PHP: rfc:linking_in_stream_wrappers

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====== PHP RFC: Support linking in stream wrappers ======
  * Version: 0.9
  * Date: 2014-10-25
  * Author: Timm Friebe, thekid@php.net
  * Contributor: Frank Kleine, https://github.com/mikey179/
  * Status: Inactive
  * First Published at: https://wiki.php.net/rfc/linking_in_stream_wrappers

===== Introduction =====
This RFC suggests adding support for links in stream wrappers by adding appropriate hooks.

===== Proposal =====
Userland stream wrappers must be able to hook into:

   1. Creating links
   2. Testing for links' existance
   3. Returning file type
   4. Resolving links

Currently, #2 and #3 are already supported by [[http://php.net/manual/en/streamwrapper.url-stat.php|streamWrapper::url_stat()]]. With this accomplished, frameworks such as [[http://vfs.bovigo.org/|vfsStream]] can support testability of symlinking correctly.

Use case #1 - **new functionality**:
<PHP> 
class wrapper {

  /**
   * Creates a hard link to the existing target with the specified name link. 
   *
   * @param  string $target
   * @param  string $link
   * @return bool Whether the operation succeeded or not.
   */
  function url_link($target, $link) {
    // ...
  }

  /**
   * Creates a symbolic link to the existing target with the specified name link. 
   *
   * @param  string $target
   * @param  string $link
   * @return bool Whether the operation succeeded or not.
   */
  function url_symlink($target, $link) {
    // ...
  }
}
</PHP>

Use cases #2 and #3 - //already possible today//:
<PHP>
class wrapper {

  /**
   * See http://php.net/manual/en/streamwrapper.url-stat.php
   *
   * @param  string $url
   * @param  int $flags
   * @return array or FALSE on error
   */
  function url_stat($url, $flags) {
    if ($flags & STREAM_URL_STAT_LINK) {
      // Stat the link, not the file it points to
    }

    // ...
 
    return [
      // ...
      'mode' => $permissions & 0120000  // S_ISLINK
    ]; 
  }
}
</PHP>

Use case #4 - **new functionality**:
<PHP>
class wrapper {

  /**
   * Reads a link and returns the target
   *
   * @param  string $link
   * @return string The link target or FALSE if the does not exist
   */
  function url_readlink($link) {
    // ...
  }
}
</PHP>

  * When url_readlink() returns a string, it will be used as readlink()'s return value.
  * When url_readlink() returns FALSE, readlink() quietly returns FALSE
  * For all other return values of url_readlink(), a warning is raised and readlink() returns FALSE
  * When url_readlink() is not implemented, a warning is raised an readlink() returns FALSE.

==== Limitations ====

It will not be possible to create links between two different stream wrappers like this: 

<PHP>
symlink('foo://some/resource', 'bar://another/resource'); // will yield a PHP_WARNING and return false
</PHP>

===== Backward Incompatible Changes =====
This RFC introduces no BC breaks.

===== Proposed PHP Version(s) =====
The next PHP 5.x version, or PHP 7.

===== RFC Impact =====
==== To SAPIs ====
No impact.

==== To Existing Extensions ====
No impact.

==== To Opcache ====
No impact.

==== New Constants ====
No new constants are introduced.

===== Open Issues =====
TODO: Check whether realpath() can be hooked into easily. Maybe it needs to be omitted...

===== Unaffected PHP Functionality =====
TODO: 

===== Future Scope =====
This sections details areas where the feature might be improved in future, but that are not currently proposed in this RFC.

===== Proposed Voting Choices =====
This RFC requires a 50%+1 majority

===== Patches and Tests =====
TODO: Implement and submit GitHub pull request.
https://github.com/thekid/php-src/compare/rfc/linking_in_stream_wrappers

===== References =====
  * http://php.net/manual/en/class.streamwrapper.php
  * [[https://github.com/mikey179/vfsStream/wiki/Known-Issues|List of known issues in vfsStream]]
  * [[https://github.com/mikey179/vfsStream/tree/linking|vfsStream reference userland implementation]]
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