It's a copy of http://codereview.chromium.org/155772, with the fix for valgri...

chrome/browser/process_singleton_linux.cc

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // On Linux, when the user tries to launch a second copy of chrome, we check
 // for a socket in the user's profile directory.  If the socket file is open we
 // send a message to the first chrome browser process with the current
 // directory and second process command line flags.  The second process then
 // exits.
+//
+// The socket file's name contains the process id of chrome's browser process,
+// eg. "SingletonSocket-9156". A symbol link named "SingletonSocket" will be
+// created and pointed to the real socket file, so they would look like:
+//
+// SingletonSocket -> SingletonSocket-9156
+// SingletonSocket-9156
+//
+// So that the socket file can be connected through "SingletonSocket" and the
+// process id can also be retrieved from it by calling readlink().
+//
+// When the second process sends the current directory and command line flags to
+// the first process, it waits for an ACK message back from the first process
+// for a certain time. If there is no ACK message back in time, then the first
+// process will be considered as hung for some reason. The second process then
+// retrieves the process id from the symbol link and kills it by sending
+// SIGKILL. Then the second process starts as normal.
+//
+// TODO(james.su@gmail.com): Add unittest for this class.
 
 #include "chrome/browser/process_singleton.h"
 
 #include <errno.h>
 #include <fcntl.h>
+#include <signal.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
 #include <sys/types.h>
-#include <sys/socket.h>
 #include <sys/un.h>
+#include <unistd.h>
+#include <cstring>
 #include <set>
+#include <string>
 
 #include "base/base_paths.h"
 #include "base/basictypes.h"
 #include "base/command_line.h"
 #include "base/eintr_wrapper.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
 #include "base/path_service.h"
+#include "base/process_util.h"
 #include "base/stl_util-inl.h"
 #include "base/string_util.h"
 #include "base/time.h"
 #include "base/timer.h"
 #include "chrome/browser/browser_init.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chrome_thread.h"
 #include "chrome/browser/profile.h"
 #include "chrome/browser/profile_manager.h"
 #include "chrome/common/chrome_constants.h"
 #include "chrome/common/chrome_paths.h"
 
+const int ProcessSingleton::kTimeoutInSeconds;
+
 namespace {
 
 const char kStartToken[] = "START";
+const char kACKToken[] = "ACK";
+const char kShutdownToken[] = "SHUTDOWN";
 const char kTokenDelimiter = '\0';
-const int kTimeOutInSeconds = 5;
 const int kMaxMessageLength = 32 * 1024;
+const int kMaxACKMessageLength = arraysize(kShutdownToken) - 1;
 
 // Set a file descriptor to be non-blocking.
 // Return 0 on success, -1 on failure.
 int SetNonBlocking(int fd) {
   int flags = fcntl(fd, F_GETFL, 0);
   if (-1 == flags)
     return flags;
   if (flags & O_NONBLOCK)
     return 0;
   return fcntl(fd, F_SETFL, flags | O_NONBLOCK);
 }
 
 // Set the close-on-exec bit on a file descriptor.
 // Returns 0 on success, -1 on failure.
 int SetCloseOnExec(int fd) {
   int flags = fcntl(fd, F_GETFD, 0);
   if (-1 == flags)
     return flags;
   if (flags & FD_CLOEXEC)
     return 0;
   return fcntl(fd, F_SETFD, flags | FD_CLOEXEC);
 }
 
