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ChildProcess: support bidirectional (duplex) additional file descriptors

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A pull request for this has already been merged.

  • #8905 by @effect-bot — merged

Assessment

Difficulty
3/5
Estimated time
Half a day
Newbie friendliness
25/100
Issue type
Feature
Clarity
Clearly specified
Activity status
Stale
Tech stack
node.js, typescript
Domain
backend

Research direction

Start from the ChildProcessOptions.additionalFds type and NodeChildProcessSpawner implementation in the platform-node-shared package. Add a duplex variant to AdditionalFdConfig that provides both a Stream and a Sink for the same descriptor, then extend ChildProcessHandle with a getDuplexFd method. Verify the change with the existing child process test suite.

Written by the indexing model from the issue text.

Description

What is the problem this feature would solve?

ChildProcessOptions.additionalFds only lets you configure each extra descriptor in one direction:

type AdditionalFdConfig =
  | { readonly type: "input"; readonly stream?: Stream<Uint8Array, PlatformError> }
  | { readonly type: "output"; readonly sink?: Sink<Uint8Array, Uint8Array, never, PlatformError> }

The Node spawner (@effect/platform-node-shared/NodeChildProcessSpawner) sets each configured fd to "pipe". For fd ≥ 3, Node backs that with a bidirectional Unix domain socket. Effect then exposes it either as a Sink (getInputFd) or as a Stream (getOutputFd), never both. And because additionalFds is keyed by fd name, one descriptor can't be declared as both input and output.

So there's no way to run a request/response protocol between parent and child over a single inherited socket, e.g. a sidecar speaking a framed protocol on fd 3. Code that needs this has to call node:child_process spawn with stdio: [..., "pipe"] directly and drive child.stdio[3] itself. That means giving up ChildProcessSpawner and doing manual lifecycle, exit and error handling plus raw event listeners. The workaround of using two descriptors (fd 3 in, fd 4 out) changes the child's protocol contract and isn't always an option.

What is the feature you are proposing to solve the problem?

Add a duplex variant to AdditionalFdConfig, for example:

additionalFds: { fd3: { type: "duplex" } }

ChildProcessHandle would then expose both directions for that fd, e.g. a getDuplexFd(fd) that returns { stream, sink }. Alternatively, getInputFd and getOutputFd could both work on the same "duplex" descriptor.

What alternatives have you considered?

  • Using node:child_process spawn directly and driving child.stdio[3] manually, which is what we do today.
  • Splitting the protocol across two unidirectional descriptors. This needs a change on the child side and doesn't match socket semantics such as half-close and shutdown.

Version: [email protected], @effect/[email protected].

Dominant language
TypeScript
Stars
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Forks
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Avg merge
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Merged PRs (30d)
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