sch connection: prevent double-free on bus members

StoreImplicitConnection accepted a raw SCH_CONNECTION* and
~CONNECTION_SUBGRAPH always deleted everything in
m_bus_element_connections. A double‑free could happen if that same
pointer was also deleted somewhere else. Make the ownership semantics
more clear. Hopefully this fixes / prevents future issues.

Fixes: https://gitlab.com/kicad/code/kicad/-/issues/17413

eeschema/connection_graph.cpp

@@ -1746,17 +1746,18 @@ void CONNECTION_GRAPH::generateBusAliasMembers()
                 // This connection cannot form a part of the item because the item is not, itself
                 // connected to this subgraph.  It exists as part of a virtual item that may be
                 // connected to other items but is not in the schematic.
-                SCH_CONNECTION* new_conn = new SCH_CONNECTION( item, subgraph->m_sheet );
+                auto new_conn = std::make_unique<SCH_CONNECTION>( item, subgraph->m_sheet );
                 new_conn->SetGraph( this );
                 new_conn->SetName( name );
                 new_conn->SetType( CONNECTION_TYPE::NET );
-                subgraph->StoreImplicitConnection( new_conn );
-                int code = assignNewNetCode( *new_conn );
+
+                SCH_CONNECTION* new_conn_ptr = subgraph->StoreImplicitConnection( std::move( new_conn ) );
+                int             code = assignNewNetCode( *new_conn_ptr );
 
                 wxLogTrace( ConnTrace, wxS( "SG(%ld), Adding full local name (%s) with sg (%d) on subsheet %s" ),
                             subgraph->m_code, name, code, subgraph->m_sheet.PathHumanReadable() );
 
-                new_sg->m_driver_connection = new_conn;
+                new_sg->m_driver_connection = new_conn_ptr;
                 new_sg->m_code = m_last_subgraph_code++;
                 new_sg->m_sheet = subgraph->GetSheet();
                 new_sg->m_is_bus_member = true;

eeschema/connection_graph.h

@@ -22,6 +22,7 @@
 #ifndef _CONNECTION_GRAPH_H
 #define _CONNECTION_GRAPH_H
 
+#include <memory>
 #include <mutex>
 #include <utility>
 #include <vector>
@@ -93,11 +94,6 @@ public:
               m_driver_connection( nullptr )
     {}
 
-    ~CONNECTION_SUBGRAPH()
-    {
-        for( SCH_CONNECTION* connection : m_bus_element_connections )
-            delete connection;
-    }
 
     friend class CONNECTION_GRAPH;
 
@@ -219,9 +215,13 @@ public:
 
     // Use this to keep a connection pointer that is not owned by any item
     // This will be destroyed with the subgraph
-    void StoreImplicitConnection( SCH_CONNECTION* aConnection )
+    SCH_CONNECTION* StoreImplicitConnection( std::unique_ptr<SCH_CONNECTION> aConnection )
     {
-        m_bus_element_connections.insert( aConnection );
+        SCH_CONNECTION* raw = aConnection.get();
+
+        m_bus_element_connections.insert( std::move( aConnection ) );
+
+        return raw;
     }
 
 private:
@@ -317,9 +317,19 @@ private:
     /// Cache for driver connection.
     SCH_CONNECTION* m_driver_connection;
 
+    // A comparator for unique_ptr<SCH_CONNECTION> to allow storage in a set
+    struct CompareConnectionPtr
+    {
+        bool operator()( const std::unique_ptr<SCH_CONNECTION>& aLeft,
+                         const std::unique_ptr<SCH_CONNECTION>& aRight ) const
+        {
+            return aLeft.get() < aRight.get();
+        }
+    };
+
     /// A cache of connections that are part of this subgraph but that don't have
     /// an owning element (i.e. bus members)
-    std::set<SCH_CONNECTION*> m_bus_element_connections;
+    std::set<std::unique_ptr<SCH_CONNECTION>, CompareConnectionPtr> m_bus_element_connections;
 
     std::mutex m_driver_mutex;
 };