+// Close a socket and check return value.
+void CloseSocket(int fd) {
+  int rv = HANDLE_EINTR(close(fd));
+  DCHECK_EQ(0, rv) << "Error closing socket: " << strerror(errno);
+}
+
+// Write a message to a socket fd.
+bool WriteToSocket(int fd, const char *message, size_t length) {
+  DCHECK(message);
+  DCHECK(length);
+  size_t bytes_written = 0;
+  do {
+    ssize_t rv = HANDLE_EINTR(
+        write(fd, message + bytes_written, length - bytes_written));
+    if (rv < 0) {
+      if (errno == EAGAIN || errno == EWOULDBLOCK) {
+        // The socket shouldn't block, we're sending so little data.  Just give
+        // up here, since NotifyOtherProcess() doesn't have an asynchronous api.
+        LOG(ERROR) << "ProcessSingleton would block on write(), so it gave up.";
+        return false;
+      }
+      LOG(ERROR) << "write() failed: " << strerror(errno);
+      return false;
+    }
+    bytes_written += rv;
+  } while (bytes_written < length);
+
+  return true;
+}
+
+// Wait a socket for read for a certain timeout in seconds.
+// Returns -1 if error occurred, 0 if timeout reached, > 0 if the socket is
+// ready for read.
+int WaitSocketForRead(int fd, int timeout) {
+  fd_set read_fds;
+  struct timeval tv;
+
+  FD_ZERO(&read_fds);
+  FD_SET(fd, &read_fds);
+  tv.tv_sec = timeout;
+  tv.tv_usec = 0;
+
+  return HANDLE_EINTR(select(fd + 1, &read_fds, NULL, NULL, &tv));
+}
+
+// Read a message from a socket fd, with an optional timeout in seconds.
+// If |timeout| <= 0 then read immediately.
+// Return number of bytes actually read, or -1 on error.
+ssize_t ReadFromSocket(int fd, char *buf, size_t bufsize, int timeout) {
+  if (timeout > 0) {
+    int rv = WaitSocketForRead(fd, timeout);
+    if (rv <= 0)
+      return rv;
+  }
+
+  size_t bytes_read = 0;
+  do {
+    ssize_t rv = HANDLE_EINTR(read(fd, buf + bytes_read, bufsize - bytes_read));
+    if (rv < 0) {
+      if (errno != EAGAIN && errno != EWOULDBLOCK) {
+        LOG(ERROR) << "read() failed: " << strerror(errno);
+        return rv;
+      } else {
+        // It would block, so we just return what has been read.
+        return bytes_read;
+      }
+    } else if (!rv) {
+      // No more data to read.
+      return bytes_read;
+    } else {
+      bytes_read += rv;
+    }
+  } while (bytes_read < bufsize);
+
+  return bytes_read;
+}
+
+// Set up a socket and sockaddr appropriate for messaging.
+void SetupSocket(const std::string& path, int* sock, struct sockaddr_un* addr) {
+  *sock = socket(PF_UNIX, SOCK_STREAM, 0);
+  CHECK(*sock >= 0) << "socket() failed: " << strerror(errno);
+
+  int rv = SetNonBlocking(*sock);
+  DCHECK_EQ(0, rv) << "Failed to make non-blocking socket.";
+  rv = SetCloseOnExec(*sock);
+  DCHECK_EQ(0, rv) << "Failed to set CLOEXEC on socket.";
+
+  addr->sun_family = AF_UNIX;
+  CHECK(path.length() < arraysize(addr->sun_path))
+      << "Socket path too long: " << path;
+  base::strlcpy(addr->sun_path, path.c_str(), arraysize(addr->sun_path));
+}
+
+// Read a symbol link, return empty string if given path is not a symbol link.
+std::string ReadLink(const std::string& path) {
+  struct stat statbuf;
+
+  if (lstat(path.c_str(), &statbuf) < 0) {
+    DCHECK_EQ(errno, ENOENT);
+    return std::string();
+  }
+
+  if (S_ISLNK(statbuf.st_mode)) {
+    char buf[PATH_MAX + 1];
+    ssize_t len = readlink(path.c_str(), buf, PATH_MAX);
+    if (len > 0) {
+      buf[len] = '\0';
+      FilePath real_path(buf);
+      // If it's not an absolute path, then it's necessary to prepend the
+      // original path's dirname.
+      if (!real_path.IsAbsolute()) {
+        real_path = FilePath(path).DirName().Append(real_path);
+      }
+      return real_path.value();
+    } else {
+      LOG(ERROR) << "readlink(" << path << ") failed: " << strerror(errno);
+    }
+  }
+
+  return std::string();
+}
+
+// Unlink a socket path. If the path is a symbol link, then the symbol link
+// and the real path referenced by the symbol link will be unlinked together.
+bool UnlinkSocketPath(const std::string& path) {
+  std::string real_path = ReadLink(path);
+
+  bool ret = true;
+  if (real_path.length())
+    ret = UnlinkSocketPath(real_path);
+
+  int rv = unlink(path.c_str());
+  if (rv < 0)
+    DCHECK_EQ(errno, ENOENT);
+
+  return rv == 0 && ret;
+}
+
+// Extract the process's pid from a symbol link path and kill it.
+// The pid will be appended to the end of path with a preceding dash, such as:
+// .../SingletonSocket-1234
+void KillProcessBySocketPath(const std::string& path) {
+  std::string real_path = ReadLink(path);
+
+  // If the path is not a symbol link, try to extract pid from the path itself.
+  if (real_path.empty())
+    real_path = path;
+
+  // Only extract pid from the base name, to avoid finding wrong value from its
+  // parent path name.
+  std::string base_name = FilePath(real_path).BaseName().value();
+  DCHECK(base_name.length());
+
+  std::string::size_type pos = base_name.rfind('-');
+  if (pos != std::string::npos) {
+    std::string pid_str = base_name.substr(pos + 1);
+    int pid;
+    if (StringToInt(pid_str, &pid)) {
+      // TODO(james.su@gmail.com): Is SIGKILL ok?
+      int rv = kill(static_cast<base::ProcessHandle>(pid), SIGKILL);
+      DCHECK_EQ(0, rv) << "Error killing process:" << strerror(errno);
+      return;
+    }
+  }
+
+  LOG(ERROR) << "Failed to extract pid from path: " << real_path;
+}
+
+// A helper class to close a socket automatically.
+class SocketCloser {
+ public:
+  explicit SocketCloser(int fd) : fd_(fd) { }
+  ~SocketCloser() { CloseSocket(fd_); }
+ private:
+  int fd_;
+};
+
 }  // namespace
 
