net/http: add Transport.MaxConnsPerHost knob

Add field to http.Transport which limits connections per host,
including dial-in-progress, in-use and idle (keep-alive) connections.

For HTTP/2, this field only controls the number of dials in progress.

Fixes #13957

Change-Id: I7a5e045b4d4793c6b5b1a7191e1342cd7df78e6c
Reviewed-on: https://go-review.googlesource.com/71272
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 11 insertions, 3 deletions

diff --git a/src/net/http/export_test.go b/src/net/http/export_test.go
index 7c7b5d5667..7cdb51b05b 100644
--- a/src/net/http/export_test.go
+++ b/src/net/http/export_test.go
@@ -133,9 +133,11 @@ func (t *Transport) IdleConnStrsForTesting_h2() []string {
 	return ret
 }
 
-func (t *Transport) IdleConnCountForTesting(cacheKey string) int {
+func (t *Transport) IdleConnCountForTesting(scheme, addr string) int {
 	t.idleMu.Lock()
 	defer t.idleMu.Unlock()
+	key := connectMethodKey{"", scheme, addr}
+	cacheKey := key.String()
 	for k, conns := range t.idleConn {
 		if k.String() == cacheKey {
 			return len(conns)
@@ -160,13 +162,19 @@ func (t *Transport) RequestIdleConnChForTesting() {
 	t.getIdleConnCh(connectMethod{nil, "http", "example.com"})
 }
 
-func (t *Transport) PutIdleTestConn() bool {
+func (t *Transport) PutIdleTestConn(scheme, addr string) bool {
 	c, _ := net.Pipe()
+	key := connectMethodKey{"", scheme, addr}
+	select {
+	case <-t.incHostConnCount(key):
+	default:
+		return false
+	}
 	return t.tryPutIdleConn(&persistConn{
 		t:        t,
 		conn:     c,                   // dummy
 		closech:  make(chan struct{}), // so it can be closed
-		cacheKey: connectMethodKey{"", "http", "example.com"},
+		cacheKey: key,
 	}) == nil
 }