Fix zombie process on process destruction

[xtrime-ru]

Summary

Prevents POSIX wrapper shells created by proc_open() from remaining as zombie processes when a process is killed or destroyed.

Also handles failures returned by posix_kill() without blocking while waiting for a process that was not terminated.

Root cause

proc_get_status($proc)['pid'], stored as $shellPid, identifies the wrapper shell created by proc_open(). This shell is a direct child of PHP.

The PID stored as $pid belongs to the command started by the wrapper shell. Waiting for $pid cannot reap $shellPid.

Reaping $shellPid previously relied on an event-loop callback. If the event loop did not run after the shell exited, no subsequent waitpid() call was made and the shell remained a zombie.

Additionally, posix_kill() failures were ignored. A failed signal followed by a blocking waitpid() could make Process::kill() wait for the command to exit naturally.

Changes

• Reap $shellPid after kill() and during handle destruction.
• Keep the global shutdown callback non-blocking.
• Retry waitpid() when interrupted by EINTR.
• Throw when posix_kill() fails.
• Treat ESRCH as successful termination because the OS process has already exited.
• Add regression tests for kill, destruction, completed processes, shutdown, and signal failures.

This completes the POSIX shutdown handling introduced in 6c711ee.

Testing

• Targeted regression run: 6 tests, 14 assertions passed.
• Psalm passed.
• PHP CS Fixer passed.