
  var_dump($p->get_tag());  
}
// Need to check error status.
var_dump( $p->get_last_error() );
var_dump( $p->get_unsupported_exception()->getMessage() );

When these APIs throw errors that callers are supposed to handle, it's just too easy to bring down users' sites with errors that aren't actionable for them. It's true that superficially "end of document" is the same as "error." It seems preferable that a document silently fail to fully parse instead of crashing and bringing down a site.

In either case, the developer should do another thing that's not obvious (add exception handling with try/catch or check error status after iteration with the available method). Relying on error status is the least impactful for a site if a developer overlooks this.

[westonruter]

This is only in the WP_HTML_Processor. WP_HTML_Tag_Processor should not throw any errors.

Yes, sorry, I meant WP_HTML_Processor.

In either case, the developer should do another thing that's not obvious (add exception handling with try/catch or check error status after iteration with the available method). Relying on error status is the least impactful for a site if a developer overlooks this.

OK, makes sense to me.

[sirreal]

These exceptions really aren't helpful here and we want them to remain internal to the class. The affected methods are all private so theres some flexibility.

I'd consider returning false or null and handling those types of values instead of using the blanket try/catch.