# Go Concurrency Patterns

Patterns extracted from the Go standard library source code.

---

## 1. sync.Mutex — The Basic Lock

### Source: `src/sync/mutex.go:18-34`, `src/sync/mutex.go:42-67`

```go
// src/sync/mutex.go:18-34
// A Mutex is a mutual exclusion lock.
// The zero value for a Mutex is an unlocked mutex.
//
// A Mutex must not be copied after first use.
type Mutex struct {
    _ noCopy
    mu isync.Mutex
}

// src/sync/mutex.go:36-39
type Locker interface {
    Lock()
    Unlock()
}

// src/sync/mutex.go:43-46
func (m *Mutex) Lock() {
    m.mu.Lock()
}

// src/sync/mutex.go:64-67
func (m *Mutex) Unlock() {
    m.mu.Unlock()
}
```

### Why

- **Zero value is ready to use** — no constructor needed
- **Must not be copied** — enforced by `noCopy` field (go vet detects copies)
- **Not associated with a goroutine** — one goroutine can Lock, another can Unlock
- **Locker interface** — abstracts over Mutex and RWMutex

### Idiomatic Usage

```go
var mu sync.Mutex
mu.Lock()
defer mu.Unlock()
// ... critical section ...
```

### Anti-pattern

```go
// DON'T: Copy a mutex
type Config struct {
    mu sync.Mutex
    data map[string]string
}
c2 := *c1  // COPIES the mutex — data race waiting to happen

// DON'T: Forget defer
mu.Lock()
// if this panics, the mutex stays locked forever
doSomething()
mu.Unlock()
```

---

## 2. sync.Once — Exactly-Once Initialization

### Source: `src/sync/once.go:12-36`, `src/sync/once.go:56-79`

```go
// src/sync/once.go:12-23
type Once struct {
    _ noCopy
    done atomic.Bool
    m    Mutex
}

// src/sync/once.go:56-63
func (o *Once) Do(f func()) {
    if !o.done.Load() {
        o.doSlow(f)
    }
}

// src/sync/once.go:65-72
func (o *Once) doSlow(f func()) {
    o.m.Lock()
    defer o.m.Unlock()
    if !o.done.Load() {
        defer o.done.Store(true)
        f()
    }
}
```

### Why

The implementation reveals a subtle guarantee: **when Do returns, f has finished**. The naive CAS-only approach (documented in the comment at line 56-63) would let the second caller return before f completes. The mutex ensures all callers wait.

The `done` field is first in the struct for hot-path performance on amd64/386 (noted in comment at line 24-27).

### Idiomatic Usage

```go
var (
    instance *DB
    once     sync.Once
)

func GetDB() *DB {
    once.Do(func() {
        instance = connectToDB()
    })
    return instance
}
```

### Anti-pattern

```go
// DON'T: Implement once yourself with a bool
var initialized bool
var mu sync.Mutex
func init() {
    mu.Lock()
    if !initialized {
        // ... setup ...
        initialized = true
    }
    mu.Unlock()
}

// DON'T: Call Do recursively (deadlocks)
var once sync.Once
once.Do(func() {
    once.Do(func() { /* deadlock */ })
})
```

---

## 3. sync.WaitGroup — Waiting for Goroutine Completion

### Source: `src/sync/waitgroup.go:14-43`, `src/sync/waitgroup.go:236-260`

```go
// src/sync/waitgroup.go:14-43
// A WaitGroup is a counting semaphore typically used to wait
// for a group of goroutines or tasks to finish.
//
// Typically, a main goroutine will start tasks, each in a new
// goroutine, by calling WaitGroup.Go and then wait for all tasks to
// complete by calling WaitGroup.Wait. For example:
//
//   var wg sync.WaitGroup
//   wg.Go(task1)
//   wg.Go(task2)
//   wg.Wait()
type WaitGroup struct {
    noCopy noCopy
    state  atomic.Uint64
    sema   uint32
}
```

### Go 1.25+: WaitGroup.Go

```go
// src/sync/waitgroup.go:236-260
func (wg *WaitGroup) Go(f func()) {
    wg.Add(1)
    go func() {
        defer func() {
            if x := recover(); x != nil {
                panic(x)  // don't call Done — let panic propagate
            }
            wg.Done()
        }()
        f()
    }()
}
```

### Why

`WaitGroup.Go` (new in Go 1.25) encapsulates the Add/go/Done pattern. Key design: if `f` panics, it re-panics **without** calling Done, preventing the main goroutine from racing to exit.

