Improve cdc.py error handling

When the CDC service reponds with an error, the program prints the error
and exits.

server/modules/protocol/examples/cdc.py

@@ -20,7 +20,10 @@ import selectors
 import binascii
 import os
 
+schema_read = False
+
 def read_data():
+    global schema_read
     sel = selectors.DefaultSelector()
     sel.register(sock, selectors.EVENT_READ)
 
@@ -29,8 +32,17 @@ def read_data():
             events = sel.select(timeout=int(opts.read_timeout) if int(opts.read_timeout) > 0 else None)
             buf = sock.recv(4096, socket.MSG_DONTWAIT)
             if len(buf) > 0:
+                # If the request for data is rejected, an error will be sent instead of the table schema
+                if not schema_read:
+                    if "err" in buf.decode().lower():
+                        print(buf.decode(), file=sys.stderr)
+                        exit(1)
+                    else:
+                        schema_read = True
+
                 os.write(sys.stdout.fileno(), buf)
                 sys.stdout.flush()
+
             else:
                 raise Exception('Socket was closed')
 
@@ -40,6 +52,13 @@ def read_data():
             print(ex, file=sys.stderr)
             break
 
+
+def check_for_err(err):
+    if "err" in err.lower().strip():
+        print(err.strip(), file=sys.stderr)
+        exit(1)
+
+
 parser = argparse.ArgumentParser(description = "CDC Binary consumer", conflict_handler="resolve")
 parser.add_argument("-h", "--host", dest="host", help="Network address where the connection is made", default="localhost")
 parser.add_argument("-P", "--port", dest="port", help="Port where the connection is made", default="4001")
@@ -60,13 +79,17 @@ auth_string += bytes(hashlib.sha1(opts.password.encode("utf_8")).hexdigest().enc
 sock.send(auth_string)
 
 # Discard the response
-response = str(sock.recv(1024)).encode('utf_8')
+response = sock.recv(1024).decode()
+
+check_for_err(response)
 
 # Register as a client as request Avro format data
 sock.send(bytes(("REGISTER UUID=XXX-YYY_YYY, TYPE=" + opts.format).encode()))
 
 # Discard the response again
-response = str(sock.recv(1024)).encode('utf_8')
+response = sock.recv(1024).decode()
+
+check_for_err(response)
 
 # Request a data stream
 sock.send(bytes(("REQUEST-DATA " + opts.FILE + (" " + opts.GTID if opts.GTID else "")).encode()))