Merge pull request #286 from roidelapluie/http2-env

Add an environment variable to enable HTTP2

Author: Julien Pivotto

config/http_config.go

@@ -27,6 +27,7 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"os"
 	"strings"
 	"sync"
 	"time"
@@ -379,18 +380,23 @@ func NewRoundTripperFromConfig(cfg HTTPClientConfig, name string, optFuncs ...HT
 			ExpectContinueTimeout: 1 * time.Second,
 			DialContext:           dialContext,
 		}
-		if opts.http2Enabled {
+		if opts.http2Enabled || os.Getenv("PROMETHEUS_COMMON_ENABLE_HTTP2") != "" {
 			// HTTP/2 support is golang has many problematic cornercases where
 			// dead connections would be kept and used in connection pools.
 			// https://github.com/golang/go/issues/32388
 			// https://github.com/golang/go/issues/39337
 			// https://github.com/golang/go/issues/39750
-			// TODO: Re-Enable HTTP/2 once upstream issue is fixed.
-			// TODO: use ForceAttemptHTTP2 when we move to Go 1.13+.
-			err := http2.ConfigureTransport(rt.(*http.Transport))
+
+			// Enable HTTP2 if the environment variable
+			// PROMETHEUS_COMMON_ENABLE_HTTP2 is set.
+			// This is a temporary workaround so that users can safely test this
+			// and validate that HTTP2 can be enabled Prometheus-Wide again.
+
+			http2t, err := http2.ConfigureTransports(rt.(*http.Transport))
 			if err != nil {
 				return nil, err
 			}
+			http2t.ReadIdleTimeout = time.Minute
 		}
 
 		// If a authorization_credentials is provided, create a round tripper that will set the