Add Socket::ReadIfReady()

net/socket/socket.h

Index: net/socket/socket.h
diff --git a/net/socket/socket.h b/net/socket/socket.h
index cda54063f3059e76726c896b5e6ae87508a5f483..306c40d62726d5860604676e6c05fa1b1876e7c6 100644
--- a/net/socket/socket.h
+++ b/net/socket/socket.h
@@ -7,6 +7,7 @@
 
 #include <stdint.h>
 
+#include "base/feature_list.h"
 #include "net/base/completion_callback.h"
 #include "net/base/net_export.h"
 
@@ -17,6 +18,9 @@ class IOBuffer;
 // Represents a read/write socket.
 class NET_EXPORT Socket {
  public:
+  // Name of the field trial for using ReadyIfReady() instead of Read().
+  static const base::Feature kReadIfReadyExperiment;
+
   virtual ~Socket() {}
 
   // Reads data, up to |buf_len| bytes, from the socket.  The number of bytes
@@ -33,6 +37,19 @@ class NET_EXPORT Socket {
   virtual int Read(IOBuffer* buf, int buf_len,
                    const CompletionCallback& callback) = 0;
 
+  // Reads as much data as possible into |buf| without blocking. Default
+  // implementation returns ERR_READ_IF_READY_NOT_IMPLEMENTED. Caller should
+  // fall back to Read() if receives ERR_READ_IF_READY_NOT_IMPLEMENTED.
+  // Upon synchronous completion, returns the number of bytes read or an error
+  // code if an error happens; If read cannot be completed synchronously,
+  // returns ERR_IO_PENDING and does not hold on to |buf|. |callback| will be
+  // invoked with OK when data can be read, at which point, caller can call
+  // ReadIfReady() again; if an error occurs asynchronously, |callback| will be
+  // invoked with the error code.

[davidben, 2017/02/10 23:33:48]

Hrm. One thing I noticed that's actually quite subtle about this API:

If ReadIfReady *synchronously* returns 0, that means EOF.

If it *asynchronously* returns 0, that means "please call me again".

[xunjieli, 2017/02/13 20:28:18]

Acknowledged. Yes, that's right. I have edited the doc. Hopefully it's clearer?

+  virtual int ReadIfReady(IOBuffer* buf,
+                          int buf_len,
+                          const CompletionCallback& callback);
+
   // Writes data, up to |buf_len| bytes, to the socket.  Note: data may be
   // written partially.  The number of bytes written is returned, or an error
   // is returned upon failure.  ERR_SOCKET_NOT_CONNECTED should be returned if