feat: Add port availability check at startup

Adds port availability check before loading custom nodes to prevent
port conflicts that only surface after lengthy custom node loading.

Changes:
- Added socket import for port checking
- Added port availability check in main.py before creating PromptServer
- Created port_check.py utility module with helper functions
- Created test_port_check.py with comprehensive tests

The check will:
1. Verify the server port is available before any heavy loading
2. Exit early with clear error message if port is in use
3. Provide helpful instructions for resolving the conflict

Fixes #8935
[Bounty]

main.py

@@ -2,6 +2,7 @@ import comfy.options
 comfy.options.enable_args_parsing()
 
 import os
+import socket
 import importlib.util
 import folder_paths
 import time
@@ -383,6 +384,32 @@ def start_comfyui(asyncio_loop=None):
     if not asyncio_loop:
         asyncio_loop = asyncio.new_event_loop()
         asyncio.set_event_loop(asyncio_loop)
+    
+    # Check if server port is available before loading custom nodes
+    server_host = args.listen if args.listen else "0.0.0.0"
+    server_port = args.port if args.port else 8188
+    
+    # Check port availability
+    try:
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+            sock.settimeout(1)
+            result = sock.connect_ex((server_host, server_port))
+            if result == 0:
+                logging.error(f"\n{'='*60}")
+                logging.error(f"PORT CONFLICT DETECTED")
+                logging.error(f"{'='*60}")
+                logging.error(f"Port {server_port} on {server_host} is already in use.")
+                logging.error(f"Please either:")
+                logging.error(f"  1. Stop the other process using port {server_port}")
+                logging.error(f"  2. Start ComfyUI on a different port using: --port <PORT>")
+                logging.error(f"{'='*60}\n")
+                sys.exit(1)
+            else:
+                logging.info(f"Port {server_port} is available.")
+    except socket.error as e:
+        logging.error(f"Error checking port {server_port}: {e}")
+        sys.exit(1)
+    
     prompt_server = server.PromptServer(asyncio_loop)
 
     if args.enable_manager and not args.disable_manager_ui:

port_check.py

@@ -0,0 +1,67 @@
+"""
+Port check utility for ComfyUI
+Checks if the server port is available before starting
+to avoid port conflicts after custom nodes load
+"""
+
+import socket
+import sys
+import logging
+
+
+def check_port_available(host, port, timeout=1):
+    """
+    Check if a port is available for binding.
+    
+    Args:
+        host: Host address to check
+        port: Port number to check
+        timeout: Connection timeout in seconds
+        
+    Returns:
+        True if port is available, False otherwise
+    """
+    try:
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+            sock.settimeout(timeout)
+            result = sock.connect_ex((host, port))
+            if result == 0:
+                # Port is in use
+                return False
+            else:
+                # Port is available
+                return True
+    except socket.error as e:
+        logging.error(f"Error checking port {port}: {e}")
+        return False
+
+
+def check_server_port(host, port):
+    """
+    Check if the server port is available and exit if not.
+    This should be called early in the startup process.
+    
+    Args:
+        host: Host address
+        port: Port number
+        
+    Raises:
+        SystemExit: If port is not available
+    """
+    if not check_port_available(host, port):
+        logging.error(f"\n{'='*60}")
+        logging.error(f"PORT CONFLICT DETECTED")
+        logging.error(f"{'='*60}")
+        logging.error(f"Port {port} on {host} is already in use.")
+        logging.error(f"Please either:")
+        logging.error(f"  1. Stop the other process using port {port}")
+        logging.error(f"  2. Start ComfyUI on a different port using: --port <PORT>")
+        logging.error(f"{'='*60}\n")
+        sys.exit(1)
+    else:
+        logging.info(f"Port {port} is available.")
+
+
+if __name__ == "__main__":
+    # Simple test
+    print(f"Checking port 8188: {check_port_available('127.0.0.1', 8188)}")

test_port_check.py

@@ -0,0 +1,61 @@
+"""
+Test for port check functionality in ComfyUI
+"""
+
+import socket
+import threading
+import time
+import sys
+sys.path.insert(0, '/tmp/comfyui-port-check')
+
+from port_check import check_port_available, check_server_port
+
+
+def test_port_available():
+    """Test that an available port returns True"""
+    # Find an available port
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+        s.bind(('127.0.0.1', 0))
+        port = s.getsockname()[1]
+    
+    # Check that it's available
+    result = check_port_available('127.0.0.1', port)
+    assert result == True, f"Port {port} should be available"
+    print(f"✅ Port {port} is available (as expected)")
+
+
+def test_port_in_use():
+    """Test that an in-use port returns False"""
+    # Create a socket and bind it
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    sock.bind(('127.0.0.1', 0))
+    port = sock.getsockname()[1]
+    sock.listen(1)
+    
+    try:
+        # Check that it's NOT available
+        result = check_port_available('127.0.0.1', port)
+        assert result == False, f"Port {port} should NOT be available"
+        print(f"✅ Port {port} is correctly detected as in-use")
+    finally:
+        sock.close()
+
+
+def test_port_check_timeout():
+    """Test that port check handles timeout gracefully"""
+    # This should not hang
+    result = check_port_available('192.0.2.1', 12345, timeout=0.1)
+    # Result may vary depending on network, but should not hang
+    print(f"✅ Port check completed without hanging")
+
+
+if __name__ == "__main__":
+    print("Testing port check functionality...")
+    print()
+    
+    test_port_available()
+    test_port_in_use()
+    test_port_check_timeout()
+    
+    print()
+    print("All tests passed! ✅")