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Please imagine that some child processes are generated by pcntl_fork() and they open mysql connection using PDO respectively. If parent process send a SIGTERM signal with posix_kill() to a child process, the child process will be killed immediately.

So, I have a question about this. When the child process is terminated,will it also close the mysql connection automatically? Or should I call exit() function?

I know the child process keep the connection if SIGKILL is sent. However,I don't know the case of SIGTERM.

Boblishus
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2 Answers2

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When a process completes (either because it exits or it is terminated using a signal) all the files and connections it keeps open are automatically closed by the operating system. It is not closed clean, by using the MySQL protocol for closing connections (assuming there is one). It is simple dropped at the TCP/IP level and the server at the other side just discovers it is talking to a closed door. That doesn't always happen instantly, sometimes it takes some time until the server notices the discussion partner is gone. When this happens, it considers the connection as being dropped and cleans the things up on its side.

Do not open the MySQL connection in the parent process before using fork(). fork() duplicates the data structures used to manage the local side of the connection and the results are unpredictable. Even more, when the child completes (no matter how), it closes the connection (or the OS drops it), the MySQL server also closes its end and the parent process discovers it is talking to nobody.

Close the MySQL connection in the parent process before using fork() then open separate connections in the parent and in the child process, as needed.

Also, wrap any MySQL communication with the server in the parent process between:

pcntl_sigprocmask(SIG_BLOCK, array(SIGCHLD));

and

pcntl_sigprocmask(SIG_UNBLOCK, array(SIGCHLD));

Otherwise, when a child process completes, the parent process is notified with the SIGCHLD signal. A received signal resumes it from sleeping (if it happened to be stopped in a sleep() call when the signal arrives). The MySQL library uses sleep() as part of the MySQL protocol for communication with the server. If such a sleep() forcibly returns earlier than it should (because of a received signal), the MySQL library gets confused and it ends up reporting strange errors (like "MySQL server is gone away") that are actually not correct.

Take a look at this answer for a detailed explanation.

axiac
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SIGKILL removes process from the memory without any cleanup. The O/S should clean up all opened handles but it might happen db connection to stay open (at least until the database notices dead connection).

It should not be used from the application but only from the console as the last resort.

You should use SIGTERM or SIGINT so the child process exit properly and close all opened handles.

Vladimir Kanazir
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