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>

7 files changed, 860 insertions, 0 deletions

diff --git a/contrib/persistent-https/LICENSE b/contrib/persistent-https/LICENSE
new file mode 100644
index 000000000..d64569567
--- /dev/null
+++ b/contrib/persistent-https/LICENSE
@@ -0,0 +1,202 @@
+
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diff --git a/contrib/persistent-https/Makefile b/contrib/persistent-https/Makefile
new file mode 100644
index 000000000..92baa3bee
--- /dev/null
+++ b/contrib/persistent-https/Makefile
@@ -0,0 +1,38 @@
+# 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.
+
+BUILD_LABEL=$(shell date +"%s")
+TAR_OUT=$(shell go env GOOS)_$(shell go env GOARCH).tar.gz
+
+all: git-remote-persistent-https git-remote-persistent-https--proxy \
+	git-remote-persistent-http
+
+git-remote-persistent-https--proxy: git-remote-persistent-https
+	ln -f -s git-remote-persistent-https git-remote-persistent-https--proxy
+
+git-remote-persistent-http: git-remote-persistent-https
+	ln -f -s git-remote-persistent-https git-remote-persistent-http
+
+git-remote-persistent-https:
+	go build -o git-remote-persistent-https \
+		-ldflags "-X main._BUILD_EMBED_LABEL $(BUILD_LABEL)"
+
+clean:
+	rm -f git-remote-persistent-http* *.tar.gz
+
+tar: clean all
+	@chmod 555 git-remote-persistent-https
+	@tar -czf $(TAR_OUT) git-remote-persistent-http* README LICENSE
+	@echo
+	@echo "Created $(TAR_OUT)"
diff --git a/contrib/persistent-https/README b/contrib/persistent-https/README
new file mode 100644
index 000000000..f784dd2e6
--- /dev/null
+++ b/contrib/persistent-https/README
@@ -0,0 +1,62 @@
+git-remote-persistent-https
+
+The git-remote-persistent-https binary speeds up SSL operations
+by running a daemon job (git-remote-persistent-https--proxy) that
+keeps a connection open to a server.
+
+
+PRE-BUILT BINARIES
+
+Darwin amd64:
+https://commondatastorage.googleapis.com/git-remote-persistent-https/darwin_amd64.tar.gz
+
+Linux amd64:
+https://commondatastorage.googleapis.com/git-remote-persistent-https/linux_amd64.tar.gz
+
+
+INSTALLING
+
+Move all of the git-remote-persistent-http* binaries to a directory
+in PATH.
+
+
+USAGE
+
+HTTPS requests can be delegated to the proxy by using the
+"persistent-https" scheme, e.g.
+
+git clone persistent-https://kernel.googlesource.com/pub/scm/git/git
+
+Likewise, .gitconfig can be updated as follows to rewrite https urls
+to use persistent-https:
+
+[url "persistent-https"]
+	insteadof = https
+[url "persistent-http"]
+	insteadof = http
+
+
+#####################################################################
+# BUILDING FROM SOURCE
+#####################################################################
+
+LOCATION
+
+The source is available in the contrib/persistent-https directory of
+the Git source repository. The Git source repository is available at
+git://git.kernel.org/pub/scm/git/git.git/
+https://kernel.googlesource.com/pub/scm/git/git
+
+
+PREREQUISITES
+
+The code is written in Go (http://golang.org/) and the Go compiler is
+required. Currently, the compiler must be built and installed from tip
+of source, in order to include a fix in the reverse http proxy:
+http://code.google.com/p/go/source/detail?r=a615b796570a2cd8591884767a7d67ede74f6648
+
+
+BUILDING
+
+Run "make" to build the binaries. See the section on
+INSTALLING above.
diff --git a/contrib/persistent-https/client.go b/contrib/persistent-https/client.go
new file mode 100644
index 000000000..71125b583
--- /dev/null
+++ b/contrib/persistent-https/client.go
@@ -0,0 +1,189 @@
+// 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.
+
+package main
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"net"
+	"net/url"
+	"os"
+	"os/exec"
+	"strings"
+	"syscall"
+	"time"
+)
+
+type Client struct {
+	ProxyBin string
+	Args     []string
+
+	insecure bool
+}
+
+func (c *Client) Run() error {
+	if err := c.resolveArgs(); err != nil {
+		return fmt.Errorf("resolveArgs() got error: %v", err)
+	}
+
+	// Connect to the proxy.
+	uconn, hconn, addr, err := c.connect()
+	if err != nil {
+		return fmt.Errorf("connect() got error: %v", err)
+	}
+	// Keep the unix socket connection open for the duration of the request.
