docs: Add related.md documenting prior art for fd passing IPC

[cgwalters]

Document related protocols and implementations that support file descriptor passing over IPC, including:

• systemd's varlink fd passing extension (SCM_RIGHTS-based, index references in JSON, used by mountfsd/machined/nsresourced/etc.)
• containerd's ttrpc fd passing efforts
• D-Bus UNIX_FD support
• Comparison with jsonrpc-fdpass approach

This provides context for the design decisions in jsonrpc-fdpass and helps users understand the landscape of fd-passing IPC protocols.

Assisted-by: OpenCode (Claude claude-opus-4-5)

[gemini-code-assist]

Summary of Changes

Hello @cgwalters, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces new documentation to contextualize the design decisions behind jsonrpc-fdpass by outlining various existing IPC protocols that support file descriptor passing. It aims to help users understand the landscape of such protocols and how jsonrpc-fdpass fits within it, particularly by detailing systemd's varlink extension and comparing it with the jsonrpc-fdpass approach.

Highlights

• New Documentation File: Added a new documentation file, docs/related.md, which details prior art and related protocols for file descriptor passing over Inter-Process Communication (IPC).
• Systemd Varlink FD Passing: The document provides an in-depth look at systemd's specific extension to the varlink protocol for FD passing, including its API, wire protocol, JSON index references, and production usage examples.
• Comparison with jsonrpc-fdpass: A direct comparison table highlights key differences between systemd's varlink FD passing and the jsonrpc-fdpass approach, focusing on FD indication, batching, and language support.
• Other FD Passing Protocols: Briefly covers containerd's ttrpc FD passing efforts and D-Bus's UNIX_FD support, providing a broader context of existing solutions.

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Changelog

• docs/related.md
  • Added a new document detailing related protocols and implementations for IPC with file descriptor passing.

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[gemini-code-assist]

Code Review

This pull request adds a new documentation file, docs/related.md, which provides a comprehensive overview of related protocols and implementations for file descriptor passing. The document is well-structured and clearly explains the approaches taken by systemd's varlink, containerd's ttrpc, and D-Bus, comparing them with jsonrpc-fdpass. This is a valuable addition for users to understand the design choices and the broader context. I have one suggestion to slightly expand the section on containerd ttrpc to make it even more informative.

[gemini-code-assist]

The section on containerd ttrpc is quite brief. To provide more context for the reader, consider expanding it with a short summary of the proposed approach for FD passing, similar to the detail provided for systemd's varlink.

For example, you could mention:

• The underlying mechanism used (e.g., SCM_RIGHTS).
• How the protocol signals the presence of FDs.
• The status of the linked pull request (e.g., open, stalled).

This would make the comparison richer and more informative.