Testing your code

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Code that shells out is miserable to test — unless the subprocess is behind a seam. In processkit that seam is one small trait. Only output_string is required; output_bytes (raw-byte stdout) and start (a live handle for streaming/probes) are defaulted, so a minimal double implements just output_string:

#[async_trait]
pub trait ProcessRunner: Send + Sync {
    async fn output_string(&self, command: &Command) -> Result<ProcessResult<String>>;
    // Defaulted (route through `start`); override for byte/streaming support:
    async fn output_bytes(&self, command: &Command) -> Result<ProcessResult<Vec<u8>>>;
    async fn start(&self, command: &Command) -> Result<RunningProcess>;
}

Production code takes a runner (generically or as &dyn ProcessRunner); tests hand it a double. Four doubles ship with the crate, plus a macro that makes whole CLI wrappers testable for free.

The ProcessRunner seam

JobRunner is the real implementation (each run in a fresh private group); a ProcessGroup is also a runner (runs land in that shared group); and impl ProcessRunner for &R means a borrowed runner works wherever an owned one does — inject &group or &recording without giving ownership away.

Every runner — real or double — gets the convenience helpers of ProcessRunnerExt for free: run (trimmed stdout, success required), run_unit, exit_code, probe (exit code as a boolean), checked (success-checked full result), and parse/try_parse (feed stdout to a closure; like first_line, generic over the closure so unavailable on a &dyn ProcessRunner). Retry policies work through the seam too, so a double exercises your retry handling hermetically.

The seam covers streaming as well as bulk runs: ProcessRunner::start returns a live RunningProcess, and a ScriptedRunner's start hands back a scripted handle whose canned lines flow through the same pump machinery a real child uses — stdout_lines, wait_for_line, and finish behave identically, with no subprocess (see Scripted streaming below). An output_string-only custom runner keeps compiling: start is defaulted to Error::Unsupported.

#![allow(unused)]
fn main() {
use processkit::{Command, ProcessRunner, ProcessRunnerExt, Result};

// Production code: generic over the runner.
async fn current_branch(runner: &impl ProcessRunner) -> Result<String> {
    runner
        .run(&Command::new("git").args(["branch", "--show-current"]))
        .await
}
}

Scripting replies

ScriptedRunner returns canned Replys for matched commands — the work-horse double:

#![allow(unused)]
fn main() {
use processkit::{Command, ProcessRunnerExt};
use processkit::testing::{Reply, ScriptedRunner};

#[tokio::test]
async fn detects_the_branch() {
    let runner = ScriptedRunner::new()
        // Match by program + argument PREFIX (element-wise; first element is
        // the program name, in registration order):
        .on(["git", "branch", "--show-current"], Reply::ok("main\n"))
        // …or by any predicate over the full Command:
        .when(
            |cmd| cmd.working_dir().is_some(),
            Reply::fail(128, "fatal: not a git repository"),
        )
        // …with an optional catch-all:
        .fallback(Reply::ok(""));

    assert_eq!(current_branch(&runner).await.unwrap(), "main");
}
}

The pieces:

  • Reply::ok(stdout) — exit 0. Reply::fail(code, stderr) — non-zero with stderr. Reply::lines(["a", "b"]) — exit 0 with the lines joined (and streamed one by one on a scripted start). Reply::timeout() — a timed-out run (the checking helpers raise Error::Timeout from it, carrying the command's own configured deadline). On a scripted start it resolves immediately as timed-out; to exercise a real deadline race, use Reply::pending() + a Command::timeout. .with_stdout(text) — attach stdout to any of them (e.g. the CONFLICT … text git prints on a failing merge). .with_line_delay(d) — pace a scripted stream's lines.
  • Reply::pending() — parks the call until the command's cancellation token (per-command cancel_on or the client-level default_cancel_on) fires, then resolves with Error::Cancelled — so a test can prove an orchestration actually cancels a blocked call, not just that it formats a canned error. With no token it parks forever, like a hung child.
  • Rules are tried in registration order; first match wins. Prefix matching is element-wise over the program name then the arguments (the first element is the program) — on(["git", "foo"]) matches git foo bar but not git foobar (and not rm foo). Use on_sequence to serve an ordered sequence of replies (each once, then the last repeats) for a fail-then-succeed scenario.
  • No match and no fallback is a loud error (Error::Spawn, not-found) — an unexpected invocation can't slip through a test silently.
  • Bulk runs also replay the canned lines through the command's on_stdout_line/on_stderr_line handlers, so a wrapper's progress-reporting path is exercised without a subprocess.