+	defer uconn.Close()
+	// Keep a connection to the HTTP server open, so no other user can
+	// bind on the same address so long as the process is running.
+	defer hconn.Close()
+
+	// Start the git-remote-http subprocess.
+	cargs := []string{"-c", fmt.Sprintf("http.proxy=%v", addr), "remote-http"}
+	cargs = append(cargs, c.Args...)
+	cmd := exec.Command("git", cargs...)
+
+	for _, v := range os.Environ() {
+		if !strings.HasPrefix(v, "GIT_PERSISTENT_HTTPS_SECURE=") {
+			cmd.Env = append(cmd.Env, v)
+		}
+	}
+	// Set the GIT_PERSISTENT_HTTPS_SECURE environment variable when
+	// the proxy is using a SSL connection.  This allows credential helpers
+	// to identify secure proxy connections, despite being passed an HTTP
+	// scheme.
+	if !c.insecure {
+		cmd.Env = append(cmd.Env, "GIT_PERSISTENT_HTTPS_SECURE=1")
+	}
+
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		if eerr, ok := err.(*exec.ExitError); ok {
+			if stat, ok := eerr.ProcessState.Sys().(syscall.WaitStatus); ok && stat.ExitStatus() != 0 {
+				os.Exit(stat.ExitStatus())
+			}
+		}
+		return fmt.Errorf("git-remote-http subprocess got error: %v", err)
+	}
+	return nil
+}
+
+func (c *Client) connect() (uconn net.Conn, hconn net.Conn, addr string, err error) {
+	uconn, err = DefaultSocket.Dial()
+	if err != nil {
+		if e, ok := err.(*net.OpError); ok && (os.IsNotExist(e.Err) || e.Err == syscall.ECONNREFUSED) {
+			if err = c.startProxy(); err == nil {
+				uconn, err = DefaultSocket.Dial()
+			}
+		}
+		if err != nil {
+			return
+		}
+	}
+
+	if addr, err = c.readAddr(uconn); err != nil {
+		return
+	}
+
+	// Open a tcp connection to the proxy.
+	if hconn, err = net.Dial("tcp", addr); err != nil {
+		return
+	}
+
+	// Verify the address hasn't changed ownership.
+	var addr2 string
+	if addr2, err = c.readAddr(uconn); err != nil {
+		return
+	} else if addr != addr2 {
+		err = fmt.Errorf("address changed after connect. got %q, want %q", addr2, addr)
+		return
+	}
+	return
+}
+
+func (c *Client) readAddr(conn net.Conn) (string, error) {
+	conn.SetDeadline(time.Now().Add(5 * time.Second))
+	data := make([]byte, 100)
+	n, err := conn.Read(data)
+	if err != nil {
+		return "", fmt.Errorf("error reading unix socket: %v", err)
+	} else if n == 0 {
+		return "", errors.New("empty data response")
+	}
+	conn.Write([]byte{1}) // Ack
+
+	var addr string
+	if addrs := strings.Split(string(data[:n]), "\n"); len(addrs) != 2 {
+		return "", fmt.Errorf("got %q, wanted 2 addresses", data[:n])
+	} else if c.insecure {
+		addr = addrs[1]
+	} else {
+		addr = addrs[0]
+	}
+	return addr, nil
+}
+
+func (c *Client) startProxy() error {
+	cmd := exec.Command(c.ProxyBin)
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return err
+	}
+	defer stdout.Close()
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+	result := make(chan error)
+	go func() {
+		bytes, _, err := bufio.NewReader(stdout).ReadLine()
+		if line := string(bytes); err == nil && line != "OK" {
+			err = fmt.Errorf("proxy returned %q, want \"OK\"", line)
+		}
+		result <- err
+	}()
+	select {
+	case err := <-result:
+		return err
+	case <-time.After(5 * time.Second):
+		return errors.New("timeout waiting for proxy to start")
+	}
+	panic("not reachable")
+}
+
+func (c *Client) resolveArgs() error {
+	if nargs := len(c.Args); nargs == 0 {
+		return errors.New("remote needed")
+	} else if nargs > 2 {
+		return fmt.Errorf("want at most 2 args, got %v", c.Args)
+	}
+
+	// Rewrite the url scheme to be http.
