net: add mechanism to wait for readability on a TCPConn

[bradfitz]

EDIT: this proposal has shifted. See #15735 (comment) below.

Old:

The net/http package needs a way to wait for readability on a TCPConn without actually reading from it. (See #15224)

http://golang.org/cl/22031 added such a mechanism, making Read(0 bytes) do a wait for readability, followed by returning (0, nil). But maybe that is strange. Windows already works like that, though. (See new tests in that CL)

Reconsider this for Go 1.8.

Maybe we could add a new method to TCPConn instead, like WaitRead.

[bradfitz]

/cc @ianlancetaylor @rsc

[gopherbot]

CL https://golang.org/cl/23227 mentions this issue.

[RalphCorderoy]

read(2) with a count of zero may be used to detect errors. Linux man page confirms, as does POSIX's read(3p) here. Mentioning it in case it influences this subverting of a Read(0 bytes) not calling syscall.Read.

[bradfitz]

I found a way to do without this in net/http, so punting to Go 1.9.

[bradfitz]

Actually, the more I think about this, I don't even want my idle HTTP/RPC goroutines to stick around blocked in a read call. In addition to the array memory backed by the slice given to Read, the goroutine itself is ~4KB of wasted memory.

What I'd really like is a way to register a func() to run when my *net.TCPConn is readable (when a Read call wouldn't block). By analogy, I want the time.AfterFunc efficiency of running a func in a goroutine later, rather than running a goroutine just to block in a time.Sleep.

My new proposal is more like:

package net

// OnReadable runs f in a new goroutine when c is readable;
// that is, when a call to c.Read will not block.
func (c *TCPConn) OnReadable(f func()) {
   // ...
}

Yes, maybe this is getting dangerously into event-based programming land.

Or maybe just the name ("OnWhatever") is offensive. Maybe there's something better.

I would use this in http, http2, and grpc.

/cc @ianlancetaylor @rsc

[ianlancetaylor]

Sounds like you are getting close to #15021.

I'm worried that the existence of such a method will encourage people to start writing their code as callbacks rather than as straightforward goroutines.

[bradfitz]

Yeah. I'm conflicted. I see the benefits and the opportunity for overuse.