process: drop pipe after child exits in wait_with_output

[Darksonn]

Drop the stdout/stderr pipes after the child has exited in wait_with_output. This will supposedly avoid a crash in some rare circumstances as discussed in #4309.

[ipetkov]

I'm still not entirely sure why delaying the IO handle drop would help, but we can easily try it out.

[iitalics]

I'm with you, I'd rather understand what's going on before jumping to a fix like this.

[Darksonn]

@iitalics You should be able to try out the version of Tokio available in this PR by adding the following to your Cargo.toml.

[patch.crates-io]
tokio = { git = "https://github.com/tokio-rs/tokio.git", branch = "wait_with_output_oom" }

If you're able to compare how it performs with and without this change without any other changes, that would be cool.

[iitalics]

I've tested this for about an hour with no crash observed.

[Darksonn]

Okay, if adding/removing this patch makes the crash go away and come back, then it seems reasonable to merge this. Not that it makes any sense.