Scripted streaming

ScriptedRunner::start returns a live RunningProcess backed by the canned reply instead of an OS child. The canned stdout/stderr feed the same pump machinery a real child uses, so the whole streaming surface works hermetically — stdout_lines yields the lines, wait_for_line probes them, finish reports the canned outcome and stderr:

#![allow(unused)]
fn main() {
use processkit::{Command, Outcome, ProcessRunner, StreamExt, Finished};
use processkit::testing::{Reply, ScriptedRunner};
use std::time::Duration;

#[tokio::test]
async fn server_becomes_ready() {
    let runner = ScriptedRunner::new()
        .on(["server", "serve"], Reply::lines(["booting", "listening on 8080"]));

    let mut run = runner.start(&Command::new("server").arg("serve")).await.unwrap();
    run.wait_for_line(|l| l.contains("listening"), Duration::from_secs(5))
        .await
        .unwrap(); // satisfied by the canned banner — no subprocess

    let Finished { outcome, .. } = run.finish().await.unwrap();
    assert_eq!(outcome, Outcome::Exited(0));
}
}

Reply::lines([...]) scripts the stdout lines; .with_line_delay(d) paces them (deterministic under #[tokio::test(start_paused = true)]), and the scripted run "exits" after the last line. The honest boundaries: a scripted handle has no OS identity (pid() is None, profile reports empty samples), does not compose into a real Pipeline, and does not model interactive stdin. Reply::pending() scripts a run that never exits on its own — cancel or time it out through the command's own knobs. A command timeout does bound a scripted stream (it ends at the deadline and reports Outcome::TimedOut, like a real child), but a scripted handle has no signal tier, so — like on Windows — it ignores timeout_grace and ends at once.

Asserting invocations

RecordingRunner wraps another runner and records every Invocation — what was asked — so a test asserts inputs, not just outputs:

#![allow(unused)]
fn main() {
use processkit::{Command, ProcessRunnerExt};
use processkit::testing::{RecordingRunner, Reply, ScriptedRunner};

#[tokio::test]
async fn passes_the_right_flags() {
    let runner = RecordingRunner::new(
        ScriptedRunner::new().fallback(Reply::ok("done")),
    );

    runner
        .run(&Command::new("gh").args(["pr", "create", "--draft"]).current_dir("/repo"))
        .await
        .unwrap();

    let call = runner.only_call(); // panics unless exactly one call
    assert_eq!(call.args_str(), ["pr", "create", "--draft"]);
    assert!(call.has_flag("--draft"));
    assert_eq!(call.cwd.as_deref().map(|c| c.to_str().unwrap()), Some("/repo"));
    assert!(!call.has_stdin);
}
}

An Invocation captures the routing knobs — program, args, cwd, envs (explicit overrides, None = removal), has_stdin — not the I/O-shaping ones (timeout, encodings, buffer policy); assert those through a when predicate over the Command itself. calls() returns the full list when more than one run is expected.

Expectation-style: MockRunner

With the mock feature, mockall generates a MockRunner for expectation-style tests (call counts, argument matchers, ordered expectations) — the right tool when the interaction is the contract.

Note: MockRunner's expect_* surface is generated by mockall and is exempt from this crate's semver guarantees — it tracks the mockall dependency, not a frozen API. For a stable double, prefer ScriptedRunner (canned replies) or RecordingRunner (input assertions) above.

use processkit::testing::MockRunner;

let mut mock = MockRunner::new();
mock.expect_output_string()
    .times(1)
    .returning(|_cmd| /* build a Result<ProcessResult<String>> */ …);

MockRunner does not inherit the defaults. Unlike a hand-written runner (where output_bytes/start are defaulted), mockall::automock replaces every method with an expectation — so a verb that routes through start or output_bytes needs its own expect_start() / expect_output_bytes(), or the unset call panics ("no expectation"). ScriptedRunner provides the defaults and the streaming seam out of the box.

For most tests ScriptedRunner/RecordingRunner read better; reach for the mock when you need mockall's matching machinery.

