Fix get_connected to detect lost network connections

Add check_connection() to ISOTCPConnection that performs a non-blocking
socket peek to detect broken TCP connections. Update Client.get_connected()
to use this active check instead of just returning a cached flag.

Previously, get_connected() would return True even after the network
connection was lost, leading to crashes on subsequent operations.

Fixes #111

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

Author: gijzelaerr

snap7/client.py

@@ -184,8 +184,14 @@ def destroy(self) -> None:
         self.disconnect()
 
     def get_connected(self) -> bool:
-        """Check if client is connected to PLC."""
-        return self.connected and self.connection is not None and self.connection.connected
+        """Check if client is connected to PLC.
+
+        Performs an active check on the underlying TCP socket to detect
+        broken connections, rather than just checking a cached flag.
+        """
+        if not self.connected or self.connection is None:
+            return False
+        return self.connection.check_connection()
 
     def db_read(self, db_number: int, start: int, size: int) -> bytearray:
         """

snap7/connection.py

@@ -382,6 +382,34 @@ def _recv_exact(self, size: int) -> bytes:
 
         return bytes(data)
 
+    def check_connection(self) -> bool:
+        """Check if the TCP connection is still alive.
+
+        Uses a non-blocking socket peek to detect broken connections.
+        """
+        if not self.connected or self.socket is None:
+            return False
+
+        try:
+            original_timeout = self.socket.gettimeout()
+            self.socket.settimeout(0)
+            try:
+                data = self.socket.recv(1, socket.MSG_PEEK)
+                if not data:
+                    self.connected = False
+                    return False
+                return True
+            except BlockingIOError:
+                # No data available but connection is still alive
+                return True
+            except (socket.error, OSError):
+                self.connected = False
+                return False
+            finally:
+                self.socket.settimeout(original_timeout)
+        except Exception:
+            return False
+
     def __enter__(self) -> "ISOTCPConnection":
         """Context manager entry."""
         return self