[Feature] Process replacement API

[weissi]

Describe the solution you'd like

Many command line utilities (example xcrun) allow you to do something and finally replace the process with the target process. Maybe xcrun -sdk ios swiftc -c -o iosfile.o iosfile.swift or similar.

To function properly, this requires process replacement, typically using execve on UNIX. I couldn't find an API in Subprocess that would let me run a process in a way that it replaces my own process.

[jakepetroules]

Can you provide a little more detail on the use cases where you absolutely need to replace the process instead of spawning a new one?

And what should the story be here on Windows? execve exists, but doesn't really work in quite the same way.

[weissi]

Can you provide a little more detail on the use cases where you absolutely need to replace the process instead of spawning a new one?

And what should the story be here on Windows? execve exists, but doesn't really work in quite the same way.

I'm working on a tool that works like xcrun, uvx, npmx etc where I need to do a bunch of work and then execve that other process such that my callers have a consistent pid. I know that this doesn't work like-for-like on Windows but on UNIX that's a very important pattern.

[jakepetroules]

such that my callers have a consistent pid

Why is this important? Not pushing back, just want to understand more about the use case.

[weissi]

such that my callers have a consistent pid

Why is this important? Not pushing back, just want to understand more about the use case.

So they can observe termination and kill it if necessary. If a parent process spawns say xcrun swift build, then they expect this pid to exit when swift build finishes and not when swift build gets invoked. Similarly, if they don't want to wait anymore and send a SIGTERM to their child, they would expect this to work.

In theory this is also implementable by keeping the xcrun processes around, capturing all signals and forwarding them to the child. But that still fails for SIGKILL and keeps lots of unnecessary resources alive. It's just expected that this works for 'trampoline' binaries.