Reintroduce ExitType

This introduces `ExitType=main|cgroup` for services.
Similar to how `Type` specifies the launch of a service, `ExitType` is
concerned with how systemd determines that a service exited.

- If set to `main` (the current behavior), the service manager will consider
  the unit stopped when the main process exits.

- The `cgroup` exit type is meant for applications whose forking model is not
  known ahead of time and which might not have a specific main process.
  The service will stay running as long as at least one process in the cgroup
  is running. This is intended for transient or automatically generated
  services, such as graphical applications inside of a desktop environment.

Motivation for this is #16805. The original PR (#18782) was reverted (#20073)
after realizing that the exit status of "the last process in the cgroup" can't
reliably be known (#19385)

This version instead uses the main process exit status if there is one and just
listens to the cgroup empty event otherwise.

The advantages of a service with `ExitType=cgroup` over scopes are:
- Integrated logging / stdout redirection
- Avoids the race / synchronisation issue between launch and scope creation
- More extensive use of drop-ins and thus distro-level configuration:
  by moving from scopes to services we can have drop ins that will affect
  properties that can only be set during service creation,
  like `OOMPolicy` and security-related properties
- It makes systemd-xdg-autostart-generator usable by fixing [1], as obviously
  only services can be used in the generator, not scopes.

[1] https://bugs.kde.org/show_bug.cgi?id=433299

1 files changed, 11 insertions, 0 deletions

diff --git a/src/core/service.h b/src/core/service.h
index eaad95df6d..70ce70fba5 100644
--- a/src/core/service.h
+++ b/src/core/service.h
@@ -35,6 +35,13 @@ typedef enum ServiceType {
         _SERVICE_TYPE_INVALID = -EINVAL,
 } ServiceType;
 
+typedef enum ServiceExitType {
+        SERVICE_EXIT_MAIN,    /* we consider the main PID when deciding if the service exited */
+        SERVICE_EXIT_CGROUP,  /* we wait for the last process in the cgroup to exit */
+        _SERVICE_EXIT_TYPE_MAX,
+        _SERVICE_EXIT_TYPE_INVALID = -EINVAL,
+} ServiceExitType;
+
 typedef enum ServiceExecCommand {
         SERVICE_EXEC_CONDITION,
         SERVICE_EXEC_START_PRE,
@@ -97,6 +104,7 @@ struct Service {
         Unit meta;
 
         ServiceType type;
+        ServiceExitType exit_type;
         ServiceRestart restart;
         ExitStatusSet restart_prevent_status;
         ExitStatusSet restart_force_status;
@@ -227,6 +235,9 @@ ServiceRestart service_restart_from_string(const char *s) _pure_;
 const char* service_type_to_string(ServiceType i) _const_;
 ServiceType service_type_from_string(const char *s) _pure_;
 
+const char* service_exit_type_to_string(ServiceExitType i) _const_;
+ServiceExitType service_exit_type_from_string(const char *s) _pure_;
+
 const char* service_exec_command_to_string(ServiceExecCommand i) _const_;
 ServiceExecCommand service_exec_command_from_string(const char *s) _pure_;