First, a better name for SingletonProcess could be SingletonProcessGuard or even just singleton_process::Guard (comparable API from std).
A builder type SingletonProcessGuardBuilder/singleton_process::GuardBuilder that build()s a singleton_process::Guard would allow to extend the functionality. Setters on the builder type:
fn name(self, name: &str) -> Selffn exit_code(self, code: u8) -> Self (u8 as in ExitCode's From implementation; default: 0, or ExitCode::SUCCESS if that type would be usable [only in Nightly])fn exit_msg(self, msg: &str) -> Self (printed with println!() before process::exit())fn terminate(self, process: Option<RedundantProcess>) -> Self accepts an enum RedundantProcess with the variants Current and Other/Original (default: None). None (no termination at all) would be useful if the app developer wants to return from their main() function instead of accepting the implications of process::exit() that destructors aren't running, or they want to use something like the interprocess crate to tell the other process - which could be an async app - to shut down gracefully (if this still requires this crate).
singleton_process::Guard::new() would then need to fail with a new Error variant OtherProcessExists/AlreadyRunning that the caller can handle how they see fit.
First, a better name for
SingletonProcesscould beSingletonProcessGuardor even justsingleton_process::Guard(comparable API fromstd).A builder type
SingletonProcessGuardBuilder/singleton_process::GuardBuilderthatbuild()s asingleton_process::Guardwould allow to extend the functionality. Setters on the builder type:fn name(self, name: &str) -> Selffn exit_code(self, code: u8) -> Self(u8as inExitCode'sFromimplementation; default: 0, orExitCode::SUCCESSif that type would be usable [only in Nightly])fn exit_msg(self, msg: &str) -> Self(printed withprintln!()beforeprocess::exit())fn terminate(self, process: Option<RedundantProcess>) -> Selfaccepts an enumRedundantProcesswith the variantsCurrentandOther/Original(default:None).None(no termination at all) would be useful if the app developer wants to return from theirmain()function instead of accepting the implications ofprocess::exit()that destructors aren't running, or they want to use something like theinterprocesscrate to tell the other process - which could be an async app - to shut down gracefully (if this still requires this crate).singleton_process::Guard::new()would then need to fail with a newErrorvariantOtherProcessExists/AlreadyRunningthat the caller can handle how they see fit.