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95 lines
3.9 KiB
95 lines
3.9 KiB
3 months ago
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Daemon helpers
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==============
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Daemonization
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-------------
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When programs run as daemons, they frequently forget to detach themselves
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from the parent environment. LibUCW therefore offers a simple daemonization
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helper which performs the necessary actions. Namely:
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* Establishing a new session via a call to `setsid()`.
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* Switching the user and group ID if needed. The user and group can be specified
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by either a name or `#uid`. If only the user is given, the relevant groups are
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set according to `/etc/passwd` and `/etc/group`.
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* Redirecting standard input and output from `/dev/null`. Standard error
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output is left open, so that error messages can be printed before you
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set up proper <<log:,logging>>. If you are not sure that your log streams
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replaced stderr, you can call <<log:log_drop_stderr()>> to do that.
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* Setting the `umask()` to a fixed value (022).
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* Switching from the current directory to `/`, so that it is not kept busy.
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* Writing a PID file. While the daemon is running, the PID file is kept locked
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by `flock()`, so we can easily detect stale PID files.
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Example
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-------
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#include <ucw/lib.h>
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#include <ucw/daemon.h>
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void daemon_body(struct daemon_params *dp)
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{
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// ... daemon's code ...
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// When we are done, release the PID file and exit:
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daemon_exit(dp);
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}
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int main(int argc, char **argv)
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{
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struct daemon_params dp = { .pid_file = "/var/run/example.pid" };
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// ... parse arguments ...
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daemon_init(&dp);
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// ... initialize everything ...
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// Let us fork off the daemon:
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daemon_run(daemon_body);
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// Here continues the parent process
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return 0;
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}
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Daemon control
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--------------
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Daemons using the LibUCW helpers can be controlled by traditional mechanisms present
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in most UNIX-like operating systems. These are usually based on PID files, but they
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are inherently racy, since they do not perform any locking. For example, attempts to
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start the daemon, while another process is trying to stop it often lead to undefined
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states. Also, detection of running daemons fails when the daemon dies and its PID
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is recycled. Checking of process name and similar tricks do not avoid the problem,
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since there can be multiple instances of the daemon running simultaneously.
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We therefore recommend the following daemon control protocol, which prevents all such
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race conditions. Its implementation is available in form of the @daemon_control() library
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function or the `ucw-daemon-control` stand-alone utility.
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* There exist two files:
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** PID file (usually `/var/run/daemon.pid`), which contains the PID of the daemon
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process and when the daemon runs, it keeps the file locked by `flock()`.
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** Guard file (usually `/var/run/daemon.lock`), which is locked (again by `flock()`)
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whenever we want to perform an action on the daemon.
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* When we want to start the daemon:
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** Lock the guard file, creating it if it did not exist.
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** Try to lock the PID file. If it fails, the daemon is already running, so we are done.
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** Unlock the PID file.
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** Run the daemon. The daemon locks the PID file and when it has everything initialized,
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it forks and the parent process writes the child's PID and exits.
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** Unlock the guard file.
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* When we want to stop it:
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** Lock the guard file, creating it if it did not exist.
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** Try to lock the PID file. If it succeeds, the daemon is not running, so we are done.
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** Read the PID from the PID file.
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** Send a signal to the process. [This is the only place when we can race. The daemon could
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have exited in the meantime and its PID could have been recycled. Hopefully, the time window
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between checking the lock and sending the signal will be short enough. Using 32-bit PIDs
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is advisable anyway.]
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** Lock the PID file. This will wait until the daemon finishes and releases the lock.
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** Unlock the guard file.
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* When we want to query the daemon's status:
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** Lock the guard file, creating it if it did not exist.
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** Try to lock the PID file. If it succeeds, the daemon was not running.
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** Unlock everything.
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ucw/daemon.h
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------------
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!!ucw/daemon.h
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