Plugin API: expose the party leader

Code/client/PluginAPI.cpp

@@ -17,3 +17,9 @@ extern "C" __declspec(dllexport) SkyrimTogetherAPI::ChannelHandle SkyrimTogether
 {
     return PluginService::Get().RegisterScopedChannel(apName, aScope, aOnMessage, aOnPeersChanged);
 }
+
+// Also outside the V1 vtable, for the same reason.
+extern "C" __declspec(dllexport) uint32_t SkyrimTogetherGetPartyLeaderId()
+{
+    return PluginService::Get().GetPartyLeaderId();
+}

Code/client/Services/Generic/PluginService.cpp

@@ -68,6 +68,12 @@ bool PluginService::IsInParty() const noexcept
     return m_inParty;
 }
 
+uint32_t PluginService::GetPartyLeaderId() const noexcept
+{
+    std::lock_guard cLock(m_lock);
+    return m_partyLeaderId;
+}
+
 SendResult PluginService::Queue(ChannelHandle aChannel, uint32_t aTargetPlayerId, const char* apBytes, size_t aLength) noexcept
 {
     if (aLength > kMaxMessageSize || (aLength != 0 && !apBytes))
@@ -138,6 +144,7 @@ void PluginService::Update() noexcept
     const uint32_t cLocalId = cOnline ? world.GetTransport().GetLocalPlayerId() : 0;
     const auto& party = world.GetPartyService();
     const bool cInParty = cOnline && party.IsInParty();
+    const uint32_t cLeaderId = cInParty ? party.GetLeaderPlayerId() : 0;
     Vector<uint32_t> members, nearby;
     if (cInParty)
         for (uint32_t id : party.GetPartyMembers())
@@ -162,6 +169,7 @@ void PluginService::Update() noexcept
         std::lock_guard cLock(m_lock);
         m_localPlayerId = cLocalId;
         m_inParty = cInParty;
+        m_partyLeaderId = cLeaderId;
         m_members = members;
         m_nearby = nearby;
         for (Channel& channel : m_channels)

Code/client/Services/PluginService.h

@@ -26,6 +26,8 @@ struct PluginService final : SkyrimTogetherAPI::IVSkyrimTogether1
     SkyrimTogetherAPI::SendResult SendTo(SkyrimTogetherAPI::ChannelHandle aChannel, uint32_t aPlayerId, const char* apBytes, size_t aLength) noexcept override;
     uint32_t GetLocalPlayerId() const noexcept override;
     bool IsInParty() const noexcept override;
+    // Not part of the V1 vtable, exported as SkyrimTogetherGetPartyLeaderId. 0 while not in a party.
+    [[nodiscard]] uint32_t GetPartyLeaderId() const noexcept;
 
     /**
      * @brief Hands a relayed Mod Message to the plugin on its channel, if any. Runs during the World update.
@@ -55,4 +57,5 @@ struct PluginService final : SkyrimTogetherAPI::IVSkyrimTogether1
     Vector<uint32_t> m_nearby;
     uint32_t m_localPlayerId{0};
     bool m_inParty{false};
+    uint32_t m_partyLeaderId{0};
 };

Code/plugin_api/README.md

@@ -36,6 +36,23 @@ send means the target is not in the current nearby snapshot (the existing result
 The server independently checks every recipient: party equality, or matching interior/worldspace
 and loaded-grid range. Both client and server must run the new protocol build.
 
+## Party leader
+
+The party leader hosts the world: NPCs, scenes and quest progression run on the leader's client. Plugins
+that move NPCs or start scenes, or that need one authoritative client, can look up
+`SkyrimTogetherGetPartyLeaderId` using `GetPartyLeaderId_t` from the header:
+
+```cpp
+auto getLeader = reinterpret_cast<SkyrimTogetherAPI::GetPartyLeaderId_t>(
+    GetProcAddress(GetModuleHandleW(nullptr), SkyrimTogetherAPI::kGetPartyLeaderIdName));
+const uint32_t leader = getLeader ? getLeader() : 0;
+const bool isLeader = leader != 0 && leader == api->GetLocalPlayerId();
+```
+
+It returns 0 while not in a party, and is absent on older executables, so plugins need a fallback (for
+example: act alone when not in a party). It reads the same locked snapshot as the other getters, so a
+leadership change shows up after the next World update.
+
 ## Lifetime and resynchronization
 
 The registration callback reports audience joins/leaves. Nearby membership follows loaded remote player

Code/plugin_api/include/SkyrimTogetherAPI.h

@@ -83,4 +83,10 @@ using RequestPluginAPI_t = void* (*)(InterfaceVersion aVersion);
 // Callbacks run in the World update context; queue engine work through SKSE AddTask.
 using RegisterScopedChannel_t = ChannelHandle (*)(const char*, ChannelScope, MessageCallback, PartyChangedCallback);
 constexpr const char* kRegisterScopedChannelName = "SkyrimTogetherRegisterScopedChannel";
+// Optional extension, independent of the V1 vtable. Older executables return no symbol.
+// The player ID of the party leader, who hosts the world's NPCs and quest scenes; 0 while not in a party.
+// Compare with GetLocalPlayerId() to know whether this client leads. Callable from any thread (locked snapshot,
+// refreshed every World update), so it can briefly lag a leadership change.
+using GetPartyLeaderId_t = uint32_t (*)();
+constexpr const char* kGetPartyLeaderIdName = "SkyrimTogetherGetPartyLeaderId";
 } // namespace SkyrimTogetherAPI