### Classic Pattern (pre-Go 1.25)

```go
var wg sync.WaitGroup
for _, item := range items {
    wg.Add(1)
    go func() {
        defer wg.Done()
        process(item)
    }()
}
wg.Wait()
```

### Anti-pattern

```go
// DON'T: Add inside the goroutine (race with Wait)
for _, item := range items {
    go func() {
        wg.Add(1)  // might run after Wait is called!
        defer wg.Done()
        process(item)
    }()
}
wg.Wait()

// DON'T: Forget Done (Wait blocks forever)
wg.Add(1)
go func() {
    process()
    // forgot wg.Done()
}()
wg.Wait()  // hangs
```

---

## 4. sync.Pool — Object Reuse for GC Pressure

### Source: `src/sync/pool.go:44-63`

```go
// src/sync/pool.go:44-63
// A Pool is a set of temporary objects that may be individually saved and
// retrieved.
//
// Any item stored in the Pool may be removed automatically at any time without
// notification. If the Pool holds the only reference when this happens, the
// item might be deallocated.
//
// Pool's purpose is to cache allocated but unused items for later reuse,
// relieving pressure on the garbage collector.
type Pool struct {
    noCopy noCopy
    local     unsafe.Pointer
    localSize uintptr
    victim     unsafe.Pointer
    victimSize uintptr
    New func() any
}
```

### Why

Pool is **not** a general cache. Items can vanish between GC cycles. It's for reducing allocation pressure on hot paths — `fmt` uses it for print buffers, `encoding/json` for encoder state.

### Idiomatic Usage (from fmt package)

```go
var ppFree = sync.Pool{
    New: func() any { return new(pp) },
}

func newPrinter() *pp {
    p := ppFree.Get().(*pp)
    p.reset()
    return p
}

func (p *pp) free() {
    p.buf = p.buf[:0]  // reset before returning
    ppFree.Put(p)
}
```

### Anti-pattern

```go
// DON'T: Use Pool for connection pooling (items disappear!)
var connPool = sync.Pool{
    New: func() any { return connectToDB() },
}
// Connections may be GC'd at any time — use database/sql's pool instead

// DON'T: Put dirty objects back without resetting
pool.Put(buf)  // still has data from last use — memory leak or data leak
```

---

## 5. Channel as Done Signal (Context Pattern)

### Source: `src/context/context.go:83-100` (Done channel), `src/io/pipe.go:42-45`

```go
// src/context/context.go:83-100
// Done returns a channel that's closed when work done on behalf of this
// context should be canceled.
Done() <-chan struct{}

// src/io/pipe.go:42-45
type pipe struct {
    wrMu sync.Mutex
    wrCh chan []byte
    rdCh chan int
    once sync.Once
    done chan struct{}  // closed on pipe close
    rerr onceError
    werr onceError
}
```

### Why

A `chan struct{}` costs zero bytes per send and closing it broadcasts to all receivers simultaneously. This is the canonical "done" signal in Go.

```go
select {
case <-ctx.Done():
    return ctx.Err()
case result := <-work:
    return result, nil
}
```

### Anti-pattern

```go
// DON'T: Use chan bool for done signals
done := make(chan bool)  // wastes 1 byte per signal, true/false meaningless

// DON'T: Send to done (only works once, only one receiver)
done <- struct{}{}  // only unblocks one goroutine

// DO: Close the channel (broadcasts to all)
close(done)
```

---

## 6. Context Propagation

### Source: `src/context/context.go:37-48` (rules), `src/net/http/request.go:368-380`

From the package doc:
```go
// src/context/context.go:37-48
// Programs that use Contexts should follow these rules:
//
// Do not store Contexts inside a struct type; instead, pass a Context
// explicitly to each function that needs it. The Context should be the first
// parameter, typically named ctx:
//
//   func DoSomething(ctx context.Context, arg Arg) error {
//       // ... use ctx ...
//   }
//
// Do not pass a nil Context, even if a function permits it. Pass context.TODO
// if you are unsure about which Context to use.
```

### Request context in net/http:

```go
// src/net/http/request.go:368-380
func (r *Request) WithContext(ctx context.Context) *Request {
    if ctx == nil {
        panic("nil context")
    }
    r2 := new(Request)
    *r2 = *r
    r2.ctx = ctx
    return r2
}
```

### Why

Context flows **down** the call chain, never stored in structs. `WithContext` returns a shallow copy — the original request is not mutated. This is the immutable-context pattern.

