Project 14: Plugin System or Hooks

Build a hook system that runs scripts on events without blocking the server.

Quick Reference

Attribute Value
Difficulty Level 4: Expert
Time Estimate 2 weeks
Main Programming Language C (Alternatives: Rust, Go)
Alternative Programming Languages Rust, Go
Coolness Level Level 4: Extensibility Architect
Business Potential 2: The “Ecosystem Play”
Prerequisites process execution, command dispatch
Key Topics hooks, async exec, timeouts

1. Learning Objectives

By completing this project, you will:

  1. Build a working implementation of plugin system or hooks and verify it with deterministic outputs.
  2. Explain the underlying Unix and terminal primitives involved in the project.
  3. Diagnose common failure modes with logs and targeted tests.
  4. Extend the project with performance and usability improvements.

2. All Theory Needed (Per-Concept Breakdown)

Event Hooks and Safe Process Execution

  • Fundamentals Event Hooks and Safe Process Execution is the core contract that makes the project behave like a real terminal tool. It sits at the boundary between raw bytes and structured state, so you must treat it as both a protocol and a data model. The goal of the fundamentals is to understand what assumptions the system makes about ordering, buffering, and ownership, and how those assumptions surface as user-visible behavior. Key terms include: hook, fork/exec, timeout, env vars. In practice, the fastest way to gain intuition is to trace a single input through the pipeline and note where it can be delayed, reordered, or transformed. That exercise reveals why Event Hooks and Safe Process Execution needs explicit invariants and why even small mistakes can cascade into broken rendering or stuck input.

  • Deep Dive into the concept A deep understanding of Event Hooks and Safe Process Execution requires thinking in terms of state transitions and invariants. You are not just implementing functions; you are enforcing a contract between producers and consumers of bytes, and that contract persists across time. Most failures in this area are caused by violating ordering guarantees, dropping state updates, or misunderstanding how the operating system delivers events. This concept is built from the following pillars: hook, fork/exec, timeout, env vars. A reliable implementation follows a deterministic flow: Register hook -> On event, fork child -> Set env context -> Enforce timeout -> Collect status. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. When you design the project, treat each key term as a source of constraints. For example, if a term implies buffering, decide the buffer size and how overflow is handled. If a term implies state, decide how that state is initialized, updated, and reset. Finally, validate your assumptions with deterministic fixtures so you can reproduce bugs. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness. From a systems perspective, the tricky part is coordinating concurrency without introducing races. Even in a single-threaded loop, multiple events can arrive in the same tick, so you need deterministic ordering. This is why many implementations keep a strict sequence: read, update state, compute diff, render. Another subtlety is error handling and recovery. A robust design treats errors as part of the normal control flow: EOF is expected, partial reads are expected, and transient failures must be retried or gracefully handled. The deep dive should also cover how to observe the system, because without logs and trace points, you cannot reason about correctness.

  • How this fit on projects This concept is the backbone of the project because it defines how data and control flow move through the system.

  • Definitions & key terms

    • hook -> user-defined script executed on events
    • fork/exec -> process creation sequence to run an external command
    • timeout -> maximum time a hook is allowed to run
    • env vars -> environment variables passed to hook processes

  • Mental model diagram (ASCII)

[Input] -> [Event Hooks and Safe Process Execution] -> [State] -> [Output]

  • How it works (step-by-step, with invariants and failure modes)

    1. Register hook
    2. On event, fork child
    3. Set env context
    4. Enforce timeout
    5. Collect status

  • Minimal concrete example

hook add client-attached ./hooks/notify.sh

  • Common misconceptions

    • “Hooks can run in-process” -> they must not block the server.

  • Check-your-understanding questions

    • How do you avoid blocking the main loop?
    • What context should hooks receive?

  • Check-your-understanding answers

    • Run hooks in child processes.
    • Provide env vars like session name and pane id.

  • Real-world applications

    • tmux hooks
    • git hooks

  • Where you’ll apply it

  • References

    • APUE Ch. 8

  • Key insights Event Hooks and Safe Process Execution works best when you treat it as a stateful contract with explicit invariants.

  • Summary You now have a concrete mental model for Event Hooks and Safe Process Execution and can explain how it affects correctness and usability.

  • Homework/Exercises to practice the concept

    • Write a hook that logs events to a file.

  • Solutions to the homework/exercises

    • Use fork+exec and redirect output.

3. Project Specification

3.1 What You Will Build

A hook system that runs external scripts on events with timeout and context variables.

3.2 Functional Requirements

  1. Requirement 1: Register hooks per event
  2. Requirement 2: Execute hooks asynchronously
  3. Requirement 3: Provide environment variables
  4. Requirement 4: Timeout long-running hooks

3.3 Non-Functional Requirements

  • Performance: Avoid blocking I/O; batch writes when possible.
  • Reliability: Handle partial reads/writes and cleanly recover from disconnects.
  • Usability: Provide clear CLI errors, deterministic output, and helpful logs.

3.4 Example Usage / Output

    $ ./mytmux hook add client-attached ./hooks/notify.sh
[hook] registered client-attached
[exit code: 0]

$ ./mytmux hook add pane-exit ./hooks/missing.sh
[error] hook script not executable
[exit code: 2]

3.5 Data Formats / Schemas / Protocols

    Hook config: event -> command string.

3.6 Edge Cases

  • Hook hangs
  • Hook not executable

3.7 Real World Outcome

This section defines a deterministic, repeatable outcome. Use fixed inputs and set TZ=UTC where time appears.

3.7.1 How to Run (Copy/Paste)

make
./mytmux hook add client-attached ./hooks/notify.sh

3.7.2 Golden Path Demo (Deterministic)

The “success” demo below is a fixed scenario with a known outcome. It should always match.