Record/replay cassettes

With the record feature, RecordReplayRunner closes the loop: record real runs to a JSON cassette once, then replay them deterministically — fast, hermetic, byte-stable, no subprocess in CI:

#![allow(unused)]
fn main() {
use processkit::{Command, JobRunner, ProcessRunnerExt};
use processkit::testing::RecordReplayRunner;

// Record once against the real tool (an opt-in `--record` test run, say):
let runner = RecordReplayRunner::record("fixtures/git.json", JobRunner::new());
let version = runner.run(&Command::new("git").arg("--version")).await?;
runner.save()?;                                  // the error-surfacing flush
                                                 // (best-effort on drop too)

// Replay everywhere else:
let runner = RecordReplayRunner::replay("fixtures/git.json")?;
assert_eq!(runner.run(&Command::new("git").arg("--version")).await?, version);
}

Semantics worth knowing before you commit a cassette:

AspectBehavior
Match keyprogram + args + cwd + a stdin source digest (hashed, never persisted: in-memory bytes hash their content, a from_file source hashes its path) — no stdin (absent or Stdin::empty()) keys distinctly; lossy UTF-8 on the text parts
Environmentvalues never reach the file — only sorted variable names, so env secrets can't leak through a committed fixture; env is not matched, so env differences can't cause spurious misses
Duplicates of one keyreplay in capture order, then the last entry repeats — a recorded sequence (git rev-parse HEAD before/after a commit) replays faithfully, while retry/probe loops keep getting a stable final answer
Missstrict Error::CassetteMiss (distinct from a missing program — is_not_found() is false) — replay never spawns a surprise subprocess; a stale cassette fails loudly
Timeoutsa recorded timed-out run replays as one, surfacing Error::Timeout with the replaying command's deadline
Formatpretty-printed JSON with a version field; unknown versions / corrupt files / an entry with a contradictory outcome / a file over 64 MiB are Error::Io(InvalidData), a missing file keeps NotFound
Err resultsnot recorded — only completed runs (non-zero exits and captured timeouts are results and are recorded)

Only env values are redacted. program, args, cwd, stdout, and stderr are stored verbatim and can carry secrets (a --password=… flag, a token echoed to output), so review a fixture before committing it. On Unix the file is written 0600 and the write refuses to follow a symlink at the cassette path (O_NOFOLLOW, so a planted link can't redirect the secret-bearing write — it fails loud instead). On Windows the file inherits the containing directory's ACL, so restrict that directory (or use a per-user temp dir, not a world-writable shared one) for secret-bearing fixtures.

A neat trick: in tests, record against a ScriptedRunner instead of JobRunner — the whole record→save→replay round trip is then itself hermetic.

Wrapping a CLI tool

CliClient is the foundation for typed wrappers around external tools (git, jj, gh, kubectl, …): it owns the program name, per-client defaults, and the runner; your wrapper contributes only commands and parsers. The cli_client! macro generates the boilerplate:

#![allow(unused)]
fn main() {
use processkit::{cli_client, Error, ProcessRunner, Result};
use std::path::Path;
use std::time::Duration;

cli_client!(
    /// A typed `git` client.
    pub struct Git => "git"
);

impl<R: ProcessRunner> Git<R> {
    /// HEAD's commit id.
    pub async fn head(&self, repo: &Path) -> Result<String> {
        self.core.run(self.core.command_in(repo, ["rev-parse", "HEAD"])).await
    }

    /// Is the work tree clean? (exit code IS the answer)
    pub async fn is_clean(&self, repo: &Path) -> Result<bool> {
        self.core.probe(self.core.command_in(repo, ["diff", "--quiet"])).await
    }

    /// Branch list, parsed — the parser is fallible and returns the crate's
    /// `Result`, typically an `Error::Parse` naming the program.
    pub async fn branches(&self, repo: &Path) -> Result<Vec<String>> {
        self.core
            .try_parse(
                self.core.command_in(repo, ["branch", "--format=%(refname:short)"]),
                |out| {
                    let list: Vec<String> = out.lines().map(str::to_owned).collect();
                    if list.is_empty() {
                        Err(Error::Parse {
                            program: "git".into(),
                            message: "no branches".into(),
                        })
                    } else {
                        Ok(list)
                    }
                },
            )
            .await
    }
}

// Production: the real runner, with per-client defaults.
let git = Git::new().default_timeout(Duration::from_secs(30));
let head = git.head(Path::new(".")).await?;
}

The generated type is Git<R: ProcessRunner = JobRunner> with Git::new(), Git::with_runner(runner), default_timeout / default_env / default_env_remove builders, and a public core: CliClient<R> whose helpers speak the crate-wide verb vocabulary: run (trimmed stdout), output_string (full result), run_unit (success only), exit_code, probe, plus parse (infallible) and try_parse (fallible → Error::Parse).

And the payoff — the wrapper tests hermetically with any double:

#![allow(unused)]
fn main() {
#[tokio::test]
async fn head_is_trimmed() {
    let git = Git::with_runner(
        ScriptedRunner::new().on(["git", "rev-parse", "HEAD"], Reply::ok("abc123\n")),
    );
    assert_eq!(git.head(Path::new("/repo")).await.unwrap(), "abc123");
}
}

…or with a cassette recorded against the real tool once.

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