net/http: merge http2 Transport dials

Instead of multiplexing streams, concurrent longpoll HTTP2 connections always create new connection, unless it's preceded by a complete roundtrip.

This code will reuse connection

getOnce()
for i:=0;i<10;i++{
        go blockingLongPoll()        
}

while this code doesn't (as shown by netstat)

for i:=0;i<10;i++{
        go blockingLongPoll()        
}

complete test is in this gist

go version devel +e5956bc linux/amd64

[bradfitz]

This has nothing to do with long polling, and nothing to do with whether RoundTrip completes or not. This is simply the case of the Transport not merging outgoing dials when a connection doesn't exist. There was even a TODO in the code for this already. Fix coming.

[gopherbot]

CL https://golang.org/cl/17134 mentions this issue.