Add persistent-https to contrib

Git over HTTPS has a high request startup latency, since the SSL
negotiation can take up to a second. In order to reduce this latency,
connections should be left open to the Git server across requests
(or invocations of the git commandline).

Reduce SSL startup latency by running a daemon job that keeps
connections open to a Git server. The daemon job
(git-remote-persistent-https--proxy) is started on the first request
through the client binary (git-remote-persistent-https) and remains
running for 24 hours after the last request, or until a new daemon
binary is placed in the PATH. The client determines the daemon's
HTTP address by communicating over a UNIX socket with the daemon.
From there, the rest of the Git protocol work is delegated to the
"git-remote-http" binary, with the environment's http_proxy set to
the daemon.

Accessing /pub/scm/linux/kernel/git/torvalds/linux repository hosted
at kernel.googlesource.com with "git ls-remote" over https:// and
persistent-https:// 5 times shows that the first request takes about
the same time (0.193s vs 0.208s---there is a slight set-up cost for
the local proxy); as expected, the other four requests are much
faster (~0.18s vs ~0.08s).

Incidentally, this also has the benefit of HTTP keep-alive working
across Git command invocations. Its common for servers to use a 5
minute keep-alive on an HTTP 1.1 connection. Git-over-HTTP commonly
uses Transfer-Encoding: chunked on replies, so keep-alive will
generally just work, even though a pack stream's length isn't known
in advance. Because the helper is an external process holding that
connection open, we also benefit from being able to reuse an
existing TCP connection to the server.  The same "git ls-remote"
test against http:// vs persistent-https:// URL shows that the
former takes ~0.09s while the first request for the latter is about
0.134s with set-up cost, and subsequent requests are ~0.065s,
shaving around one RTT to the server.

Signed-off-by: Colby Ranger <cranger@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 82 insertions, 0 deletions

diff --git a/contrib/persistent-https/main.go b/contrib/persistent-https/main.go
new file mode 100644
index 0000000000..fd1b107743
--- /dev/null
+++ b/contrib/persistent-https/main.go
@@ -0,0 +1,82 @@
+// Copyright 2012 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// The git-remote-persistent-https binary speeds up SSL operations by running
+// a daemon job that keeps a connection open to a Git server. This ensures the
+// git-remote-persistent-https--proxy is running and delegating execution
+// to the git-remote-http binary with the http_proxy set to the daemon job.
+// A unix socket is used to authenticate the proxy and discover the
+// HTTP address. Note, both the client and proxy are included in the same
+// binary.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+	"time"
+)
+
+var (
+	forceProxy = flag.Bool("proxy", false, "Whether to start the binary in proxy mode")
+	proxyBin   = flag.String("proxy_bin", "git-remote-persistent-https--proxy", "Path to the proxy binary")
+	printLabel = flag.Bool("print_label", false, "Prints the build label for the binary")
+
+	// Variable that should be defined through the -X linker flag.
+	_BUILD_EMBED_LABEL string
+)
+
+const (
+	defaultMaxIdleDuration    = 24 * time.Hour
+	defaultPollUpdateInterval = 15 * time.Minute
+)
+
+func main() {
+	flag.Parse()
+	if *printLabel {
+		// Short circuit execution to print the build label
+		fmt.Println(buildLabel())
+		return
+	}
+
+	var err error
+	if *forceProxy || strings.HasSuffix(os.Args[0], "--proxy") {
+		log.SetPrefix("git-remote-persistent-https--proxy: ")
+		proxy := &Proxy{
+			BuildLabel:         buildLabel(),
+			MaxIdleDuration:    defaultMaxIdleDuration,
+			PollUpdateInterval: defaultPollUpdateInterval,
+		}
+		err = proxy.Run()
+	} else {
+		log.SetPrefix("git-remote-persistent-https: ")
+		client := &Client{
+			ProxyBin: *proxyBin,
+			Args:     flag.Args(),
+		}
+		err = client.Run()
+	}
+	if err != nil {
+		log.Fatalln(err)
+	}
+}
+
+func buildLabel() string {
+	if _BUILD_EMBED_LABEL == "" {
+		log.Println(`unlabeled build; build with "make" to label`)
+	}
+	return _BUILD_EMBED_LABEL
+}