Forcola

Leak-free external process execution for the BEAM.

Forcola runs OS processes through a small Rust shim that puts each child in its own process group and kills the whole group, SIGTERM then SIGKILL, when the run times out or the BEAM dies. Children and grandchildren that remain in the group die with the command.

Named for the forcola, the carved oarlock of a Venetian gondola.

Installation

Add forcola to your dependencies:

def deps do
  [
    {:forcola, "~> 0.4"}
  ]
end

Requires Elixir 1.18+ and OTP 27+. No Rust toolchain is needed on the five precompiled targets (macOS arm64 and x86-64, Linux x86-64 and arm64 glibc, x86-64 musl): the shim binary is downloaded from the matching GitHub Release and verified against a SHA256 checksum at compile time. On other targets, or to opt out of the download, set FORCOLA_BUILD=1 to build from source with cargo. See the getting started guide.

For Mix escripts, include Forcola's priv files, extract the bundled shim to a trusted executable path, and pass shim_path: path to any execution mode. The escript installation example covers extraction, permissions, and cleanup.

Observe a bounded run's output

Pass output_observer: {observer_pid, run_ref} to Forcola.run/2 to receive raw output while the command is still running:

Forcola.run(["my-cli", "--json"],
  timeout_ms: 60_000,
  output_observer: {observer_pid, run_ref}
)
# observer_pid receives {run_ref, {:stdout, bytes}} or
# {run_ref, {:stderr, bytes}} for each output frame.

The target must be a local PID and the tag a reference. On legacy runs, the collector sends notifications before returning the same final Forcola.Result it would return without an observer. Exit, timeout, and cleanup behavior stay the same; a dead observer does not affect the run.

Frames are bytes, not lines or decoded messages. A frame can split a line or UTF-8 character. Notifications are opt-in and unacknowledged; the observer must drain its mailbox. They provide neither backpressure nor durable delivery. With merge_stderr: true, merged stderr is observed as stdout; leave merging disabled when the original channel matters.

Bound output, idle time, and environment

Forcola.run(["/absolute/path/to/cli", "--json"],
  timeout_ms: 60_000,
  idle_timeout_ms: 10_000,
  max_output_bytes: 1_048_576,
  env_mode: :replace,
  env: [{"PATH", "/usr/bin:/bin"}],
  interrupt_signal: :int,
  interrupt_grace_ms: 500
)

The new integer options must be positive and at most 4_294_967_295. Output bytes count stdout and stderr together, including partial lines. Exactly the limit is allowed; an additional byte cancels. The captured and observed prefix never exceeds it. Finite credits keep at most 8 KiB per output pipe in flight. The independent whole-run deadline never resets on output; the idle deadline does. Without max_output_bytes, captured output has no cumulative memory limit. env_mode: :inherit remains the default; :replace clears the environment before applying the validated {name, value} string pairs. Without a new strict option, legacy env: nil, maps, and atom names keep their existing encoding and spawn-error behavior.

The optional signal/grace pair sends process-group SIGINT before the existing TERM/KILL sequence on timeout, explicit cancellation, and owner or BEAM death. A cooperative parent can use that opportunity to stop children in other groups. An uncooperative escaped child remains outside group cleanup on macOS.

These options require a matching shim capability exchange before child execution. Older shims return {:error, {:spawn, :incompatible_shim}}. Handshake stalls return {:error, {:spawn, :handshake_timeout}}, malformed replies return {:error, {:spawn, :invalid_handshake}}, and transport exit before SPAWN returns {:error, {:spawn, {:shim_exited, status}}}. With new options, timeout_ms is an integer in 0..4_294_967_295 and includes startup negotiation; SPAWN receives only its remaining budget. Cleanup and guard close can extend beyond that execution deadline (see Forcola.run/2). cgroup: :required alone keeps legacy collection and errors without HELLO; its existing required-containment wire encoding remains enforced. Stream, Duplex, and Daemon reject the new one-shot environment and interrupt options before opening a shim. Stream and Daemon also reject max_output_bytes; Duplex keeps its existing support with delivery: :pull. Duplex and Daemon reject idle_timeout_ms; Stream keeps its existing idle option. These existing mode options keep their own validation and semantics. Bounded failures return {:error, {reason, partial_result}}, with reason :timeout, :idle_timeout, :max_output_bytes, or :observer_overflow. Strict observers cancel at 1,024 queued messages rather than growing the recipient's mailbox indefinitely; notifications remain unacknowledged.

result.evidence keeps raw child_status and child_signal, termination, reason, output_truncated, cleanup, scope, and guard_close independent of the legacy status. Only explicit valid EXIT evidence confirms cleanup. Strict drain waits for pipe closure after root exit. An escaped descendant holding a pipe can cause reason: :timeout or :idle_timeout even with child_status: 0; wrappers must consume those facts independently. :process_group confirmation never proves whole-tree containment. Early JSON on stdout is partial output until the terminal result arrives. See Forcola.Result for field values and Forcola.run/2 for error shapes.

