[WIP] Fix improper socket teardown without shutdown()

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>Socket closed without shutdown() causes TIME_WAIT accumulation and data loss risk</issue_title>
<issue_description># Socket closed without shutdown() causes TIME_WAIT accumulation and data loss risk

Summary

The clickhouse-cpp client calls close() / closesocket() directly on the TCP socket without first calling shutdown(). This is an improper socket teardown that causes two problems:

1. TIME_WAIT socket accumulation on the client side when the client initiates the close, which can exhaust ephemeral ports under high connection churn.
2. Potential data loss — close() on a socket with unread data in the receive buffer causes the OS to send a RST instead of a graceful FIN, which can discard in-flight data and confuse the peer.

The issue affects both plain TCP and SSL connections (no SSL_shutdown() either).

Root Cause

Socket::Close() in socket.cpp:

void Socket::Close() {
    CloseSocket(handle_);
    handle_ = INVALID_SOCKET;
}

Where CloseSocket is:

void CloseSocket(SOCKET socket) {
    if (socket == INVALID_SOCKET)
        return;
#if defined(_win_)
    closesocket(socket);
#else
    close(socket);
#endif
}

There is no shutdown(fd, SHUT_RDWR) or shutdown(fd, SHUT_WR) call before close() anywhere in the library. A search of the entire clickhouse/ source tree confirms zero uses of shutdown().

For SSL sockets, SSLSocket inherits from Socket and its destructor chain also never calls SSL_shutdown() before closing the underlying file descriptor.

Affected Code Paths

Every socket close goes through Socket::Close() → CloseSocket():

• Client::~Client() → ~Impl() → ~socket_ → Socket::~Socket() → Close()
• Client::Impl::ResetConnection() → InitializeStreams() → old socket swapped out → ~Socket()
• SocketRAIIWrapper::~SocketRAIIWrapper() in SocketConnect() (on connection failure)

Consequences

1. TIME_WAIT accumulation

When the client closes first without shutdown(), the TCP stack enters TIME_WAIT on the client side. Under high connection churn (many short-lived client instances), this can exhaust the available ephemeral port range (~28,000 ports on Linux default), causing connect() failures with EADDRNOTAVAIL.

2. RST instead of graceful FIN

Per POSIX/RFC 793, calling close() on a socket that has unread data in the kernel receive buffer causes the OS to send a TCP RST to the peer instead of a graceful FIN. This can happen when:

• The server sent data the client never read (e.g., client destroys Client object mid-query)
• The server sent an exception or log packet the client didn't consume

The RST causes the peer to see ECONNRESET, and any data the client sent that is still in-flight may be discarded by the peer's TCP stack.

3. SSL session not terminated properly

For SSL connections, SSL_shutdown() is never called. This means the TLS close_notify alert is never sent, which:

• Prevents SSL session resumption/caching
• May cause the server to log TLS errors
• Violates the TLS specification (RFC 5246 §7.2.1)

Reproduction

void tcp_server(std::atomic<bool>& running, int port) {
    int server_fd = socket(AF_INET, SOCK_STREAM, 0);
    // ... bind, listen ...
    while (running) {
        int client_fd = accept(server_fd, nullptr, nullptr);
        if (client_fd >= 0) {
            // Read Hello, then close properly from server side
            recv(client_fd, buffer, sizeof(buffer), 0);
            shutdown(client_fd, SHUT_RDWR);
            close(client_fd);
        }
    }
}

// Client side — creates and destroys many connections
for (int i = 0; i < 15000; ++i) {
    try {
        clickhouse::Client client(opts);
    } catch (...) {}
}

// Check TIME_WAIT count — will be very high
// Linux: grep "06 " /proc/net/tcp | wc -l

Suggested Fix

Add shutdown() before close() in Socket::Close():

void Socket::Close() {
    if (handle_ == INVALID_SOCKET)
        return;

#if defined(_win_)
    shutdown(handle_, SD_BOTH);
    closesocket(handle_);
#else
    shutdown(handle_, SHUT_RDWR);
    close(handle_);
#endif
    handle_ = INVALID_SOCKET;
}

For SSL sockets, add SSL_shutdown() in the SSLSocket destructor before the base Socket::Close() runs:

SSLSocket::~SSLSocket() {
    if (ssl_) {
        SSL_shutdown(ssl_.get());
    }
}

Note: shutdown() may fail if the socket is already in an error state — the return value can be safely ignored here since we are closing regardless.

Environment

• Library: clickhouse-cpp (all current versions)
• Affected platforms: All (Linux, macOS, Windows)
• Affected files: clickhouse/base/socket.cpp, clickhouse/base/sslsocket.cpp</issue_description>

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