wait: support ptrace events for Linux

[abbradar]

Part of #270 -- see abbradar@b2d3b4a for previous discussion.

[kamalmarhubi]

Copying part of my comment on the previous discussion:

I'm also a bit put off by adding a meaningless member on non-Linux systems. Trying to think of alternatives...

I'm going to hold off on this for a bit while I think about stuff related to #190. :-)

[homu]

☔ The latest upstream changes (presumably #362) made this pull request unmergeable. Please resolve the merge conflicts.

[kamalmarhubi]

Coming back to this, I'd be happier if the extra variant and methods were cfg'ed to only be present on Linux, or whichever systems support it.

[luser]

I was thinking about this again, and how to handle this in a way that isn't terrible across platforms. What would you think about adding another entry to the WaitStatus enum, like WaitStatus::PtraceEvent? Then the if status::stopped case could check for this and return that value. It'd still be useless on non-Linux systems, but it seems like it'd be easier to work with a platform-conditional enum variant than a conditional field inside an enum variant. #[cfg(...)] seems to work OK on enum variants and match arms:
https://play.rust-lang.org/?gist=15519135f09722bf6d8653912592c079&version=stable&backtrace=0

[abbradar]

Sigh, I still haven't returned to this (haven't actually got a chance to touch Rust for a long time already). I understand that this can be frustrating, and also -- if anyone wants to continue this work please do so!

I hope I can return to this someday...

[kamalmarhubi]

@abbradar this is not frustrating at all! And thanks for your contributions :-)

[kamalmarhubi]

@luser

What would you think about adding another entry to the WaitStatus enum, like WaitStatus::PtraceEvent?

Could you sketch out what you mean a tiny bit?

[luser]

Oh, I can do you one better! I actually wrote the patch because I wanted to use it for something, I just forgot to do anything with it. Here it is:
luser@92a3cf7

Here's the code I wrote that uses it:
https://github.com/luser/tracetree/blob/master/src/main.rs#L65

I didn't actually try to make that code portable yet, but making it build should be straightforward. (Making it actually work right is probably harder).

[Susurrus]

Is this still relevant? If so, @abbradar would you be willing to update this patch?

[luser]

This was superseded by #438

You can see an example of using it here: https://github.com/luser/tracetree/blob/2dcde59a9b2bd8dd6343a147a6d09f5d4428068e/src/main.rs#L141

[Susurrus]

Okay, then I'm going to close this. Thanks, @luser.