Qt: Ask user to use standard port on startup if specified port is in use

If the port (specified by -port or GUI setting) is already in use when
starting the GUI client, and it's not the standard port, ask the user if
they want to listen via the standard network port instead.

Github-Pull: bitcoin#7107
Rebased-From: 1f37c87

src/net.cpp

@@ -2258,6 +2258,27 @@ bool CConnman::InitBinds(const std::vector<CService>& binds, const std::vector<C
         struct in6_addr inaddr6_any = IN6ADDR_ANY_INIT;
         fBound |= Bind(CService(inaddr6_any, GetListenPort()), BF_NONE);
         fBound |= Bind(CService(inaddr_any, GetListenPort()), !fBound ? BF_REPORT_ERROR : BF_NONE);
+
+        if (!fBound) {
+            int defaultPort = Params().GetDefaultPort();
+            // If listening failed and another port than the standard port was specified,
+            // ask if the user wants to connect via the standard port for the network instead
+            if (GetListenPort() != defaultPort) {
+                bool fRet = uiInterface.ThreadSafeQuestion(
+                    _("Do you want to use the standard network port for ") + _(PACKAGE_NAME) + " (port " + i64tostr(defaultPort) + ") instead?",
+                    _("Listen on port ") + i64tostr(GetListenPort()) + _(" failed."),
+                    "", CClientUIInterface::MSG_INFORMATION | CClientUIInterface::MODAL | CClientUIInterface::BTN_OK | CClientUIInterface::BTN_ABORT);
+
+                if (fRet) {
+                    gArgs.ForceSetArg("-port", defaultPort);
+                    // Attempt to use standard port
+                    struct in_addr inaddr_any;
+                    inaddr_any.s_addr = INADDR_ANY;
+                    fBound |= Bind(CService(in6addr_any, defaultPort), BF_NONE);
+                    fBound |= Bind(CService(inaddr_any, defaultPort), BF_NONE);
+                }
+            }
+        }
     }
     return fBound;
 }