The problem

The common Elixir timeout pattern leaks processes:

task = Task.async(fn -> System.cmd(binary, args) end)

case Task.yield(task, timeout) || Task.shutdown(task) do
  {:ok, result} -> result
  nil -> {:error, :timeout}
end

Task.shutdown kills the BEAM task, which closes the Erlang port. Closing a port closes pipes; it sends no signal. The external process keeps running until it next writes to a closed pipe, and any children it spawned are never signaled at all. The caller gets {:error, :timeout} while the command keeps running.

The design

A port program, not a NIF: the shim is a separate OS process, so a bug in it cannot crash the BEAM, and BEAM death reaches it as stdin EOF.

BEAM <--stdin/stdout pipes--> forcola_shim <--forks--> child (own process group)
                                   |                      |- grandchild
                                   |                      |- grandchild
                                   |
                     on timeout or stdin EOF:
                     kill(-pgid, SIGTERM), then SIGKILL

• The shim calls setsid before exec, so the child leads a new process group. Kill means the whole group: the CLI and everything it forked.
• Timeout is mandatory on bounded runs. On expiry the caller receives {:error, {:timeout, partial_result}} with output captured so far, and the group is confirmed dead before the call returns (or the result is explicitly marked {:signal, :unconfirmed}).
• If the BEAM dies, even by kill -9, the shim sees stdin EOF and kills the group before exiting.
• Shim binaries ship precompiled per target via GitHub Releases with SHA256 verification. Consumers need no Rust, C, or C++ toolchain.

Execution modes

The getting started guide has a runnable example, options, and return/message shapes for each mode.

Process groups and cleanup

The group kill covers the child and everything it keeps in its process group: ordinary grandchildren die with the command. Deliberate daemonizers (double-fork plus setsid) leave the group; on Linux the opt-in cgroup: true layer contains them when active; cgroup: :required refuses to execute without it. Daemon control channels like docker and work handed to system schedulers stay out of reach of any process-based mechanism. The process groups guide covers the kill sequence, cgroup containment, the confirmation guarantee and its exceptions, and the full "What group kill cannot reach" audit.

Adopting in a wrapper library

Forcola slots into existing CLI wrapper libraries without becoming a mandatory dependency: the wrapper defines a small runner behaviour, keeps its System.cmd/3 path as the default, and accepts a Forcola-backed one via config, with Forcola as an optional dep. The adoption guide covers the pattern, a worked example against a real wrapper, the mode mapping, and migration notes for erlexec-based wrappers.

Prior art

• erlexec has opt-in process-group kill and best-effort Linux cgroup attachment, but compiles C++ on the consumer's machine.
• MuonTrap has the port-program architecture, default stdout/stderr flow control, and cgroup v2 controls; without a cgroup, it signals only the direct child.
• Rambo proved a Rust shim works in a hex package; its x86-64-only binary distribution is the cautionary tale the release workflow here is designed around.

The alternatives guide compares these in detail.

Development

The local quality gate mirrors CI:

mix deps.get
mix format --check-formatted
mix compile --warnings-as-errors
mix test
mix credo --strict
mix dialyzer
cargo test --manifest-path native/forcola_shim/Cargo.toml
cargo clippy --manifest-path native/forcola_shim/Cargo.toml --all-targets -- -D warnings

CI runs the Elixir integration suite on Ubuntu and macOS, and runs the privilege-drop tests under passwordless sudo on its Linux runner. Real cgroup containment needs a delegated writable cgroup v2 subtree, which hosted runners do not always provide; the platform job reports availability in its job summary. Set FORCOLA_REQUIRE_ROOT_TESTS=1 or FORCOLA_REQUIRE_CGROUP=1 to turn either platform prerequisite into a hard failure on a suitable dedicated runner.

License

MIT