### Anti-pattern

```go
// DON'T: Store context in a struct
type Server struct {
    ctx context.Context  // stale context persists beyond request lifecycle
}

// DON'T: Pass nil context
doWork(nil, data)  // use context.TODO() if unsure

// DON'T: Put context anywhere other than first parameter
func doWork(data Data, ctx context.Context)  // wrong position
```

---

## 7. Context Cancellation (WithCancel/WithTimeout)

### Source: `src/context/context.go:242-249` (WithCancel), `src/net/http/server.go:4007-4050` (TimeoutHandler)

```go
// src/context/context.go:242-249
func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
    c := withCancel(parent)
    return c, func() { c.cancel(true, Canceled, nil) }
}
```

Real-world use — net/http TimeoutHandler:
```go
// src/net/http/server.go:4011-4014
func (h *timeoutHandler) ServeHTTP(w ResponseWriter, r *Request) {
    ctx, cancelCtx := context.WithTimeout(r.Context(), h.dt)
    defer cancelCtx()
    r = r.WithContext(ctx)
    done := make(chan struct{})
    // ...
    go func() {
        h.handler.ServeHTTP(tw, r)
        close(done)
    }()
    select {
    case <-done:
        // handler completed
    case <-ctx.Done():
        // timeout
    }
}
```

### Why

`defer cancelCtx()` is critical — it releases resources (timers, goroutines) when the parent returns, even if the child hasn't timed out yet. The go vet tool checks for this.

### Anti-pattern

```go
// DON'T: Forget to call cancel (leaks goroutines)
ctx, _ := context.WithCancel(parent)  // cancel function discarded!

// DON'T: Cancel before work starts
ctx, cancel := context.WithTimeout(parent, 5*time.Second)
cancel()  // immediately cancels — no work can happen
doWork(ctx)
```

---

## 8. Select with Done Channel

### Source: `src/context/context.go:83-100` (Done in select), `src/io/pipe.go:51-60`

```go
// src/io/pipe.go:51-60
func (p *pipe) read(b []byte) (n int, err error) {
    select {
    case <-p.done:
        return 0, p.readCloseError()
    default:
    }

    select {
    case bw := <-p.wrCh:
        nr := copy(b, bw)
        p.rdCh <- nr
        return nr, nil
    case <-p.done:
        return 0, p.readCloseError()
    }
}
```

### Why

The double-select pattern: first a non-blocking check (with `default`), then a blocking wait. The non-blocking check prevents a race where `done` was closed between the last operation and the current one.

### Standard Context Select Pattern

```go
// From context package doc (line 83-100)
func Stream(ctx context.Context, out chan<- Value) error {
    for {
        v, err := DoSomething(ctx)
        if err != nil {
            return err
        }
        select {
        case <-ctx.Done():
            return ctx.Err()
        case out <- v:
        }
    }
}
```

### Anti-pattern

```go
// DON'T: Check ctx.Done() in a busy loop
for {
    select {
    case <-ctx.Done():
        return
    default:
    }
    // busy-spins CPU at 100%
}
```

---

## 9. Goroutine-per-Connection (net/http Server)

### Source: `src/net/http/server.go` (conceptual — the serve loop spawns goroutines per connection)

```go
// The pattern (simplified from server.go serve loop):
for {
    conn, err := listener.Accept()
    if err != nil {
        // handle
        continue
    }
    go srv.handleConn(conn)  // one goroutine per connection
}
```

### Why

Go's goroutines are cheap (~2KB initial stack). The server doesn't need a thread pool or async/await — it spawns a goroutine per connection and lets the runtime scheduler handle multiplexing.