+	idx := len(c.Args) - 1
+	rawurl := c.Args[idx]
+	rurl, err := url.Parse(rawurl)
+	if err != nil {
+		return fmt.Errorf("invalid remote: %v", err)
+	}
+	c.insecure = rurl.Scheme == "persistent-http"
+	rurl.Scheme = "http"
+	c.Args[idx] = rurl.String()
+	if idx != 0 && c.Args[0] == rawurl {
+		c.Args[0] = c.Args[idx]
+	}
+	return nil
+}
diff --git a/contrib/persistent-https/main.go b/contrib/persistent-https/main.go
new file mode 100644
index 000000000..fd1b10774
--- /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
+}
diff --git a/contrib/persistent-https/proxy.go b/contrib/persistent-https/proxy.go
new file mode 100644
index 000000000..bb0cdba38
--- /dev/null
+++ b/contrib/persistent-https/proxy.go
@@ -0,0 +1,190 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httputil"
+	"os"
+	"os/exec"
+	"os/signal"
+	"sync"
+	"syscall"
+	"time"
+)
+
+type Proxy struct {
+	BuildLabel         string
+	MaxIdleDuration    time.Duration
+	PollUpdateInterval time.Duration
+
+	ul        net.Listener
+	httpAddr  string
+	httpsAddr string
+}
+
+func (p *Proxy) Run() error {
+	hl, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return fmt.Errorf("http listen failed: %v", err)
+	}
+	defer hl.Close()
+
+	hsl, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		return fmt.Errorf("https listen failed: %v", err)
+	}
+	defer hsl.Close()
+
+	p.ul, err = DefaultSocket.Listen()
+	if err != nil {
+		c, derr := DefaultSocket.Dial()
+		if derr == nil {
+			c.Close()
+			fmt.Println("OK\nA proxy is already running... exiting")
+			return nil
+		} else if e, ok := derr.(*net.OpError); ok && e.Err == syscall.ECONNREFUSED {
+			// Nothing is listening on the socket, unlink it and try again.
+			syscall.Unlink(DefaultSocket.Path())
+			p.ul, err = DefaultSocket.Listen()
+		}
+		if err != nil {
+			return fmt.Errorf("unix listen failed on %v: %v", DefaultSocket.Path(), err)
+		}
+	}
+	defer p.ul.Close()
+	go p.closeOnSignal()
+	go p.closeOnUpdate()
+
+	p.httpAddr = hl.Addr().String()
+	p.httpsAddr = hsl.Addr().String()
+	fmt.Printf("OK\nListening on unix socket=%v http=%v https=%v\n",
+		p.ul.Addr(), p.httpAddr, p.httpsAddr)
+
+	result := make(chan error, 2)
+	go p.serveUnix(result)
+	go func() {
+		result <- http.Serve(hl, &httputil.ReverseProxy{
+			FlushInterval: 500 * time.Millisecond,
+			Director:      func(r *http.Request) {},
+		})
+	}()
+	go func() {
+		result <- http.Serve(hsl, &httputil.ReverseProxy{
+			FlushInterval: 500 * time.Millisecond,
+			Director: func(r *http.Request) {
+				r.URL.Scheme = "https"
+			},
+		})
+	}()
+	return <-result
+}
+
+type socketContext struct {
+	sync.WaitGroup
+	mutex sync.Mutex
+	last  time.Time
+}
+
+func (sc *socketContext) Done() {
+	sc.mutex.Lock()
+	defer sc.mutex.Unlock()
+	sc.last = time.Now()
+	sc.WaitGroup.Done()
+}
+
+func (p *Proxy) serveUnix(result chan<- error) {
+	sockCtx := &socketContext{}
+	go p.closeOnIdle(sockCtx)
+
+	var err error
+	for {
+		var uconn net.Conn
+		uconn, err = p.ul.Accept()
+		if err != nil {
+			err = fmt.Errorf("accept failed: %v", err)
+			break
+		}
+		sockCtx.Add(1)
+		go p.handleUnixConn(sockCtx, uconn)
+	}
+	sockCtx.Wait()
+	result <- err
+}
+
+func (p *Proxy) handleUnixConn(sockCtx *socketContext, uconn net.Conn) {
+	defer sockCtx.Done()
+	defer uconn.Close()
+	data := []byte(fmt.Sprintf("%v\n%v", p.httpsAddr, p.httpAddr))
+	uconn.SetDeadline(time.Now().Add(5 * time.Second))
+	for i := 0; i < 2; i++ {
+		if n, err := uconn.Write(data); err != nil {
+			log.Printf("error sending http addresses: %+v\n", err)
+			return
+		} else if n != len(data) {
+			log.Printf("sent %d data bytes, wanted %d\n", n, len(data))
+			return
+		}
+		if _, err := uconn.Read([]byte{0, 0, 0, 0}); err != nil {
+			log.Printf("error waiting for Ack: %+v\n", err)
+			return
+		}
+	}
+	// Wait without a deadline for the client to finish via EOF
