net/http: add pluggable HTTP/3 support for closing idle connections

This CL defines the closeIdleConectionser interface, allowing us to call
a CloseIdleConnections method that a pluggable HTTP/3 transport might
implement.

Concretely, this allows an HTTP/3 transport implementation to clean up
resources such as an open UDP socket that is no longer needed,
preventing resources from lingering around indefinitely until the entire
program exits.

For #77440

Change-Id: I7216caee58954c3651f96a56dbf27ec74c539ad3
Reviewed-on: https://go-review.googlesource.com/c/go/+/753161
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

1 files changed, 20 insertions, 3 deletions

diff --git a/src/net/http/transport.go b/src/net/http/transport.go
index efc3838154..32bbb04759 100644
--- a/src/net/http/transport.go
+++ b/src/net/http/transport.go
@@ -388,9 +388,8 @@ type dialClientConner interface {
 	// If HTTP/3 proxies are not supported, DialClientConn should return
 	// an error wrapping [errors.ErrUnsupported].
 	//
-	// The RoundTripper returned by DialClientConn may implement
-	// any of the following methods to support the [ClientConn]
-	// method of the same name:
+	// The RoundTripper returned by DialClientConn must also implement the
+	// following methods to support [ClientConn] methods of the same name:
 	//	Close() error
 	//	Err() error
 	// 	Reserve() error
@@ -411,6 +410,16 @@ type dialClientConner interface {
 	DialClientConn(ctx context.Context, address string, proxy *url.URL, internalStateHook func()) (RoundTripper, error)
 }
 
+type closeIdleConnectionser interface {
+	// CloseIdleConnections is called by Transport.CloseIdleConnections.
+	//
+	// The transport will close idle connections created with DialClientConn
+	// before calling this method. The HTTP/3 transport should not attempt to
+	// close idle connections, but may clean up shared resources such as UDP
+	// sockets if no connections remain.
+	CloseIdleConnections()
+}
+
 // h2Transport is the interface we expect to be able to call from
 // net/http against an *http2.Transport that's either bundled into
 // h2_bundle.go or supplied by the user via x/net/http2.
@@ -956,6 +965,9 @@ func (t *Transport) CloseIdleConnections() {
 	if t2 := t.h2transport; t2 != nil {
 		t2.CloseIdleConnections()
 	}
+	if cc, ok := t.h3transport.(closeIdleConnectionser); ok {
+		cc.CloseIdleConnections()
+	}
 }
 
 // prepareTransportCancel sets up state to convert Transport.CancelRequest into context cancelation.
@@ -3088,11 +3100,16 @@ func (pc *persistConn) closeLocked(err error) {
 		pc.t.decConnsPerHost(pc.cacheKey)
 		// Close HTTP/1 (pc.alt == nil) connection.
 		// HTTP/2 closes its connection itself.
+		// Close HTTP/3 connection if it implements io.Closer.
 		if pc.alt == nil {
 			if err != errCallerOwnsConn {
 				pc.conn.Close()
 			}
 			close(pc.closech)
+		} else {
+			if cc, ok := pc.alt.(io.Closer); ok {
+				cc.Close()
+			}
 		}
 	}
 	pc.mutateHeaderFunc = nil