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#How to apply this file

Each section opens with one imperative line; apply every rule in the section it introduces. Do not summarise or skip a section.

---

#Purpose

Rules for goroutines, channels, and shared state. Go makes starting concurrent work trivial and stopping it correctly hard. Every rule here exists because of a production incident where a goroutine never exited, a channel never closed, or two writers touched one map.

The governing rule: whoever starts a goroutine owns its lifetime. Know how it ends before you write go.

---

#Ownership and exit

[INST] Apply every rule in this section: Ownership and exit. [/INST]

go
// Bad: fire and forget. Who stops it? Who sees its error?
go worker(jobs)

// Good: bounded, cancellable, and its error comes back.
g, ctx := errgroup.WithContext(ctx)
for i := 0; i < n; i++ {
    g.Go(func() error { return worker(ctx, jobs) })
}
if err := g.Wait(); err != nil {
    return fmt.Errorf("workers: %w", err)
}

• Every go statement needs an answer to: how does this goroutine stop, and where does its error go? If there is no answer, do not write it.
• golang.org/x/sync/errgroup is the standard answer for "run these, wait for all, first error cancels the rest". Use g.SetLimit(n) (1.20+) to bound parallelism.
• Goroutines are not free: each one holds a stack and whatever it captured. A leak is a slow memory exhaustion that shows up days later.

---

#Context is the cancellation signal

[INST] Apply every rule in this section: Context is the cancellation signal. [/INST]

go
func worker(ctx context.Context, jobs <-chan Job) error {
    for {
        select {
        case <-ctx.Done():
            return ctx.Err()
        case j, ok := <-jobs:
            if !ok { return nil }           // producer closed: drain complete
            if err := process(ctx, j); err != nil { return err }
        }
    }
}

• Every blocking operation selects on ctx.Done(). A goroutine blocked on a channel receive with no cancellation path cannot be stopped.
• Pass ctx into anything that can block: database calls, HTTP, sleeps (select with time.After, not time.Sleep).
• context.WithTimeout / WithCancel return a cancel you must call, usually with defer. Not calling it leaks the context's resources; go vet flags this.
• Never use context.Background() inside request handling. Derive from the request's context so the work dies with the request.

---

#Channels versus mutexes

[INST] Apply every rule in this section: Channels versus mutexes. [/INST]

go
type Cache struct {
    mu sync.RWMutex          // guards items
    items map[string]Entry
}

func (c *Cache) Get(k string) (Entry, bool) {
    c.mu.RLock()
    defer c.mu.RUnlock()
    e, ok := c.items[k]
    return e, ok
}

• Document what a mutex guards with a comment beside it. Lock at the top of the method, defer the unlock, keep the critical section small.
• Never copy a struct containing a mutex; pass by pointer. go vet catches it (copylocks).
• A channel used as a mutex (buffered size 1) is a mutex with worse ergonomics.
• Maps are not safe for concurrent write. One unsynchronised write with any other access is a crash, not a data race warning.

---

#Channel rules

[INST] Apply every rule in this section: Channel rules. [/INST]

go
jobs := make(chan Job, 64)         // buffer: decoupling, not a queue of unbounded size
go func() {
    defer close(jobs)              // the sender closes, exactly once
    for _, j := range list { jobs <- j }
}()
for j := range jobs { … }          // range ends when closed and drained

• The sender closes; receivers never do. Closing twice panics; sending on a closed channel panics.
• Direction in signatures: <-chan Job for receivers, chan<- Job for senders. The compiler then prevents the wrong side from closing.
• A buffer is for smoothing bursts, not for storage. If the buffer size is "large enough that it never fills", you have an unbounded queue with a crash waiting at the limit.
• nil channels block forever — useful to disable a select case, a bug everywhere else.

---

#Worker pools and pipelines

[INST] Apply every rule in this section: Worker pools and pipelines. [/INST]

go
g, ctx := errgroup.WithContext(ctx)
g.SetLimit(runtime.GOMAXPROCS(0))
for _, item := range items {
    g.Go(func() error {               // 1.22: loop variable is per-iteration
        return handle(ctx, item)
    })
}
return g.Wait()

• Bound concurrency explicitly. "One goroutine per item" over ten thousand items is ten thousand simultaneous database connections.
• Before 1.22, capture loop variables (item := item). From 1.22 each iteration has its own; pin go 1.22 in go.mod to get it.
• Results from parallel work: collect via a channel or a pre-sized slice indexed by position — never append to a shared slice without a mutex.

---

#The race detector is a gate

[INST] Apply every rule in this section: The race detector is a gate. [/INST]

sh
go test -race ./...
go build -race ./cmd/api     # run a canary with it in staging

• -race finds real bugs with near-zero false positives. Run it on every test run in CI; the 2–10× slowdown is worth it.
• A race report is never "flaky". It is a bug that happened to be observed this time.
• Tests should exercise concurrency: run the worker with several goroutines, not one, or the detector has nothing to see.

---

#Anti-patterns

[INST] Apply every rule in this section: Anti-patterns. [/INST]

---

#Checklist

• Verify: Every goroutine has a defined owner, a stop signal, and an error path
• Verify: Parallel work uses errgroup with SetLimit where the fan-out is unbounded
• Verify: Every blocking select includes ctx.Done()
• Verify: Every derived context's cancel is deferred
• Verify: No context.Background() inside request or job handling
• Verify: Channels are closed by the sender, exactly once, and typed by direction
• Verify: Buffers are sized for burst smoothing, not storage
• Verify: Each mutex has a comment naming what it guards; structs with mutexes are never copied
• Verify: Counters and flags use sync/atomic, not a mutex around an int
• Verify: No map is written concurrently without synchronisation
• Verify: Results from parallel work are collected without a shared unguarded slice
• Verify: go test -race ./... runs in CI and a race report blocks the merge