fix: announce LDP on listening interface (#5875)

* fix: make sure LDP announces are sent on the listening interface

* fix: add implementation for test mediator

* fix: set SO_REUSEADDR for send socket

libtransmission/session.h

@@ -257,6 +257,11 @@ private:
         {
         }
 
+        [[nodiscard]] tr_address bind_address(tr_address_type type) const override
+        {
+            return session_.bind_address(type);
+        }
+
         [[nodiscard]] tr_port port() const override
         {
             return session_.advertisedPeerPort();

libtransmission/tr-lpd.cc

@@ -100,9 +100,7 @@ auto makeAnnounceMsg(std::string_view cookie, tr_port port, std::vector<std::str
         ret += fmt::format("cookie: {:s}\r\n", cookie);
     }
 
-    ret += "\r\n\r\n";
-
-    return ret;
+    return ret + "\r\n\r\n";
 }
 
 struct ParsedAnnounce
@@ -363,6 +361,22 @@ private:
                 return false;
             }
 
+            if (setsockopt(
+                    mcast_snd_socket_,
+                    SOL_SOCKET,
+                    SO_REUSEADDR,
+                    reinterpret_cast<char const*>(&opt_on),
+                    sizeof(opt_on)) == -1)
+            {
+                return false;
+            }
+
+            if (auto [ss, sslen] = mediator_.bind_address(TR_AF_INET).to_sockaddr({});
+                bind(mcast_snd_socket_, reinterpret_cast<sockaddr*>(&ss), sslen) == -1)
+            {
+                return false;
+            }
+
             /* configure outbound multicast TTL */
             if (setsockopt(
                     mcast_snd_socket_,

libtransmission/tr-lpd.h

@@ -41,6 +41,8 @@ public:
 
         virtual ~Mediator() = default;
 
+        [[nodiscard]] virtual tr_address bind_address(tr_address_type type) const = 0;
+
         [[nodiscard]] virtual tr_port port() const = 0;
 
         [[nodiscard]] virtual bool allowsLPD() const = 0;

tests/libtransmission/lpd-test.cc

@@ -40,6 +40,11 @@ public:
     {
     }
 
+    [[nodiscard]] tr_address bind_address(tr_address_type /* type */) const override
+    {
+        return {};
+    }
+
     [[nodiscard]] tr_port port() const override
     {
         return port_;