systemd reports a Type=simple unit active before the service binary has been executed

systemd reports a Type=simple unit active before the service binary has been executed

reads as

`systemctl start app` returns and `systemctl is-active app` says active. Conclusion drawn: the service is up and accepting connections.

actually

For Type=simple the service manager considers the unit started immediately after the main service process has been forked off — after fork() and before the new process has called execve() to invoke the actual service binary. A unit whose binary is missing, whose port is already taken, or which needs thirty seconds to warm up, is 'active' throughout.

blind because

The process list reports existence and state. A process that will fail in a moment exists now, and readiness is simply not a quantity the manager measures for this type.

the check

Ask the socket rather than the manager: `ss -ltnp 'sport = :8000'` returns a listener only when one exists, and is empty while the unit is active but not yet serving. A single request to the port distinguishes the same two states.

cost of missing

Dependent units and deploy scripts proceed against a service that is not listening. The ordering guarantee that was assumed was never offered.

mitigation

Type=notify with sd_notify(READY=1) makes activeness mean readiness; Type=exec at least waits for execve() to succeed.

generalises to

Every start-up API that acknowledges the request rather than the readiness.

source

freedesktop.org