Agent skill

m12-lifecycle

Use when designing resource lifecycles. Keywords: RAII, Drop, resource lifecycle, connection pool, lazy initialization, connection pool design, resource cleanup patterns, cleanup, scope, OnceCell, Lazy, once_cell, OnceLock, transaction, session management, when is Drop called, cleanup on error, guard pattern, scope guard, 资源生命周期, 连接池, 惰性初始化, 资源清理, RAII 模式

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SKILL.md

Resource Lifecycle

Layer 2: Design Choices

Core Question

When should this resource be created, used, and cleaned up?

Before implementing lifecycle:

  • What's the resource's scope?
  • Who owns the cleanup responsibility?
  • What happens on error?

Lifecycle Pattern → Implementation

Pattern When Implementation
RAII Auto cleanup Drop trait
Lazy init Deferred creation OnceLock, LazyLock
Pool Reuse expensive resources r2d2, deadpool
Guard Scoped access MutexGuard pattern
Scope Transaction boundary Custom struct + Drop

Thinking Prompt

Before designing lifecycle:

  1. What's the resource cost?

    • Cheap → create per use
    • Expensive → pool or cache
    • Global → lazy singleton

  2. What's the scope?

    • Function-local → stack allocation
    • Request-scoped → passed or extracted
    • Application-wide → static or Arc

  3. What about errors?

    • Cleanup must happen → Drop
    • Cleanup is optional → explicit close
    • Cleanup can fail → Result from close

Trace Up ↑

To domain constraints (Layer 3):

"How should I manage database connections?"
    ↑ Ask: What's the connection cost?
    ↑ Check: domain-* (latency requirements)
    ↑ Check: Infrastructure (connection limits)
Question Trace To Ask
Connection pooling domain-* What's acceptable latency?
Resource limits domain-* What are infra constraints?
Transaction scope domain-* What must be atomic?

Trace Down ↓

To implementation (Layer 1):

"Need automatic cleanup"
    ↓ m02-resource: Implement Drop
    ↓ m01-ownership: Clear owner for cleanup

"Need lazy initialization"
    ↓ m03-mutability: OnceLock for thread-safe
    ↓ m07-concurrency: LazyLock for sync

"Need connection pool"
    ↓ m07-concurrency: Thread-safe pool
    ↓ m02-resource: Arc for sharing

Quick Reference

Pattern Type Use Case
RAII Drop trait Auto cleanup on scope exit
Lazy Init OnceLock, LazyLock Deferred initialization
Pool r2d2, deadpool Connection reuse
Guard MutexGuard Scoped lock release
Scope Custom struct Transaction boundaries

Lifecycle Events

Event Rust Mechanism
Creation new(), Default
Lazy Init OnceLock::get_or_init
Usage &self, &mut self
Cleanup Drop::drop()

Pattern Templates

RAII Guard

rust
struct FileGuard {
    path: PathBuf,
    _handle: File,
}

impl Drop for FileGuard {
    fn drop(&mut self) {
        // Cleanup: remove temp file
        let _ = std::fs::remove_file(&self.path);
    }
}

Lazy Singleton

rust
use std::sync::OnceLock;

static CONFIG: OnceLock<Config> = OnceLock::new();

fn get_config() -> &'static Config {
    CONFIG.get_or_init(|| {
        Config::load().expect("config required")
    })
}

Common Errors

Error Cause Fix
Resource leak Forgot Drop Implement Drop or RAII wrapper
Double free Manual memory Let Rust handle
Use after drop Dangling reference Check lifetimes
E0509 move out of Drop Moving owned field Option::take()
Pool exhaustion Not returned Ensure Drop returns

Anti-Patterns

Anti-Pattern Why Bad Better
Manual cleanup Easy to forget RAII/Drop
lazy_static! External dep std::sync::OnceLock
Global mutable state Thread unsafety OnceLock or proper sync
Forget to close Resource leak Drop impl

Related Skills

When See
Smart pointers m02-resource
Thread-safe init m07-concurrency
Domain scopes m09-domain
Error in cleanup m06-error-handling

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