Geek Answers Handbook

When SIGTERM is sent, does the child process close the mysql connection?

Imagine some child processes are generated by pcntl_fork () and they open a mysql connection using PDO respectively. If the parent process sends a SIGTERM signal using posix_kill () for the child process, the child process will be immediately destroyed.

So, I have a question about this. When the child process is completed, will it also automatically close the mysql connection? Or do I need to call exit ()?

I know that the child process supports the connection if SIGKILL is sent. However, I do not know the case of SIGTERM.

0
source share
2 answers

When a process terminates (either because of its exit, or with its completion using a signal), all files and connections that it keeps open are automatically closed by the operating system. It is not closed clean, using the MySQL protocol to close connections (assuming it is). It just drops at the TCP / IP level, and the server, on the other hand, just detects that it is talking with the door closed. This does not always happen instantly, sometimes it takes some time until the server notices that the discussion partner has disappeared. When this happens, he sees the connection as having fallen and cleans things on his side.

Do not open the MySQL connection in the parent process before using fork() . fork() duplicates the data structures used to control the local side of the connection, and the results are unpredictable. Moreover, when the child finishes (no matter how), it closes the connection (or the OS crashes), the MySQL server also closes its end, and the parent process discovers that it is not talking to anyone.

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

Also, terminate 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 the child process ends, the parent process is notified using the SIGCHLD signal. The received signal resumes it from sleep mode (if it was stopped when sleep() was called when the signal arrived). The MySQL library uses sleep() as part of the MySQL protocol to communicate with the server. If such sleep() forced to return earlier than it should (due to the received signal), the MySQL library gets confused, and as a result, it reports strange errors (for example, "the MySQL server is gone"), which are actually incorrect.

See this answer for a detailed explanation.

+2
source

SIGKILL deletes a process from memory without any cleanup. O / S should clear all open handles, but it may happen that the db connection remains open (at least until the database sees a dead connection).

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

You must use SIGTERM or SIGINT so that the child process terminates correctly and close all open descriptors.

+1
source

More articles:


All Articles