 ///////////////////////////////////////////////////////////////////////////////
 // ProcessSingleton::LinuxWatcher
 // A helper class for a Linux specific implementation of the process singleton.
 // This class sets up a listener on the singleton socket and handles parsing
 // messages that come in on the singleton socket.
 class ProcessSingleton::LinuxWatcher
     : public MessageLoopForIO::Watcher,
       public MessageLoop::DestructionObserver,
       public base::RefCountedThreadSafe<ProcessSingleton::LinuxWatcher> {
  public:
-  // We expect to only be constructed on the UI thread.
-  explicit LinuxWatcher(ProcessSingleton* parent)
-      : ui_message_loop_(MessageLoop::current()),
-        parent_(parent) {
-  }
-
-  virtual ~LinuxWatcher() {
-    STLDeleteElements(&readers_);
-  }
-
-  // Start listening for connections on the socket.  This method should be
-  // called from the IO thread.
-  void StartListening(int socket);
-
-  // This method determines if we should use the same process and if we should,
-  // opens a new browser tab.  This runs on the UI thread.
-  void HandleMessage(std::string current_dir, std::vector<std::string> argv);
-
-  // MessageLoopForIO::Watcher impl.  These run on the IO thread.
-  virtual void OnFileCanReadWithoutBlocking(int fd);
-  virtual void OnFileCanWriteWithoutBlocking(int fd) {
-    // ProcessSingleton only watches for accept (read) events.
-    NOTREACHED();
-  }
-
-  // MessageLoop::DestructionObserver
-  virtual void WillDestroyCurrentMessageLoop() {
-    fd_watcher_.StopWatchingFileDescriptor();
-  }
-
- private:
+  // A helper class to read message from an established socket.
   class SocketReader : public MessageLoopForIO::Watcher {
    public:
     SocketReader(ProcessSingleton::LinuxWatcher* parent,
                  MessageLoop* ui_message_loop,
                  int fd)
         : parent_(parent),
           ui_message_loop_(ui_message_loop),
           fd_(fd),
           bytes_read_(0) {
       // Wait for reads.
       MessageLoopForIO::current()->WatchFileDescriptor(
           fd, true, MessageLoopForIO::WATCH_READ, &fd_reader_, this);
-      timer_.Start(base::TimeDelta::FromSeconds(kTimeOutInSeconds),
+      timer_.Start(base::TimeDelta::FromSeconds(kTimeoutInSeconds),
                    this, &SocketReader::OnTimerExpiry);
     }
 
     virtual ~SocketReader() {
-      int rv = HANDLE_EINTR(close(fd_));
-      DCHECK_EQ(0, rv) << "Error closing socket: " << strerror(errno);
+      CloseSocket(fd_);
     }
 
