Deadline support for the net package

[Aram Hăvărneanu]

I will be working on adding deadline support to the net package.

Network I/O interested in timeouts will register itself with a

deadline goroutine implemented in the runtime, similar to how the Unix

code now registers with the runtime-integrated network poller.  I/O

will then proceed synchronously, as usual on Plan 9, but the deadline

goroutine (which is just a fancy timer goroutine) will post notes to

procs that exceed the timeout value.  This goroutine has to be in the

runtime because it needs access to the "m" that is blocked in the

system call.  Notice that we can't get the "m" right away, since we

are not yet scheduled on the "m" that will do the system call.  We can

get the "m" only when we try to do the I/O; until then we only save

the file descriptor.  The final registration with the deadline

goroutine is done either in entersyscall or in the syscall i/o

wrapper.

Apart from posting notes, the deadline goroutine will inform the

interrupted proc that it interrupted it, by setting some per-m flag or

something.  This has to be done to distinguish a note posted by the

deadline goroutine from a note posted by some unrelated process.

The interrupted code, having seen this flag will return whatever net

currently returns on deadlines.  If it doesn't see this flag it will

return the usual error, as normal.

This mechanism is made harder by the fact that the Go runtime expects

to own the note handlers, and we are using them in a novel way that

will break some assumptions.  In particular, because we reuse procs

instead of creating them for new I/O, we can't set up a new note

handler, so the Go-installed handler must do.  This means changing the

default handler.

-- 

Aram Hăvărneanu

[Jeff Sickel]

Send references to the CL when you have something working.

[Jeff Sickel]

See CL https://codereview.appspot.com/49920048 before proceeding. 

I've started flushing out the Dealine issues in association with that CL as it touches on the fd_plan9.go and other net and runtime source.