Make --socket-fd work with systemd socket activation #8082

Is there an existing issue for this?

• I have searched the existing issues

OS/Web Information

• Web Browser: not applicable (code-server does not serve requests)
• Local OS: not applicable
• Remote OS: Ubuntu 24.04 and Rocky Linux 9
• Remote Architecture: arm64 (Ubuntu) and amd64 (Rocky)
• code-server --version: 4.134.0 and 4.143.1

Steps to Reproduce

1. Install code-server 4.134.0 or later.
2. Run systemd-socket-activate -l 127.0.0.1:18190 code-server --socket-fd 3 --auth none. This passes a listening TCP socket as fd 3, as systemd socket activation does.
3. Run curl http://127.0.0.1:18190/healthz.
4. Repeat with a Unix socket: systemd-socket-activate -l /tmp/cs.sock code-server --socket-fd 3 --auth none, then curl --unix-socket /tmp/cs.sock http://localhost/healthz.

Expected

code-server listens on the inherited socket and answers the request.

Actual

code-server logs error listen EEXIST: file already exists and does not serve requests. This occurs for TCP and Unix sockets.

Logs

[2026-10-27T19:17:11.391Z] info  code-server 4.143.1 22ddf60dbc52f164cff13620ab515086dd08c90a
[2026-10-27T19:17:11.392Z] info  Using user-data-dir /root/.local/share/code-server
[2026-10-27T19:17:11.393Z] error listen EEXIST: file already exists

Does this bug reproduce in native VS Code?

No, this works as expected in native VS Code

Does this bug reproduce in VS Code web?

No, this works as expected in VS Code web

Does this bug reproduce in GitHub Codespaces?

No, this works as expected in GitHub Codespaces

Are you accessing code-server over a secure context?

• I am using a secure context.

Notes

#7940 added --socket-fd. The flag fails for two reasons:

1. The parent process gets the socket, but the child process calls listen. ParentProcess.spawn in src/node/wrapper.ts forks the child with stdio: ["pipe", "pipe", "pipe", "ipc"], so the child does not inherit the socket. In the child, fd 3 is the IPC channel, so listen({ fd: 3 }) fails with EEXIST.
2. A second failure occurs when the child gets a Unix socket by fd. server.address() then returns null, and ensureAddress in src/node/app.ts throws Server has no address. TCP sockets are not affected.

The unit test in #7940 calls listen() in the test process, so the test does not go through the wrapper. --socket-fd is a code-server flag, so the VS Code answers above do not apply. I will open a PR that fixes both failures.