     // MessageLoopForIO::Watcher impl.
     virtual void OnFileCanReadWithoutBlocking(int fd);
     virtual void OnFileCanWriteWithoutBlocking(int fd) {
       // SocketReader only watches for accept (read) events.
       NOTREACHED();
     }
 
+    // Finish handling the incoming message by optionally sending back an ACK
+    // message and removing this SocketReader.
+    void FinishWithACK(const char *message, size_t length);
+
    private:
     // If we haven't completed in a reasonable amount of time, give up.
     void OnTimerExpiry() {
       parent_->RemoveSocketReader(this);
       // We're deleted beyond this point.
     }
 
     MessageLoopForIO::FileDescriptorWatcher fd_reader_;
 
     // The ProcessSingleton::LinuxWatcher that owns us.
@@ skipping 10 matching lines @@
 
     // Tracks the number of bytes we've read in case we're getting partial
     // reads.
     size_t bytes_read_;
 
     base::OneShotTimer<SocketReader> timer_;
 
     DISALLOW_COPY_AND_ASSIGN(SocketReader);
   };
 
+  // We expect to only be constructed on the UI thread.
+  explicit LinuxWatcher(ProcessSingleton* parent)
+      : ui_message_loop_(MessageLoop::current()),
+        parent_(parent) {
+  }
+
+  virtual ~LinuxWatcher() {
+    STLDeleteElements(&readers_);
+  }
+
+  // Start listening for connections on the socket.  This method should be
+  // called from the IO thread.
+  void StartListening(int socket);
+
+  // This method determines if we should use the same process and if we should,
+  // opens a new browser tab.  This runs on the UI thread.
+  // |reader| is for sending back ACK message.
+  void HandleMessage(const std::string& current_dir,
+                     const std::vector<std::string>& argv,
+                     SocketReader *reader);
+
+  // MessageLoopForIO::Watcher impl.  These run on the IO thread.
+  virtual void OnFileCanReadWithoutBlocking(int fd);
+  virtual void OnFileCanWriteWithoutBlocking(int fd) {
+    // ProcessSingleton only watches for accept (read) events.
+    NOTREACHED();
+  }
+
+  // MessageLoop::DestructionObserver
+  virtual void WillDestroyCurrentMessageLoop() {
+    fd_watcher_.StopWatchingFileDescriptor();
+  }
+
+ private:
   // Removes and deletes the SocketReader.
   void RemoveSocketReader(SocketReader* reader);
 
   MessageLoopForIO::FileDescriptorWatcher fd_watcher_;
 
   // A reference to the UI message loop (i.e., the message loop we were
   // constructed on).
   MessageLoop* ui_message_loop_;
 
   // The ProcessSingleton that owns us.
   ProcessSingleton* const parent_;
 
   std::set<SocketReader*> readers_;
 
   DISALLOW_COPY_AND_ASSIGN(LinuxWatcher);
 };
 
 void ProcessSingleton::LinuxWatcher::OnFileCanReadWithoutBlocking(int fd) {
   // Accepting incoming client.
   sockaddr_un from;
   socklen_t from_len = sizeof(from);
   int connection_socket = HANDLE_EINTR(accept(
       fd, reinterpret_cast<sockaddr*>(&from), &from_len));
   if (-1 == connection_socket) {
     LOG(ERROR) << "accept() failed: " << strerror(errno);
     return;
   }
+  int rv = SetNonBlocking(connection_socket);
+  DCHECK_EQ(0, rv) << "Failed to make non-blocking socket.";
   SocketReader* reader = new SocketReader(this,
                                           ui_message_loop_,
                                           connection_socket);
   readers_.insert(reader);
 }
 
 void ProcessSingleton::LinuxWatcher::StartListening(int socket) {
   DCHECK(ChromeThread::GetMessageLoop(ChromeThread::IO) ==
          MessageLoop::current());
   // Watch for client connections on this socket.
   MessageLoopForIO* ml = MessageLoopForIO::current();
   ml->AddDestructionObserver(this);
   ml->WatchFileDescriptor(socket, true, MessageLoopForIO::WATCH_READ,
                           &fd_watcher_, this);
 }
 
