Implementing Asynchronous I/O with IOCP in Go

This challenge focuses on building a fundamental component for high-performance network applications in Go: integrating with the Windows I/O Completion Port (IOCP) mechanism. IOCP is a low-level Windows API that enables efficient asynchronous I/O operations, significantly improving scalability by allowing a small number of threads to manage a large number of concurrent I/O requests. This challenge will guide you through setting up and utilizing IOCP for basic socket operations.

Problem Description

Your task is to create a Go program that leverages the Windows I/O Completion Port (IOCP) for asynchronous socket operations. This involves setting up an IOCP, associating network sockets with it, and processing I/O completion events.

What needs to be achieved:

IOCP Creation: Create an I/O Completion Port using CreateIoCompletionPort.
Socket Association: Associate one or more network sockets (TCP listening socket and client sockets) with the created IOCP.
Asynchronous I/O Submission: For each associated socket, submit at least one asynchronous I/O operation (e.g., ReadFile or WriteFile on the socket, conceptually, but we'll simulate this).
Completion Event Processing: Create a worker thread that continuously retrieves completion packets from the IOCP using GetQueuedCompletionStatus.
Event Handling: Process the retrieved completion packets to identify the socket and the type of I/O operation that completed.

Key Requirements:

The solution must use Windows API calls directly, likely through syscall or a similar package.
A dedicated goroutine (acting as the IOCP worker thread) must be responsible for calling GetQueuedCompletionStatus.
The program should be able to demonstrate the successful submission and completion of at least one asynchronous operation for a connected client socket.
Graceful handling of socket disconnections or errors during I/O operations is expected.

Expected Behavior:

When the program runs, it should:

Create an IOCP.
Create a listening TCP socket.
Associate the listening socket with the IOCP.
Start accepting client connections in a loop.
For each accepted client connection:
- Associate the client socket with the IOCP.
- Submit an initial asynchronous read operation (even if it completes immediately with no data).
- Submit an asynchronous write operation.
The IOCP worker goroutine should pick up completion events for these operations.
The program should print messages indicating when operations are submitted and when completions are received, identifying the socket involved.

Important Edge Cases to Consider:

Zero Bytes Read: When a client disconnects gracefully, a read operation might complete with zero bytes. This should be handled as a disconnection.
Socket Errors: Errors occurring during I/O operations (e.g., connection reset) need to be detected and handled.
Linger Options: Consider how socket linger options might affect the timing of completions.
Simulating Asynchronous I/O: Since Go's standard net package abstracts away low-level OS I/O, you'll need to simulate asynchronous operations. This can be done by manually submitting overlapped I/O structures for ReadFile and WriteFile on the socket handle.

Examples

Example 1: Basic Setup and First Read Completion

Input: (No direct user input. The program starts up.)

Program Output:
[Timestamp] Creating IOCP...
[Timestamp] IOCP created. Handle: [handle_value]
[Timestamp] Creating listening socket...
[Timestamp] Listening socket created. Handle: [handle_value]
[Timestamp] Associating listening socket [handle_value] with IOCP...
[Timestamp] Listening socket associated with IOCP.
[Timestamp] Starting IOCP worker goroutine...
[Timestamp] Listening for connections on [address]...

(A client connects)

[Timestamp] Client connected. Handle: [handle_value]
[Timestamp] Associating client socket [handle_value] with IOCP...
[Timestamp] Client socket associated with IOCP.
[Timestamp] Submitting initial read for client socket [handle_value]...
[Timestamp] Read submitted. Overlapped: [overlapped_ptr]
[Timestamp] Submitting write for client socket [handle_value]...
[Timestamp] Write submitted. Overlapped: [overlapped_ptr]

(IOCP worker picks up a completion)

[Timestamp] Completion received. BytesTransferred: 0, Key: [socket_handle], Overlapped: [overlapped_ptr]
[Timestamp] Operation type: Read (based on overlapped structure)
[Timestamp] Client disconnected gracefully or no data. Handle: [socket_handle]

Explanation: The program successfully initializes IOCP, associates sockets, and submits read/write operations. When a client connects, its socket is associated, and an initial read is submitted. The IOCP worker then detects the completion of this read, which in this specific simulated case might return 0 bytes, indicating a potential disconnect or simply no immediate data.

Example 2: Successful Write Completion

Input: (Continues from Example 1. Assume the previous read completed with 0 bytes, but the write was still submitted.)

Program Output:
... (previous output) ...
[Timestamp] Completion received. BytesTransferred: [number_of_bytes_written], Key: [socket_handle], Overlapped: [overlapped_ptr]
[Timestamp] Operation type: Write (based on overlapped structure)
[Timestamp] Write operation completed successfully for client socket [socket_handle].

Explanation: The IOCP worker thread detects the completion of the previously submitted write operation. The `BytesTransferred` will indicate how many bytes were actually written. This confirms the asynchronous write was successful.

Example 3: Handling a Connection Reset

Input: (Client disconnects abruptly while data is being written)

Program Output:
... (previous output) ...
[Timestamp] Completion received. BytesTransferred: 0, Key: [socket_handle], Overlapped: [overlapped_ptr]
[Timestamp] Operation type: Write (based on overlapped structure)
[Timestamp] Write operation failed for client socket [socket_handle]. Error: [windows_error_code] (e.g., WSAECONNRESET)
[Timestamp] Closing client socket [socket_handle].

Explanation: During an asynchronous write operation, the client forcefully disconnects. The IOCP worker receives a completion packet for the write operation. The `BytesTransferred` might be 0, and importantly, a specific Windows error code (like `WSAECONNRESET`) will be associated with the completion, indicating the connection was reset. The program correctly identifies the error and closes the socket.

Constraints

This challenge is Windows-specific. The solution will rely on Windows API calls.
You are expected to use the syscall package or similar low-level Go mechanisms to interact with the Windows API.
The program should be able to handle at least one concurrent client connection.
The implementation should avoid blocking operations on the IOCP worker goroutine other than GetQueuedCompletionStatus.
Error handling for all Windows API calls is crucial.

Notes

You will need to interact with the CreateIoCompletionPort, CreateFile (with FILE_FLAG_OVERLAPPED and FILE_ATTRIBUTE_NORMAL for sockets), PostQueuedCompletionStatus, GetQueuedCompletionStatus, CloseHandle Windows API functions.
For sockets, you'll be passing the socket handle (obtained from syscall.WSASocket) to CreateIoCompletionPort.
To submit asynchronous I/O on a socket, you'll need to create OVERLAPPED structures. The hEvent field of OVERLAPPED can be NULL when using IOCP. The Offset and OffsetHigh fields are typically zero for socket operations.
You will need to find a way to associate the OVERLAPPED structure with the specific operation type (read or write) and the socket handle when it's returned by GetQueuedCompletionStatus. A common technique is to embed a custom struct (containing the socket handle and operation type) within the OVERLAPPED structure (or a structure that contains OVERLAPPED).
Go's standard net package's Listen and Accept functions do not directly expose the necessary low-level handles or allow for direct OVERLAPPED submission. You will likely need to use syscall.WSASocket and related functions to create and manage sockets at this level.
Consider using a pointer to a Go struct that contains the OVERLAPPED structure and other relevant information (like the socket handle) as the lpOverlapped argument to GetQueuedCompletionStatus.
You will need to research how to translate Windows error codes (like GetLastError) into Go errors or informative messages.