+	uconn.SetDeadline(time.Time{})
+	uconn.Read([]byte{0, 0, 0, 0})
+}
+
+func (p *Proxy) closeOnIdle(sockCtx *socketContext) {
+	for d := p.MaxIdleDuration; d > 0; {
+		time.Sleep(d)
+		sockCtx.Wait()
+		sockCtx.mutex.Lock()
+		if d = sockCtx.last.Add(p.MaxIdleDuration).Sub(time.Now()); d <= 0 {
+			log.Println("graceful shutdown from idle timeout")
+			p.ul.Close()
+		}
+		sockCtx.mutex.Unlock()
+	}
+}
+
+func (p *Proxy) closeOnUpdate() {
+	for {
+		time.Sleep(p.PollUpdateInterval)
+		if out, err := exec.Command(os.Args[0], "--print_label").Output(); err != nil {
+			log.Printf("error polling for updated binary: %v\n", err)
+		} else if s := string(out[:len(out)-1]); p.BuildLabel != s {
+			log.Printf("graceful shutdown from updated binary: %q --> %q\n", p.BuildLabel, s)
+			p.ul.Close()
+			break
+		}
+	}
+}
+
+func (p *Proxy) closeOnSignal() {
+	ch := make(chan os.Signal, 10)
+	signal.Notify(ch, os.Interrupt, os.Kill, os.Signal(syscall.SIGTERM), os.Signal(syscall.SIGHUP))
+	sig := <-ch
+	p.ul.Close()
+	switch sig {
+	case os.Signal(syscall.SIGHUP):
+		log.Printf("graceful shutdown from signal: %v\n", sig)
+	default:
+		log.Fatalf("exiting from signal: %v\n", sig)
+	}
+}
diff --git a/contrib/persistent-https/socket.go b/contrib/persistent-https/socket.go
new file mode 100644
index 000000000..193b911dd
--- /dev/null
+++ b/contrib/persistent-https/socket.go
@@ -0,0 +1,97 @@
+// 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.
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+// A Socket is a wrapper around a Unix socket that verifies directory
+// permissions.
+type Socket struct {
+	Dir string
+}
+
+func defaultDir() string {
+	sockPath := ".git-credential-cache"
+	if home := os.Getenv("HOME"); home != "" {
+		return filepath.Join(home, sockPath)
+	}
+	log.Printf("socket: cannot find HOME path. using relative directory %q for socket", sockPath)
+	return sockPath
+}
+
+// DefaultSocket is a Socket in the $HOME/.git-credential-cache directory.
+var DefaultSocket = Socket{Dir: defaultDir()}
+
+// Listen announces the local network address of the unix socket. The
+// permissions on the socket directory are verified before attempting
+// the actual listen.
+func (s Socket) Listen() (net.Listener, error) {
+	network, addr := "unix", s.Path()
+	if err := s.mkdir(); err != nil {
+		return nil, &net.OpError{Op: "listen", Net: network, Addr: &net.UnixAddr{Name: addr, Net: network}, Err: err}
+	}
+	return net.Listen(network, addr)
+}
+
+// Dial connects to the unix socket. The permissions on the socket directory
+// are verified before attempting the actual dial.
+func (s Socket) Dial() (net.Conn, error) {
+	network, addr := "unix", s.Path()
+	if err := s.checkPermissions(); err != nil {
+		return nil, &net.OpError{Op: "dial", Net: network, Addr: &net.UnixAddr{Name: addr, Net: network}, Err: err}
+	}
+	return net.Dial(network, addr)
+}
+
+// Path returns the fully specified file name of the unix socket.
+func (s Socket) Path() string {
+	return filepath.Join(s.Dir, "persistent-https-proxy-socket")
+}
+
+func (s Socket) mkdir() error {
+	if err := s.checkPermissions(); err == nil {
+		return nil
+	} else if !os.IsNotExist(err) {
+		return err
+	}
+	if err := os.MkdirAll(s.Dir, 0700); err != nil {
+		return err
+	}
+	return s.checkPermissions()
+}
+
+func (s Socket) checkPermissions() error {
+	fi, err := os.Stat(s.Dir)
+	if err != nil {
+		return err
+	}
+	if !fi.IsDir() {
+		return fmt.Errorf("socket: got file, want directory for %q", s.Dir)
+	}
+	if fi.Mode().Perm() != 0700 {
+		return fmt.Errorf("socket: got perm %o, want 700 for %q", fi.Mode().Perm(), s.Dir)
+	}
+	if st := fi.Sys().(*syscall.Stat_t); int(st.Uid) != os.Getuid() {
+		return fmt.Errorf("socket: got uid %d, want %d for %q", st.Uid, os.Getuid(), s.Dir)
+	}
+	return nil
+}