Fixes deadlock between Mouse cuts and xfideventread

[hherman1]

When cutting with the mouse it was possible for:

1. Text select to release the biglock, while maintaining a winlock
2. xfideventread to then acquire the biglock, and wait for the winlock

Deadlocking. After fixing this deadlock, the following appeared:

1. xfideventwrite releases the big lock, but holds the winlock
2. Text select reacqiures the big lock and blocks on the winlock

Another deadlock.

This was reproducible by me in https://github.com/hherman1/acme-go by running Watch echo 1 2 3 and attempting to cut anything from the Watch window.

[pedramos]

This seems to cause a segfault when pasting to acme's top panel because in that situation t.W is nil (in cmd/acme/internal/ui/text.go:550).

This should be able to fix that:

		if t.W != nil {
			wind.Winunlock(t.W)
		}

		BigUnlock()
 		for Mouse.Buttons == b {
 			*Mouse = <-Mousectl.C
 		}

		BigLock()
		if t.W != nil {
			wind.Winlock(t.W, t.W.Owner)
		}

I also had to increase the size of rch channel on draw/mouse.go:43 from 2 to 10. There is no special reason behind 10 aside from it was the first value I tried and it worked right away. Without this, for large acme windows (this happened with a 4k screen with a maximized acme window), Cut & Paste, with the mouse, on a Watch window, still freezes acme for me. Maybe this should be a different commit though.

[hherman1]

The if checking seems like a good change to this diff. I guess I literally never paste in the top bar which is why I didn’t encounter that issue. Are you able to edit this pr directly? I don’t know how often @rsc checks this repo, so I’m also willing to accept this change on my fork in the mean time if that is helpful

[pedramos]

Meanwhile I noticed my change breaks copy paste on win because wind.Winlock sets the t.W.Owner to 0.
So, this seems to fix it:

		var owner rune
		if t.W != nil  {
			owner = t.W.Owner
			wind.Winunlock(t.W)
		}

		BigUnlock()
		for Mouse.Buttons == b {
			*Mouse = <-Mousectl.C
		}

		BigLock()
		if t.W != nil {
			wind.Winlock(t.W, owner)
		}

I will keep testing it to make sure nothing else breaks.

Are you able to edit this pr directly?

I don't think I can :/