-void ProcessSingleton::LinuxWatcher::HandleMessage(std::string current_dir,
-    std::vector<std::string> argv) {
+void ProcessSingleton::LinuxWatcher::HandleMessage(
+    const std::string& current_dir, const std::vector<std::string>& argv,
+    SocketReader* reader) {
   DCHECK(ui_message_loop_ == MessageLoop::current());
+  DCHECK(reader);
   // Ignore the request if the browser process is already in shutdown path.
   if (!g_browser_process || g_browser_process->IsShuttingDown()) {
     LOG(WARNING) << "Not handling interprocess notification as browser"
                     " is shutting down";
+    // Send back "SHUTDOWN" message, so that the client process can start up
+    // without killing this process.
+    reader->FinishWithACK(kShutdownToken, arraysize(kShutdownToken) - 1);
     return;
   }
 
   // If locked, it means we are not ready to process this message because
   // we are probably in a first run critical phase.
   if (parent_->locked()) {
     DLOG(WARNING) << "Browser is locked";
+    // Send back "ACK" message to prevent the client process from starting up.
+    reader->FinishWithACK(kACKToken, arraysize(kACKToken) - 1);
     return;
   }
 
   CommandLine parsed_command_line(argv);
   PrefService* prefs = g_browser_process->local_state();
   DCHECK(prefs);
 
   FilePath user_data_dir;
   PathService::Get(chrome::DIR_USER_DATA, &user_data_dir);
   ProfileManager* profile_manager = g_browser_process->profile_manager();
   Profile* profile = profile_manager->GetDefaultProfile(user_data_dir);
   if (!profile) {
     // We should only be able to get here if the profile already exists and
     // has been created.
     NOTREACHED();
     return;
   }
 
-  // TODO(tc): Send an ACK response on success.
-
   // Run the browser startup sequence again, with the command line of the
   // signalling process.
   FilePath current_dir_file_path(current_dir);
   BrowserInit::ProcessCommandLine(parsed_command_line,
                                   current_dir_file_path.ToWStringHack(),
                                   false, profile, NULL);
+
+  // Send back "ACK" message to prevent the client process from starting up.
+  reader->FinishWithACK(kACKToken, arraysize(kACKToken) - 1);
 }
 
 void ProcessSingleton::LinuxWatcher::RemoveSocketReader(SocketReader* reader) {
   DCHECK(reader);
   readers_.erase(reader);
   delete reader;
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 // ProcessSingleton::LinuxWatcher::SocketReader
 //
 
 void ProcessSingleton::LinuxWatcher::SocketReader::OnFileCanReadWithoutBlocking(
     int fd) {
   DCHECK_EQ(fd, fd_);
   while (bytes_read_ < sizeof(buf_)) {
     ssize_t rv = HANDLE_EINTR(
         read(fd, buf_ + bytes_read_, sizeof(buf_) - bytes_read_));
     if (rv < 0) {
       if (errno != EAGAIN && errno != EWOULDBLOCK) {
         LOG(ERROR) << "read() failed: " << strerror(errno);
-        rv = HANDLE_EINTR(close(fd));
-        DCHECK_EQ(0, rv) << "Error closing socket: " << strerror(errno);
+        CloseSocket(fd);
         return;
       } else {
         // It would block, so we just return and continue to watch for the next
         // opportunity to read.
         return;
       }
     } else if (!rv) {
       // No more data to read.  It's time to process the message.
       break;
     } else {
@@ skipping 11 matching lines @@
 
   std::string str(buf_, bytes_read_);
   std::vector<std::string> tokens;
   SplitString(str, kTokenDelimiter, &tokens);
 
   if (tokens.size() < 3 || tokens[0] != kStartToken) {
     LOG(ERROR) << "Wrong message format: " << str;
     return;
   }
 
