backend: configure protocol-level connection health checks

This should detect a connection that is stuck for more than 2 minutes.

internal/backend/http_transport.go

@@ -13,6 +13,7 @@ import (
 	"github.com/peterbourgon/unixtransport"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
+	"golang.org/x/net/http2"
 )
 
 // TransportOptions collects various options which can be set for an HTTP based
@@ -74,7 +75,6 @@ func Transport(opts TransportOptions) (http.RoundTripper, error) {
 			KeepAlive: 30 * time.Second,
 			DualStack: true,
 		}).DialContext,
-		ForceAttemptHTTP2:     true,
 		MaxIdleConns:          100,
 		MaxIdleConnsPerHost:   100,
 		IdleConnTimeout:       90 * time.Second,
@@ -83,6 +83,17 @@ func Transport(opts TransportOptions) (http.RoundTripper, error) {
 		TLSClientConfig:       &tls.Config{},
 	}
 
+	// ensure that http2 connections are closed if they are broken
+	h2, err := http2.ConfigureTransports(tr)
+	if err != nil {
+		panic(err)
+	}
+	if feature.Flag.Enabled(feature.HTTPTimeouts) {
+		h2.WriteByteTimeout = 120 * time.Second
+		h2.ReadIdleTimeout = 60 * time.Second
+		h2.PingTimeout = 60 * time.Second
+	}
+
 	unixtransport.Register(tr)
 
 	if opts.InsecureTLS {