feat: retry AsyncConnection.open() indefinitely; drop IPv6 only on early startup failure

IPv4 connections are retried forever in heartbeat_sender if open() fails,
so a temporary network outage does not terminate the sender.

IPv6 connections that have never opened successfully are dropped after
IPV6_EARLY_FAIL_LIMIT (3) consecutive failures so that a network without
IPv6 support does not keep a dead sender running.

At startup all resolved connections are added to the list regardless of
whether the initial open() succeeds; the heartbeat_sender loop handles
the first real connection attempt.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/client/main.py

@@ -56,23 +56,26 @@ class AsyncConnection:
         self.transport: Optional[asyncio.DatagramTransport] = None
         self.protocol: Optional[asyncio.DatagramProtocol] = None
         self._dead = False
+        self._ever_opened = False
+        self._open_fail_count = 0   # consecutive failures before first success
 
         self.logger = logging.getLogger(f"hbc.conn.{addr}")
-    
+
     async def open(self) -> bool:
         """Open the UDP connection.
-        
+
         Returns:
             True if successful, False otherwise
         """
         try:
             loop = asyncio.get_event_loop()
-            
+
             # Create datagram endpoint
             self.transport, self.protocol = await loop.create_datagram_endpoint(
                 lambda: HeartbeatProtocol(self),
                 family=self.af
             )
+            self._ever_opened = True
             self.logger.debug(f"Opened connection to {self.addr}:{self.port}")
             return True
         except Exception as e:
@@ -262,15 +265,51 @@ async def handle_update(conn: AsyncConnection, _msg: dict):  # pyright: ignore[r
 
 
 async def heartbeat_sender(conn: AsyncConnection, interval: int):
-    """Send periodic heartbeats.
-    
+    """Send periodic heartbeats, retrying the connection if it is not open.
+
+    IPv6 connections that fail to open before their first successful send are
+    dropped after IPV6_EARLY_FAIL_LIMIT attempts so that a network without IPv6
+    does not keep a dead sender alive.  IPv4 connections are retried indefinitely.
+
     Args:
         conn: Connection to send on
         interval: Heartbeat interval in seconds
     """
     logger = logging.getLogger("hbc.heartbeat")
-    
-    while running:
+    IPV6_EARLY_FAIL_LIMIT = 3
+
+    while running and not conn._dead:
+        # Ensure transport is open before attempting to send.
+        if not conn.transport:
+            opened = await conn.open()
+            if opened:
+                conn._open_fail_count = 0
+            else:
+                conn._open_fail_count += 1
+                # Drop an IPv6 connection that has never come up within the
+                # first few attempts — it is likely unavailable on this network.
+                if (not conn._ever_opened
+                        and conn.af == socket.AF_INET6
+                        and conn._open_fail_count >= IPV6_EARLY_FAIL_LIMIT):
+                    logger.warning(
+                        f"IPv6 connection to {conn.addr} unreachable after "
+                        f"{conn._open_fail_count} attempts, disabling"
+                    )
+                    conn._dead = True
+                    break
+                # Retry after the normal interval; IPv4 retries forever.
+                try:
+                    if shutdown_event:
+                        await asyncio.wait_for(shutdown_event.wait(), timeout=interval)
+                        break
+                    else:
+                        await asyncio.sleep(interval)
+                except asyncio.TimeoutError:
+                    pass
+                except asyncio.CancelledError:
+                    raise
+                continue
+
         try:
             msg = {
                 "acks": conn.ackcount,
@@ -278,20 +317,17 @@ async def heartbeat_sender(conn: AsyncConnection, interval: int):
                 "interval": interval
             }
             await conn.sendto(msg, "HTB")
-            
-        except Exception as e:
-            logger.error(f"Error sending heartbeat: {e}", exc_info=True)
+
         except asyncio.CancelledError:
             logger.debug("Heartbeat sender cancelled")
             raise
-        
+        except Exception as e:
+            logger.error(f"Error sending heartbeat: {e}", exc_info=True)
+
         # Wait for next interval or shutdown event
         try:
             if shutdown_event:
-                await asyncio.wait_for(
-                    shutdown_event.wait(), 
-                    timeout=interval
-                )
+                await asyncio.wait_for(shutdown_event.wait(), timeout=interval)
                 break
             else:
                 await asyncio.sleep(interval)
@@ -479,14 +515,15 @@ async def async_main(args, config):
         for addr_info in addrs:
             af = addr_info[0]
             addr = addr_info[4][0]
-            
+
             conn = AsyncConnection(conn_id, addr, hb_port, af, iam)
-            if await conn.open():
-                connections.append(conn)
-                conn_id += 1
-    
+            if not await conn.open():
+                logger.warning(f"Initial open to {addr} failed, heartbeat sender will retry")
+            connections.append(conn)
+            conn_id += 1
+
     if not connections:
-        logger.error("No connections established")
+        logger.error("No connections established (DNS resolution failed for all hosts)")
         return 1
     
     logger.info(f"Created {len(connections)} connections")