+  // Stop the expiration timer to prevent this SocketReader object from being
+  // terminated unexpectly.
+  timer_.Stop();
+
   std::string current_dir = tokens[1];
   // Remove the first two tokens.  The remaining tokens should be the command
   // line argv array.
   tokens.erase(tokens.begin());
   tokens.erase(tokens.begin());
 
   // Return to the UI thread to handle opening a new browser tab.
   ui_message_loop_->PostTask(FROM_HERE, NewRunnableMethod(
       parent_,
       &ProcessSingleton::LinuxWatcher::HandleMessage,
       current_dir,
-      tokens));
+      tokens,
+      this));
   fd_reader_.StopWatchingFileDescriptor();
 
+  // LinuxWatcher::HandleMessage() is in charge of destroying this SocketReader
+  // object by invoking SocketReader::FinishWithACK().
+}
+
+void ProcessSingleton::LinuxWatcher::SocketReader::FinishWithACK(
+    const char *message, size_t length) {
+  if (message && length) {
+    // Not necessary to care about the return value.
+    WriteToSocket(fd_, message, length);
+  }
+
+  if (shutdown(fd_, SHUT_WR) < 0)
+    LOG(ERROR) << "shutdown() failed: " << strerror(errno);
+
   parent_->RemoveSocketReader(this);
   // We are deleted beyond this point.
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 // ProcessSingleton
 //
 ProcessSingleton::ProcessSingleton(const FilePath& user_data_dir)
     : locked_(false),
       foreground_window_(NULL),
       ALLOW_THIS_IN_INITIALIZER_LIST(watcher_(new LinuxWatcher(this))) {
   socket_path_ = user_data_dir.Append(chrome::kSingletonSocketFilename);
 }
 
 ProcessSingleton::~ProcessSingleton() {
 }
 
 bool ProcessSingleton::NotifyOtherProcess() {
   int socket;
   sockaddr_un addr;
-  SetupSocket(&socket, &addr);
+  SetupSocket(socket_path_.value(), &socket, &addr);
+
+  // It'll close the socket automatically when exiting this method.
+  SocketCloser socket_closer(socket);
 
   // Connecting to the socket
   int ret = HANDLE_EINTR(connect(socket,
                                  reinterpret_cast<sockaddr*>(&addr),
                                  sizeof(addr)));
   if (ret < 0)
     return false;  // Tell the caller there's nobody to notify.
 
   timeval timeout = {20, 0};
   setsockopt(socket, SOL_SOCKET, SO_SNDTIMEO, &timeout, sizeof(timeout));
@@ skipping 10 matching lines @@
 
   const std::vector<std::string>& argv =
       CommandLine::ForCurrentProcess()->argv();
   for (std::vector<std::string>::const_iterator it = argv.begin();
       it != argv.end(); ++it) {
     to_send.push_back(kTokenDelimiter);
     to_send.append(*it);
   }
 
   // Send the message
-  int bytes_written = 0;
-  const int buf_len = to_send.length();
-  do {
-    int rv = HANDLE_EINTR(
-        write(socket, to_send.data() + bytes_written, buf_len - bytes_written));
-    if (rv < 0) {
-      if (errno == EAGAIN || errno == EWOULDBLOCK) {
-        // The socket shouldn't block, we're sending so little data.  Just give
-        // up here, since NotifyOtherProcess() doesn't have an asynchronous api.
-        LOG(ERROR) << "ProcessSingleton would block on write(), so it gave up.";
-        return false;
-      }
-      LOG(ERROR) << "write() failed: " << strerror(errno);
-      return false;
-    }
-    bytes_written += rv;
-  } while (bytes_written < buf_len);
-
-  int rv = shutdown(socket, SHUT_WR);
-  if (rv < 0) {
-    LOG(ERROR) << "shutdown() failed: " << strerror(errno);
-  } else {
-    // TODO(tc): We should wait for an ACK, and if we don't get it in a certain
-    // time period, kill the other process.
+  if (!WriteToSocket(socket, to_send.data(), to_send.length())) {
+    // Try to kill the other process, because it might have been dead.
+    KillProcessBySocketPath(socket_path_.value());
+    return false;
   }
 
