[BUG] The `StartService` task hangs forever when a dependency loop exists between services

[sisungo]

When a dependency loop exists between services, starting it will cause the tasks hang forever.

Minimal Example

The following service is named self.

[service]
type = "oneshot"
dependencies = ["self"]

[exec]
start = "ls"

Expected Behavior

The dependency loop is detected, automatically broken and a warning log is triggered.

Actual Behavior

The task hangs forever.

[sisungo]

Priority: Low.

A correct service should not contain dependency loops. When the deadlock is found by someone, the StartService task can be immediately interrupted.

However, we still need to fix the bug in the future. It may cause confusing error information when stopping the service (because StartService is important and cannot be automatically interrupted).

[sisungo]

UPDATE: Now when a service is started as dependency of another service, it generates an autostart which takes the dependent service. We may analyse this chain to solve the problem.