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, 30 insertions, 0 deletions

diff --git a/src/network/socket/qnativesocketengine_unix.cpp b/src/network/socket/qnativesocketengine_unix.cpp
index cb0a521360..b380b0f7d6 100644
--- a/src/network/socket/qnativesocketengine_unix.cpp
+++ b/src/network/socket/qnativesocketengine_unix.cpp
@@ -226,6 +226,20 @@ static void convertToLevelAndOption(QNativeSocketEngine::SocketOption opt,
 #endif
         }
         break;
+
+    case QNativeSocketEngine::PathMtuInformation:
+        if (socketProtocol == QAbstractSocket::IPv6Protocol || socketProtocol == QAbstractSocket::AnyIPProtocol) {
+#ifdef IPV6_MTU
+            level = IPPROTO_IPV6;
+            n = IPV6_MTU;
+#endif
+        } else {
+#ifdef IP_MTU
+            level = IPPROTO_IP;
+            n = IP_MTU;
+#endif
+        }
+        break;
     }
 }
 
@@ -331,6 +345,20 @@ int QNativeSocketEnginePrivate::option(QNativeSocketEngine::SocketOption opt) co
         return -1;
     }
 
+    case QNativeSocketEngine::PathMtuInformation:
+#if defined(IPV6_PATHMTU) && !defined(IPV6_MTU)
+        // Prefer IPV6_MTU (handled by convertToLevelAndOption), if available
+        // (Linux); fall back to IPV6_PATHMTU otherwise (FreeBSD):
+        if (socketProtocol == QAbstractSocket::IPv6Protocol) {
+            ip6_mtuinfo mtuinfo;
+            QT_SOCKOPTLEN_T len = sizeof(mtuinfo);
+            if (::getsockopt(socketDescriptor, IPPROTO_IPV6, IPV6_PATHMTU, &mtuinfo, &len) == 0)
+                return int(mtuinfo.ip6m_mtu);
+            return -1;
+        }
+#endif
+        break;
+
     default:
         break;
     }
@@ -420,6 +448,8 @@ bool QNativeSocketEnginePrivate::setOption(QNativeSocketEngine::SocketOption opt
     }
 #endif
 
+    if (n == -1)
+        return false;
     return ::setsockopt(socketDescriptor, level, n, (char *) &v, sizeof(v)) == 0;
 }