51423: AtomParser: fixes PHP 8 fatal error in wp-includes/atomlib.php

[hellofromtonya]

Trac ticket: https://core.trac.wordpress.org/ticket/51423

Split of the larger PR for this commit.

tl;dr

This change guards Core while ensuring 3rd party developers continue to be alerted to the problem from fopen Warning.

• Guards AtomParser::parser() from PHP 8 fatal error when/if AtomParser::FILE is ever changed to an invalid or unavailable resource.
• Retains original behavior.
• Retains original Warning from fopen to alert 3rd party developer of an invalid/unavailable resource.
• Does not retain the additional warnings from fread and fclose.
  • Is it problematic to hide these additional warnings?
  • Hmm, I argue "No".
  • Why? fopen will throw a Warning. The additional warnings are due to the initial problem at fopen.
• Ensures XML Parser is freed and error handler restored.

Risk:

• Core: none
• 3rd party: possible

More Details

When AtomParser::FILE is not changed

This change is not needed if the property AtomParser::FILE is not changed.

Why?

By default, AtomParser::FILE is set to php://input for the read-only stream.

• fopen will return the resource pointer for the stream.
• If there's nothing in the stream:
  • fread returns an empty string
  • fclose closes the resource pointer.
  • There are no errors or warnings in PHP 5, 7, and 8.
• See it in action here https://3v4l.org/p4YKQ.

What if the property AtomParser::FILE is changed?

If valid and available, fopen returns the resource. Same behavior as above.

AtomParser::FILE is invalid or unavailable

If invalid/unavailable, fopen returns false and throws a Warning in PHP 5, 7, and 8. See it in action here https://3v4l.org/pYKdD.

Warning: fopen(): open_basedir restriction in effect. File(/usr/local/something.txt) is not within the allowed path(s): (/tmp:/in:/etc) in /in/pYKdD on line 12
Warning: fopen(/usr/local/something.txt): failed to open stream: Operation not permitted in /in/pYKdD on line 12

This behavior is the same in all PHP versions. We don't need to do anything for PHP 8.

What about fread and fclose?

See it in action here https://3v4l.org/HhApV.

fopen returned false. fread and fclose throws:

• PHP 5 and 7

Warning: fread() expects parameter 1 to be resource, bool given in /in/HhApV on line 13
Warning: fclose() expects parameter 1 to be resource, bool given in /in/HhApV on line 25

• PHP 8

Fatal error: Uncaught TypeError: fread(): Argument #1 ($stream) must be of type resource, bool given in /in/HhApV:13

TODO

• Validate current behavior with PHP 5-8
• Analyze and write up explanation and results
• Add fixes
  - [ ] Add code coverage for the AtomParser::parse(). There's no code coverage for the external library.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[hellofromtonya]

AtomParser is part of an external library that was added to core's source in 2.3.0. The original library has not been updated since 2007.

This PR modifies this original library's source. Do we wish to make this change in core or contribute upstream?