Use sockets with unread data if they've never been used before.

net/socket/client_socket_pool_base.h

 // Copyright (c) 2012 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.
 //
 // A ClientSocketPoolBase is used to restrict the number of sockets open at
 // a time.  It also maintains a list of idle persistent sockets for reuse.
 // Subclasses of ClientSocketPool should compose ClientSocketPoolBase to handle
 // the core logic of (1) restricting the number of active (connected or
 // connecting) sockets per "group" (generally speaking, the hostname), (2)
 // maintaining a per-group list of idle, persistent sockets for reuse, and (3)
@@ skipping 321 matching lines @@
   // NetworkChangeNotifier::IPAddressObserver methods:
   virtual void OnIPAddressChanged() OVERRIDE;
 
  private:
   friend class base::RefCounted<ClientSocketPoolBaseHelper>;
 
   // Entry for a persistent socket which became idle at time |start_time|.
   struct IdleSocket {
     IdleSocket() : socket(NULL) {}
 
+    // An idle socket can't be used if it is disconnected or has been used
+    // before and has received data unexpectedly (hence no longer idle).  The
+    // unread data would be mistaken for the beginning of the next response if
+    // we were to use the socket for a new request.
+    //
+    // Note that a socket that has never been used before (like a preconnected
+    // socket) may be used even with unread data.  This may be, e.g., a SPDY
+    // SETTINGS frame.
+    bool IsUsable() const;
+
     // An idle socket should be removed if it can't be reused, or has been idle
     // for too long. |now| is the current time value (TimeTicks::Now()).
     // |timeout| is the length of time to wait before timing out an idle socket.
-    //
-    // An idle socket can't be reused if it is disconnected or has received
-    // data unexpectedly (hence no longer idle).  The unread data would be
-    // mistaken for the beginning of the next response if we were to reuse the
-    // socket for a new request.
     bool ShouldCleanup(base::TimeTicks now, base::TimeDelta timeout) const;
 
     StreamSocket* socket;
     base::TimeTicks start_time;
   };
 
   typedef PriorityQueue<const Request*> RequestQueue;
   typedef std::map<const ClientSocketHandle*, const Request*> RequestMap;
 
   // A Group is allocated per group_name when there are idle sockets or pending
@@ skipping 507 matching lines @@
   // Histograms for the pool
   ClientSocketPoolHistograms* const histograms_;
   internal::ClientSocketPoolBaseHelper helper_;
 
   DISALLOW_COPY_AND_ASSIGN(ClientSocketPoolBase);
 };
 
 }  // namespace net
 
 #endif  // NET_SOCKET_CLIENT_SOCKET_POOL_BASE_H_