Process.start could have a mode that replaces current process with the started one.

[sigurdm]

Similar to exec in posix.

This would make signal handling more consistent if the spawning process is there as a script to run another program, also it might save memory.

This came up in the context of pub run (dart-lang/pub#2492) that wants to start the program in a subprocess, and let that subprocess handle signals without interference from pub run.

[sigurdm]

cc @jonasfj @sortie

[jonasfj]

See also: #42092

[blopker]

Hey all, at the risk of digging up an old issue, I wanted to share my thoughts on this. I think that allowing Dart to replace itself with another process would open up using Dart to a new class of CLI programs, specifically "process launchers". I am currently looking at different languages to reimplement asdf in since bash is not scaling well. Dart currently does not seem to have the equivalent of bash's exec $(command), which was surprising to me.

I thought using Process.start with ProcessStartMode.detached then calling exit(0) right after would work, but the detached process doesn't seem to inherit stdio. It can't print to the terminal, but it does continue to run after Dart exits. Using ProcessStartMode.inheritStdio lets the subprocess print, but only if exit(0) is not called. Dart must continue to run while the subprocess executes.

I guess I'm looking for something like ProcessStartMode.detachedInheritStdio, if that's even possible. Generally, I'm just looking for bash's exec, but in Dart. Is there a way to do this that I'm missing?

[sigurdm]

Perhaps exec can be called via ffi nowadays...

[blopker]

That's an interesting idea. I haven't done any C in a while though. Do you know which exec syscall I might use?

[sigurdm]

I guess you can choose from one of these depending on how you want to pass the arguments: https://en.wikipedia.org/wiki/Exec_(system_call)#Unix,_POSIX,_and_other_multitasking_systems

However I'm not sure how easy it is to close file descriptors before the call:

https://stackoverflow.com/questions/56650579/why-should-i-close-all-file-descriptors-after-calling-fork-and-prior-to-callin

Someone from the VM team will know a lot more. Eg. @mkustermann

[mkustermann]

Perhaps exec can be called via ffi nowadays...

Agreed - that may work (modulo #38832).

However I'm not sure how easy it is to close file descriptors before the call:

IIRC the VM open all files with O_CLOEXEC which will cause them to be automatically closed on exec(). So that may not be an issue.