boxmaker/docs/plans/M3b/14-brokerd-pipes-module.md

M3b task 14: move the pipe handling into its own module

Branch: m3b (run git switch m3b; git status --short must be empty, otherwise stop) Commit subject: brokerd: move the container's pipe handling into pipes.rs

Goal

container.rs is 432 lines, and the next two tasks add to the pipe handling. Before they do, move that code into its own file, without changing what it does. This is a pure move: every test passes before and after, unchanged.

Files

• Create: crates/brokerd/src/pipes.rs
• Modify: crates/brokerd/src/container.rs, crates/brokerd/src/lib.rs, docs/implementer-log.md
• No test files change.

What moves

From container.rs, cut these three items, bodies unchanged, and paste them into pipes.rs:

1. struct Io { … } (the three JoinHandle fields) → pub(crate) struct Io.
2. impl Io { fn start(…) … fn finish(…) … } → the same, with pub(crate) fn start and pub(crate) fn finish.
3. fn read_capped(…) → pub(crate) fn read_capped.

Their doc comments move with them. pipes.rs starts with a module doc comment:

//! The three pipes of one container: its arguments go in on standard input, its output and errors
//! come back, each on its own thread so no pipe can block another.

and the use lines those items need (std::io::{Read, Write}, std::process::Child, std::thread::JoinHandle). Io::start uses STDERR_KEPT: leave that constant in container.rs (the tests import it from there) and write crate::container::STDERR_KEPT in pipes.rs.

In lib.rs: pub mod pipes;, between pub mod ledger; and pub mod podman;.

In container.rs: add use crate::pipes::Io;, and remove the use lines the compiler then says are unused. Nothing else in container.rs changes.

Steps

• 1. Branch. git switch m3b.
• 2. Move the three items as above. Run cargo fmt --all.
• 3. Check it builds. cargo check -p brokerd, then cargo clippy -p brokerd --all-targets -- -D warnings. Fix only imports.
• 4. See every test pass, unchanged. cargo test -p brokerd --test container --test container_egress --test serve_runner. Expected: 11, 6 and 2 passed.
• 5. Check it moved. grep -n "struct Io\|fn read_capped" crates/brokerd/src/container.rs prints nothing; wc -l crates/brokerd/src/container.rs is under 380.
• 6. Run the gate. make gate. Expected last line: gate: ok.
• 7. Log and commit. git add crates/brokerd docs/implementer-log.md Cargo.lock && git commit

Done when

• container.rs no longer holds Io or read_capped; make gate prints gate: ok.

Stop and report if

• Any test fails after the move: a pure move cannot break one, so report instead of changing code.