fix(build): P0 regression — Operation not permitted (os error 1) on warm build

[zackees]

Summary

Fixes #129 (the os error 1 half). AVR / ESP32 / every-platform builds fail on main immediately after Toolchain: avr-gcc 7.3.0 with:

build error: build failed: io error: Operation not permitted (os error 1)

This is reproducing on CI (Build Leonardo and siblings) from 2.1.17 onwards and on every pip install fbuild==2.1.18.

Root cause

fbuild_core::containment::spawn_contained (Unix) delegated to ContainedProcessGroup::spawn_with_containment in the external running-process-core crate. That implementation stores the first spawned child's PID as the group's PGID and has every subsequent child call setpgid(0, first_child_pid) from its pre_exec hook.

Once the first child exits and is reaped — e.g. the short avr-gcc -dumpversion call that emits the Toolchain: line — the kernel tears down that process group. The very next spawn's setpgid(0, stale_pgid) then fails with EPERM, which propagates as io error: Operation not permitted (os error 1) through the build orchestrator.

Reproducing commit: #108 (added containment via the running-process crate). The failure became universal only once the daemon-centric build path in 2.1.17 / 2.1.18 always takes spawn_contained for every subprocess.

What this PR does

• On Unix, bypasses ContainedProcessGroup::spawn_with_containment and installs our own pre_exec hook that:
  • calls setpgid(0, 0) — each child gets a fresh process group, no stale-pgid dependency.
  • on Linux, calls prctl(PR_SET_PDEATHSIG, SIGKILL) so the kernel still SIGKILLs the child when the spawning daemon thread exits.
• Windows behaviour is unchanged — Job Object assignment is stateless and has no analogous failure mode.
• macOS loses the drop-time killpg backstop, which was already a no-op in practice: the global ContainedProcessGroup lives in a OnceLock that never drops. Improving macOS containment can be tracked separately.

Regression test

sequential_contained_spawns_do_not_fail_with_eperm in crates/fbuild-core/src/containment.rs — initialises the global containment group, spawns a short-lived command, waits for it to exit, sleeps briefly, then spawns a second command. On the pre-fix code path, the second spawn reliably failed with EPERM; with this fix, both succeed.

Test plan

• uv run cargo clippy --workspace --all-targets -- -D warnings clean
• uv run cargo test -p fbuild-core --lib containment — 3/3 pass
• uv run cargo test -p fbuild-daemon --test process_containment -- --ignored — 1/1 pass (Windows locally; CI will verify Linux/macOS)
• Full uv run cargo test --workspace --lib — green
• CI Build Leonardo / Build Arduino Uno (end-to-end reproducer) — verified by this PR's CI run

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between 5a1a346 and 0beb733.

📒 Files selected for processing (1)

• crates/fbuild-core/src/containment.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/p0-regression-os-error-1

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