3.7.3 If CLI: provide an exact terminal transcript

    $ ./mytmux hook add client-attached ./hooks/notify.sh
[hook] registered client-attached
[exit code: 0]

Failure Demo (Deterministic)

    $ ./mytmux hook add pane-exit ./hooks/missing.sh
[error] hook script not executable
[exit code: 2]

3.7.8 If TUI

At least one ASCII layout for the UI:

    +------------------------------+
    | Plugin System or Hooks           |
    | [content area]               |
    | [status / hints]             |
    +------------------------------+

4. Solution Architecture

4.1 High-Level Design

    +-----------+     +-----------+     +-----------+
    |  Client   | <-> |  Server   | <-> |  PTYs     |
    +-----------+     +-----------+     +-----------+

4.2 Key Components

| Component | Responsibility | Key Decisions | |-----------|----------------|---------------| | Hook registry | Stores event->command mapping. | Use a hash map. | | Executor | Runs hooks in child processes. | Fork+exec with timeout. | | Context builder | Sets env vars for hooks. | Include session/pane metadata. |

4.4 Data Structures (No Full Code)

    struct Hook { char event[64]; char cmd[256]; };

4.4 Algorithm Overview

Key Algorithm: Async hook execution

  1. Fork
  2. Exec hook
  3. Set alarm/timeout
  4. Collect exit status

Complexity Analysis:

  • O(1) per hook

5. Implementation Guide

5.1 Development Environment Setup

    cc --version
make --version

5.2 Project Structure

    hooks/
|-- src/
|   |-- hooks.c
|   `-- exec.c
`-- Makefile

5.3 The Core Question You’re Answering

“How do you let users extend behavior without modifying core code?”

5.4 Concepts You Must Understand First

  1. hooks
    • Why it matters and how it impacts correctness.
  2. async exec
    • Why it matters and how it impacts correctness.
  3. timeouts
    • Why it matters and how it impacts correctness.

5.5 Questions to Guide Your Design

  • How will you sandbox hook execution?
  • What is the timeout policy?

    5.6 Thinking Exercise

List events that should be hookable and why.

5.7 The Interview Questions They’ll Ask

  • What happens if a hook hangs?

    5.8 Hints in Layers

  • Run hooks asynchronously.

  • Use alarm or timerfd for timeouts.

5.9 Books That Will Help

| Topic | Book | Chapter | |——-|——|———| | Process execution | Advanced Programming in the UNIX Environment | Ch. 8 |

5.10 Implementation Phases

Phase 1: Foundation (2 weeks)

Goals:

  • Establish the core data structures and loop.
  • Prove basic I/O or rendering works.

Tasks:

  1. Implement the core structs and minimal main loop.
  2. Add logging for key events and errors.

Checkpoint: You can run the tool and see deterministic output.

Phase 2: Core Functionality (2 weeks)

Goals:

  • Implement the main requirements and pass basic tests.
  • Integrate with OS primitives.

Tasks:

  1. Implement remaining functional requirements.
  2. Add error handling and deterministic test fixtures.

Checkpoint: All functional requirements are met for the golden path.

Phase 3: Polish & Edge Cases (2 weeks)

Goals:

  • Handle edge cases and improve UX.
  • Optimize rendering or I/O.

Tasks:

  1. Add edge-case handling and exit codes.
  2. Improve logs and documentation.

Checkpoint: Failure demos behave exactly as specified.

5.11 Key Implementation Decisions

Decision Options Recommendation Rationale
I/O model blocking vs non-blocking non-blocking avoids stalls in multiplexed loops
Logging text vs binary text for v1 easier to inspect and debug

6. Testing Strategy

6.1 Test Categories

Category Purpose Examples
Unit Tests Validate components parser, buffer, protocol
Integration Tests Validate interactions end-to-end CLI flow
Edge Case Tests Handle boundary conditions resize, invalid input

6.2 Critical Test Cases

  1. Hook executes on event
  2. Timeout kills long hook

    6.3 Test Data

text Hook that sleeps 10s with timeout 1s should be killed.

7. Common Pitfalls & Debugging

7.1 Frequent Mistakes

| Pitfall | Symptom | Solution | |———|———|———-| | Hook blocks server | Ran in main loop | Use fork/exec with timeout. |

7.2 Debugging Strategies

  • Log hook start/end with exit status.

    7.3 Performance Traps

  • Spawning hooks for high-frequency events.

8. Extensions & Challenges

8.1 Beginner Extensions

  • Add list-hooks command.
  • Add remove-hook command.

    8.2 Intermediate Extensions

  • Add hook stdout capture.
  • Add per-hook timeout.

    8.3 Advanced Extensions

  • Add plugin discovery and load order.

9. Real-World Connections

9.1 Industry Applications

  • Editor plugins
  • CI hooks
  • tmux
  • git

    9.3 Interview Relevance

  • Event loops, terminal I/O, and state machines are common interview topics.

10. Resources

10.1 Essential Reading

11. Self-Assessment Checklist

11.1 Understanding

  • I can explain the core concept without notes
  • I can explain how input becomes output in this tool
  • I can explain the main failure modes

11.2 Implementation

  • All functional requirements are met
  • All test cases pass
  • Code is clean and well-documented
  • Edge cases are handled

11.3 Growth

  • I can identify one thing I’d do differently next time
  • I’ve documented lessons learned
  • I can explain this project in a job interview

12. Submission / Completion Criteria

Minimum Viable Completion:

  • Tool runs and passes the golden-path demo
  • Deterministic output matches expected snapshot
  • Failure demo returns the correct exit code

Full Completion:

  • All minimum criteria plus:
  • Edge cases handled and tested
  • Documentation covers usage and troubleshooting

Excellence (Going Above & Beyond):

  • Add at least one advanced extension
  • Provide a performance profile and improvement notes