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..112f119d51888cafc8977ceff05faf834227bd9f 100644
--- a/net/udp/udp_socket_posix.cc
+++ b/net/udp/udp_socket_posix.cc
@@ -425,6 +425,35 @@ 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 OK;

[mmenke, 2016/08/12 19:46:05]

It's considered a success if we can't set this?  If setting do not fragment
fails, aren't we in basically the same situations as if we can't set it at all?

[Ryan Hamilton, 2016/08/12 21:46:02]

Good point. Changed it to return NOT_IMPLEMENTED.

+#else
+  int v6_only = false;
+  if (addr_family_ == AF_INET6) {
+    socklen_t v6_only_len = sizeof(v6_only);
+    getsockopt(socket_, IPPROTO_IPV6, IPV6_V6ONLY, &v6_only, &v6_only_len);

[mmenke, 2016/08/12 19:46:05]

What if getsockopt fails?  Do we care?

[Ryan Hamilton, 2016/08/12 21:46:02]

Good point. I don't think we *really* care, but might as well be more pedantic.

+  }
+
+  int rv;
+  if (addr_family_ == AF_INET6) {

[mmenke, 2016/08/12 19:46:05]

Should this be merged with the above if block?  Could also then early return if
v6_only (And maybe only check it after setting IPPROTO_IPV6.  Think that may be
a little clearer.  It also minimizes variable scope, which is recommended by
google style guide.  And saves a system call when the setsocket fails.  Which I
guess is good?

[Ryan Hamilton, 2016/08/12 21:46:02]

Ah, yes. This code moved around so I missed that.
WFM. It ends up duplicating the:
  return rv == 0 ? OK : MapSystemError(errno);
but I'm always a fan of early return.

Done.

+    int val = IPV6_PMTUDISC_DO;

[mmenke, 2016/08/12 19:46:05]

IP_PMTUDISC_DO being defined guarantees IPV6_PMTUDISC_DO is?

[Ryan Hamilton, 2016/08/12 21:46:02]

As far as I can tell. It's true for all our trybots at least.

+    rv =
+        setsockopt(socket_, IPPROTO_IPV6, IPV6_MTU_DISCOVER, &val, sizeof(val));
+    if (rv != 0)
+      return MapSystemError(errno);
+  }
+  if (!v6_only) {
+    int val = IP_PMTUDISC_DO;
+    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());