-  rv = HANDLE_EINTR(close(socket));
-  DCHECK_EQ(0, rv) << strerror(errno);
+  if (shutdown(socket, SHUT_WR) < 0)
+    LOG(ERROR) << "shutdown() failed: " << strerror(errno);
 
-  // Assume the other process is handling the request.
+  // Read ACK message from the other process. It might be blocked for a certain
+  // timeout, to make sure the other process has enough time to return ACK.
+  char buf[kMaxACKMessageLength + 1];
+  ssize_t len =
+      ReadFromSocket(socket, buf, kMaxACKMessageLength, kTimeoutInSeconds);
+
+  // Failed to read ACK, the other process might have been frozen.
+  if (len <= 0) {
+    KillProcessBySocketPath(socket_path_.value());
+    return false;
+  }
+
+  buf[len] = '\0';
+  if (strncmp(buf, kShutdownToken, arraysize(kShutdownToken) - 1) == 0) {
+    // The other process is shutting down, it's safe to start a new process.
+    return false;
+  } else if (strncmp(buf, kACKToken, arraysize(kACKToken) - 1) == 0) {
+    // Assume the other process is handling the request.
+    return true;
+  }
+
+  NOTREACHED() << "The other process returned unknown message: " << buf;
   return true;
 }
 
 void ProcessSingleton::Create() {
   int sock;
   sockaddr_un addr;
-  SetupSocket(&sock, &addr);
 
-  if (unlink(socket_path_.value().c_str()) < 0)
-    DCHECK_EQ(errno, ENOENT);
+  // Append the process id to the socket path, so that other process can find it
+  // out.
+  std::string path = StringPrintf(
+      "%s-%u", socket_path_.value().c_str(), base::GetCurrentProcId());
+  SetupSocket(path, &sock, &addr);
+
+  UnlinkSocketPath(socket_path_.value());
+
+  // Create symbol link before binding the socket, so that the socket file can
+  // always be reached and removed by another process.
+  // The symbol link only contains the filename part of the socket file, so that
+  // the whole config directory can be moved without breaking the symbol link.
+  std::string symlink_content = FilePath(path).BaseName().value();
+  if (symlink(symlink_content.c_str(), socket_path_.value().c_str()) < 0)
+    NOTREACHED() << "Failed to create symbol link: " << strerror(errno);
 
   if (bind(sock, reinterpret_cast<sockaddr*>(&addr), sizeof(addr)) < 0) {
     LOG(ERROR) << "bind() failed: " << strerror(errno);
     LOG(ERROR) << "SingletonSocket failed to create a socket in your home "
                   "directory. This means that running multiple instances of "
                   "the Chrome binary will start multiple browser process "
                   "rather than opening a new window in the existing process.";
-    close(sock);
+    CloseSocket(sock);
     return;
   }
 
   if (listen(sock, 5) < 0)
     NOTREACHED() << "listen failed: " << strerror(errno);
 
   // Normally we would use ChromeThread, but the IO thread hasn't started yet.
   // Using g_browser_process, we start the thread so we can listen on the
   // socket.
   MessageLoop* ml = g_browser_process->io_thread()->message_loop();
   DCHECK(ml);
   ml->PostTask(FROM_HERE, NewRunnableMethod(
     watcher_.get(),
     &ProcessSingleton::LinuxWatcher::StartListening,
     sock));
 }
-
-void ProcessSingleton::SetupSocket(int* sock, struct sockaddr_un* addr) {
-  *sock = socket(PF_UNIX, SOCK_STREAM, 0);
-  if (*sock < 0)
-    LOG(FATAL) << "socket() failed: " << strerror(errno);
-
-  int rv = SetNonBlocking(*sock);
-  DCHECK_EQ(0, rv) << "Failed to make non-blocking socket.";
-  rv = SetCloseOnExec(*sock);
-  DCHECK_EQ(0, rv) << "Failed to set CLOEXEC on socket.";
-
-  addr->sun_family = AF_UNIX;
-  if (socket_path_.value().length() > sizeof(addr->sun_path) - 1)
-    LOG(FATAL) << "Socket path too long: " << socket_path_.value();
-  base::strlcpy(addr->sun_path, socket_path_.value().c_str(),
-                sizeof(addr->sun_path));
-}