Add a SetDoNotFragment() method to DatagramSocket, and call this for

net/udp/udp_socket_posix.cc

Index: net/udp/udp_socket_posix.cc
diff --git a/net/udp/udp_socket_posix.cc b/net/udp/udp_socket_posix.cc
index 797e1a0bc1352b5a4a9a43c8c663c7acb05ab374..342c94cd6612a31d762829d6f52d679be70182e0 100644
--- a/net/udp/udp_socket_posix.cc
+++ b/net/udp/udp_socket_posix.cc
@@ -425,6 +425,36 @@ int UDPSocketPosix::SetSendBufferSize(int32_t size) {
   return rv == 0 ? OK : MapSystemError(errno);
 }
 
+int UDPSocketPosix::SetDoNotFragment() {
+  DCHECK_NE(socket_, kInvalidSocket);
+  DCHECK(CalledOnValidThread());
+
+#if !defined(IP_PMTUDISC_DO)
+  return ERR_NOT_IMPLEMENTED;
+#else
+  int v6_only = false;
+  if (addr_family_ == AF_INET6) {
+    socklen_t v6_only_len = sizeof(v6_only);
+    int rv =
+        getsockopt(socket_, IPPROTO_IPV6, IPV6_V6ONLY, &v6_only, &v6_only_len);
+    if (rv != 0)
+      return MapSystemError(errno);
+
+    int val = IPV6_PMTUDISC_DO;
+    rv =
+        setsockopt(socket_, IPPROTO_IPV6, IPV6_MTU_DISCOVER, &val, sizeof(val));
+    if (rv != 0)
+      return MapSystemError(errno);
+    if (v6_only)
+      return rv == 0 ? OK : MapSystemError(errno);

[mmenke, 2016/08/12 21:55:49]

rv is always 0 here (See if statement on above line).  My suggestion is to move
the getsockopt just above the v6_only check, and then do:

if (rv != 0 || v6_only)
  return rv == 0 ? OK : MapSystemError(errno);

Note that you could just merge the two if's you have down here as things are,
but then you'd combining the results of the two platform API calls.  If you
moave the getsocketopt down here, this is would just be handling the result of
one call.  Or could just use two if lines.  Either way, think the getsocketopt
makes more sense second, since it doesn't affect the setsockopt call.

[Ryan Hamilton, 2016/08/12 22:15:26]

Done. 

I realize that we don't actually need rv here since we use errno not rv to
determine the return value so I actually simplified the code ever so slightly. 

+  }
+
+  int val = IP_PMTUDISC_DO;
+  int rv = setsockopt(socket_, IPPROTO_IP, IP_MTU_DISCOVER, &val, sizeof(val));
+  return rv == 0 ? OK : MapSystemError(errno);
+#endif
+}
+
 int UDPSocketPosix::AllowAddressReuse() {
   DCHECK_NE(socket_, kInvalidSocket);
   DCHECK(CalledOnValidThread());