### Anti-pattern

```go
// DON'T: Limit yourself to a fixed thread pool for I/O-bound work
pool := make(chan struct{}, 10)  // artificial limit on connections
for {
    pool <- struct{}{}  // blocks at 10
    conn := accept()
    go func() {
        defer func() { <-pool }()
        handle(conn)
    }()
}
// Only appropriate for CPU-bound work or resource-constrained scenarios
```

---

## 10. Channel as Synchronous Pipe (io.Pipe)

### Source: `src/io/pipe.go:38-45`, `src/io/pipe.go:195-205`

```go
// src/io/pipe.go:38-45
type pipe struct {
    wrMu sync.Mutex
    wrCh chan []byte   // writer sends data slices
    rdCh chan int      // reader returns bytes consumed
    once sync.Once
    done chan struct{}
    rerr onceError
    werr onceError
}

// src/io/pipe.go:195-205
func Pipe() (*PipeReader, *PipeWriter) {
    pw := &PipeWriter{r: PipeReader{pipe: pipe{
        wrCh: make(chan []byte),
        rdCh: make(chan int),
        done: make(chan struct{}),
    }}}
    return &pw.r, pw
}
```

### Why

`io.Pipe` connects a Writer to a Reader using **unbuffered channels** — each Write blocks until the corresponding Read consumes the data. No internal buffering means backpressure is automatic. The `done` channel signals when either end closes.

### Pattern: Channel Pipeline

```go
func generate(ctx context.Context) <-chan int {
    out := make(chan int)
    go func() {
        defer close(out)
        for i := 0; ; i++ {
            select {
            case out <- i:
            case <-ctx.Done():
                return
            }
        }
    }()
    return out
}
```

### Anti-pattern

```go
// DON'T: Forget to close channels (receivers block forever)
func produce() <-chan int {
    ch := make(chan int)
    go func() {
        for i := 0; i < 10; i++ {
            ch <- i
        }
        // forgot close(ch) — receivers hang on range
    }()
    return ch
}
```

---

## 11. database/sql Connection Opener Goroutine

### Source: `src/database/sql/sql.go:836-843`

```go
// src/database/sql/sql.go:836-843
func OpenDB(c driver.Connector) *DB {
    ctx, cancel := context.WithCancel(context.Background())
    db := &DB{
        connector: c,
        openerCh:  make(chan struct{}, connectionRequestQueueSize),
        lastPut:   make(map[*driverConn]string),
        stop:      cancel,
    }
    go db.connectionOpener(ctx)
    return db
}
```

### Why

A dedicated background goroutine (`connectionOpener`) processes connection requests from a buffered channel. The goroutine is controlled by a context — calling `cancel()` (stored as `db.stop`) shuts it down cleanly. This is the "long-lived worker goroutine with context shutdown" pattern.

### Anti-pattern

```go
// DON'T: Start background goroutines without shutdown mechanism
go func() {
    for {
        processWork()  // runs forever, no way to stop it
    }
}()
```

---

## 12. noCopy — Preventing Value Copies at Vet Time

### Source: `src/sync/cond.go:120-126`

```go
// src/sync/cond.go:120-126
type noCopy struct{}

// Lock is a no-op used by -copylocks checker from `go vet`.
func (*noCopy) Lock()   {}
func (*noCopy) Unlock() {}
```

### Why

Embedding `noCopy` in a struct makes `go vet` report an error when the struct is copied. All sync primitives use this because copying a locked mutex or in-use WaitGroup is always a bug.

### Anti-pattern

```go
// DON'T: Pass sync types by value
func doWork(wg sync.WaitGroup) {  // copies the WaitGroup!
    defer wg.Done()
}

// DO: Pass by pointer
func doWork(wg *sync.WaitGroup) {
    defer wg.Done()
}
```

---

## Summary: Concurrency Decision Guide

| Need | Use |
|------|-----|
| Protect shared state | `sync.Mutex` + `defer Unlock()` |
| One-time initialization | `sync.Once` |
| Wait for N goroutines | `sync.WaitGroup` (prefer `.Go()` in 1.25+) |
| Reduce allocation pressure | `sync.Pool` (not for connections!) |
| Signal completion/cancellation | `chan struct{}` + `close()` |
| Deadline/timeout propagation | `context.WithTimeout` / `context.WithCancel` |
| Backpressure between producer/consumer | Unbuffered channels |
| Fan-out with results | Buffered channel + WaitGroup |
| Long-lived background worker | Goroutine + context cancellation |
| Prevent struct copying | Embed `noCopy` field |