PHP: rfc:additional-context-in-pcntl-signal-handler

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====== PHP RFC: Additional Context in pcntl_signal Handler ======
  * Version: 1.0
  * Date: 2016-06-14
  * Authors: David Walker <dave@mudsite.com>, Bishop Bettini <bishop@php.net>
  * Status: Accepted
  * First Published at: http://wiki.php.net/rfc/additional-context-in-pcntl-signal-handler


===== Introduction =====
Modern UNIX kernels include additional, contextual information when delivering a signal, however the callable handler for [[http://php.net/manual/en/function.pcntl-signal.php|pcntl_signal]] does not receive this information. Developers coordinating multiple processes through signals must arrange to gather and send this same information using alternative channels, which is time-consuming, error-prone, and unnecessary.


===== Proposal =====
Currently the signal handler receives only one formal argument, the [[http://man7.org/linux/man-pages/man7/signal.7.html|number of the signal]] sent. This proposal would add a second formal argument, an array of signal info as provided by the underlying system's kernel.

One possible use case is identifying the process ID of the signal //sender//, [[http://serverfault.com/a/94995/204816|described at the C level in this StackOverflow Q&A]] and implemented in PHP as seen here:

<file php>
<?php
pcntl_signal(SIGUSR1, function ($signo, $siginfo) {
    printf('USR1 from %s', $siginfo['pid'] ?? 'unknown');
});
posix_kill(posix_getpid(), SIGUSR1);
pcntl_signal_dispatch();
</file>

Other uses cases include identifying the cause of the signal (such as asynchronous I/O or timer expiration), identifying the sending user, and finding the consumed user & system time of an exited process.

The ''$siginfo'' array may contain [[https://www.mkssoftware.com/docs/man5/siginfo_t.5.asp|one or more of the following keys]], depending upon the system support for signal information and the nature of the sent signal:

  * **''signo''**, the signal number being sent (duplicates the first argument to the handler).
  * **''code''**, the signal code, which depends upon the signal sent and provides context-specific reason //why// the signal was sent.
  * **''value''**, the value for the code, if appropriate for the code.
  * **'' errno''**, the error number associated with the signal, if appropriate for the signal //and// non-zero.
  * **''pid''**, the process ID of the signal sender, if appropriate for the signal.
  * **''uid''**, the real user ID of the signal sender, if appropriate for the signal.
  * **''addr''**, the address at which the faulting signal occurred: only faulting signals like ''SIGFPE'' include this.
  * **''status''**, the status code of an exiting process: only exiting signals like ''SIGCHLD'' include this.
  * **''band''**, the asynchronous I/O band, if appropriate for the signal.
  * **''mtime''**, the time of last data modification, if appropriate for the signal.

The naming of ''$siginfo'' and its keys comes from the ''pcntl_sigwaitinfo'' implementation. (So for example, "''signo''" in PHP comes from "''si_signo''" in the C structure, the leading "si_" having been removed by the PHP engine.) Name normalization does not apply to the values of the ''code'' field, as those represent system-specific values that follow no normative naming convention.

Some older systems not conforming to [[https://standards.ieee.org/findstds/standard/1003.1-2001.html|POSIX.1-2001]] (where this structure was formally codified) may not support passing the additional context. In such case, PHP will pass ''null'' as the second argument to the handler.


===== Backward Incompatible Changes =====

There are no backward compatibility breaks.


===== Proposed PHP Version(s) =====

Next PHP 7.x, currently 7.1.


===== RFC Impact =====
==== To SAPIs ====

None.


==== To Existing Extensions ====

The ''[[http://php.net/manual/en/book.pcntl.php|pcntl]]'' extension will be updated.


==== To Opcache ====

None.

===== Discussion =====

Instead of updating ''pcntl_signal'', it was suggested to add a method ''pcntl_sigaction'', which would "keep maximum compatibility and eliminate unnecessary additional overhead". This suggestion was later withdrawn as perhaps adding more complication than the patch itself, though a later [[https://github.com/php/php-src/pull/1985|PR]] addressed parts of the suggestion. [[https://www.mail-archive.com/internals@lists.php.net/msg86504.html|Reference.]]

Performance was raised as a concern. Having run tests through callgrind there is an additional 0.0001% cost for the feature. The profiled code defined an empty function, set the handler, and triggered the signal. Passing the additional information resulted in 2000 extra instructions, out of a total 13 million. This seems negligible compared to the cost of acquiring the same information through other means (eg message queues, temporary files, etc.).

===== Open Issues =====

None.

===== Proposed Voting Choices =====

Vote shall be Yes or No to deliver kernel-provided additional context to the ''pcntl_signal'' handler.

Requires a 50%+1 majority.

<doodle title="Provide additional context in pcntl signal handler?" auth="bishopb" voteType="single" closed="true">
   * Yes
   * No
</doodle>

Voting shall close one week after opening on Thursday, July 14, 2016 at 23:59 UTC.

===== Patches and Tests =====

  * [[https://github.com/php/php-src/pull/1993|Implementation with tests.]]


===== References =====

  * [[https://marc.info/?l=php-internals&m=146584196929126&w=2|Initial ping to php.internals]]
  * [[https://marc.info/?l=php-internals&m=146670242109688&w=2|Discussion]]
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