QAbstractSocket: Add socketOption for the Path MTU

This allow retrieving the value of the known PMTU for the current
socket. This works on Linux (IPv6 and IPv4) and FreeBSD (IPv6 only) --
the other OSes don't have the necessary API.

Note: do we need add IP_MTU_DISCOVER?

Change-Id: I6e9274c1e7444ad48c81fffd14dcaf97a18ce335
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 13 insertions, 0 deletions

diff --git a/src/network/socket/qabstractsocket.cpp b/src/network/socket/qabstractsocket.cpp
index 6d47540b75..98baa0c047 100644
--- a/src/network/socket/qabstractsocket.cpp
+++ b/src/network/socket/qabstractsocket.cpp
@@ -387,6 +387,11 @@
     (see \l{QAbstractSocket::}{setReadBufferSize()}).
     This enum value has been introduced in Qt 5.3.
 
+    \value PathMtuSocketOption Retrieves the Path Maximum Transmission Unit
+    (PMTU) value currently known by the IP stack, if any. Some IP stacks also
+    allow setting the MTU for transmission.
+    This enum value was introduced in Qt 5.11.
+
     Possible values for \e{TypeOfServiceOption} are:
 
     \table
@@ -2023,6 +2028,10 @@ void QAbstractSocket::setSocketOption(QAbstractSocket::SocketOption option, cons
         case ReceiveBufferSizeSocketOption:
             d_func()->socketEngine->setOption(QAbstractSocketEngine::ReceiveBufferSocketOption, value.toInt());
             break;
+
+        case PathMtuSocketOption:
+            d_func()->socketEngine->setOption(QAbstractSocketEngine::PathMtuInformation, value.toInt());
+            break;
     }
 }
 
@@ -2065,6 +2074,10 @@ QVariant QAbstractSocket::socketOption(QAbstractSocket::SocketOption option)
         case ReceiveBufferSizeSocketOption:
                 ret = d_func()->socketEngine->option(QAbstractSocketEngine::ReceiveBufferSocketOption);
                 break;
+
+        case PathMtuSocketOption:
+                ret = d_func()->socketEngine->option(QAbstractSocketEngine::PathMtuInformation);
+                break;
     }
     if (ret == -1)
         return QVariant();