Pocket engine architecture

Pocket is a composable task runner that separates planning from execution. The plan phase does all computation (tree walking, path resolution, flag merging) once. The execution phase is simple iteration.

Design principles

Single-pass planning. The filesystem is walked once. The composition tree is traversed once. Path patterns are resolved once. All results are cached in the Plan struct and shared throughout execution.

Composition preservation. The original Serial/Parallel tree is kept intact in the plan. Execution follows the same tree structure. This enables introspection (./pok plan -json) and CI integration without separate codepaths.

Context layering. Each nesting level can add context (path, flags, env, name suffix). Values accumulate as composition nests. CLI flags override plan flags, which override task defaults.

Cooperative concurrency. Parallel tasks use errgroup.WithContext for fail-fast. Output is buffered per-goroutine and flushed atomically to prevent interleaving. Signals propagate via context cancellation.

File ownership

Subsystem overview

Plan building

The plan is the bridge between user configuration and execution. It transforms a composition tree + filesystem into a flat, pre-computed execution plan.

Inputs: Config.Auto, Config.Manual, filesystem walk, shim config. Outputs: Plan struct with task instances, path mappings, task index.

See INTERNALS.md for the plan building algorithm and path resolution details.

Composition model

The Runnable interface has a single unexported method run(ctx) error. Four types implement it:

• •Task — leaf node, runs a function
• •serial — sequential execution, stops on first error
• •parallel — concurrent execution with buffered output
• •pathFilter — wraps a runnable, iterates over resolved paths

Serial() and Parallel() are the public constructors. WithOptions() wraps a runnable in a pathFilter with the given options.

Context propagation

Context carries execution state through the composition tree:

• •Path (PathFromContext) — current directory, set by pathFilter
• •Verbose (Verbose) — from -v CLI flag
• •Env — per-task environment overrides (set/filter)
• •Name suffix — accumulated from WithNameSuffix
• •Auto-exec — whether manual tasks should be skipped
• •Output — writer for stdout/stderr (swapped for buffered in parallel)

Exec pipeline

pk.Exec runs external commands with:

1. •.pocket/bin prepended to PATH (plus any registered dirs)
2. •Working directory from PathFromContext
3. •Color env vars forced when TTY detected
4. •Stdin set to nil (prevents CI hangs)
5. •Graceful shutdown: SIGINT on Unix, then SIGKILL after 5s
6. •Output: streamed in verbose mode, buffered in normal mode (shown only on error or when warning patterns detected)

Deduplication

Tasks are deduplicated by (effectiveName, path) tuple. A task at the same path only runs once per invocation. Global tasks (Global: true) use a fixed path of ".", so they run at most once total. WithForceRun() bypasses this.

Parallel output buffering

When multiple tasks run in parallel, each goroutine gets a bufferedOutput that captures all writes. On completion, the buffer is flushed to the parent output under a mutex. This prevents interleaved output while preserving first-to-complete ordering. Single-item parallel runs skip buffering entirely.

Shim generation

Shims are generated at the repo root and at each unique include/detect path. Three variants: POSIX (pok), Windows batch (pok.cmd), PowerShell (pok.ps1). Each shim computes a relative path back to .pocket and sets TASK_SCOPE for path-scoped task visibility.

See INTERNALS.md for details on each subsystem.