net/http: mechanism to dynamically change MaxIdleConnsPerHost?

[go-loco]

Please answer these questions before submitting your issue. Thanks!

• What version of Go are you using (go version)?

1.6

• What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/hernan/apps/go"
GORACE=""
GOROOT="/Users/hernan/apps/go/go-src/go"
GOTOOLDIR="/Users/hernan/apps/go/go-src/go/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/vw/ht0lksb16l199rbp98p7hhfr0000gn/T/go-build001484838=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

• What did you do?

It is not possible for clients of http.Transport to dynamically change MaxIdleConnsPerHost attribute. It would be great to have a sync.RWMutex as an attribute of http.Transport, so this is possible.

The use case is that for HTTP/1.1 Hosts, it would be nice for the client to have the possibility of adjusting max-idle connections base on usage.

The following gist shows the problem:
https://gist.github.com/go-loco/810fe53d8aee8879b8d1

Making the following changes to http.Transport would add this feature:

    // MaxIdleConnsPerHost, if non-zero, controls the maximum idle
    // (keep-alive) connections to keep per-host. If zero,
    // DefaultMaxIdleConnsPerHost is used.
    MaxIdleConnsPerHost int

    // MaxIdleConnsMutex, let you dynamically change the
    // MaxIdleConnsPerHost without creating a data race.
    //
    // Usage:
    //
    //  MaxIdleConnsMutex.Lock()
    //  MaxIdleConnsPerHost = newValue
    //  MaxIdleConnsMutex.Unlock()
    //
    MaxIdleConnsMutex sync.RWMutex

func (t *Transport) tryPutIdleConn(pconn *persistConn) error {

    t.MaxIdleConnsMutex.RLock()
    max := t.MaxIdleConnsPerHost
    t.MaxIdleConnsMutex.RUnlock()

    if t.DisableKeepAlives || max < 0 {
        return errKeepAlivesDisabled
    }
    if pconn.isBroken() {
        return errConnBroken
    }
    key := pconn.cacheKey

    if max == 0 {
        max = DefaultMaxIdleConnsPerHost
    }
...

[bradfitz]

Exposing a RWMutex is weird. There's no precedent for that. If anything it would be a setter method, and/or a pluggable conn pool mechanism for others to do whatever they want.

[go-loco]

I completely agree.

A better solution would be, as you describe, adding the following method to http.Transport:

func (t *Transport) SetMaxIdleConnections(maxIdleConns int) {

    t.idleMu.Lock()
    defer t.idleMu.Unlock()

    t.MaxIdleConnsPerHost = maxIdleConns
}

and changing tryPutIdleConn

func (t *Transport) tryPutIdleConn(pconn *persistConn) error {

    t.idleMu.Lock()
    defer t.idleMu.Unlock()

    max := t.MaxIdleConnsPerHost

    if t.DisableKeepAlives || max < 0 {
        return errKeepAlivesDisabled
    }
    if pconn.isBroken() {
        return errConnBroken
    }
    key := pconn.cacheKey

    if max == 0 {
        max = DefaultMaxIdleConnsPerHost
    }
    pconn.markReused()

    waitingDialer := t.idleConnCh[key]
...
}

[bradfitz]

Related: #13801 and #13957 and #12580 and #12289