fn: expand the ContextGuard and add tests

fn/context_guard.go

@@ -3,6 +3,7 @@ package fn
 import (
 	"context"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -11,103 +12,234 @@ var (
 	DefaultTimeout = 30 * time.Second
 )
 
-// ContextGuard is an embeddable struct that provides a wait group and main quit
-// channel that can be used to create guarded contexts.
+// ContextGuard is a struct that provides a wait group and main quit channel
+// that can be used to create guarded contexts.
 type ContextGuard struct {
-	DefaultTimeout time.Duration
-	Wg             sync.WaitGroup
-	Quit           chan struct{}
+	mu sync.Mutex
+	wg sync.WaitGroup
+
+	quit    chan struct{}
+	stopped sync.Once
+
+	// id is used to generate unique ids for each context that should be
+	// cancelled when the main quit signal is triggered.
+	id atomic.Uint32
+
+	// cancelFns is a map of cancel functions that can be used to cancel
+	// any context that should be cancelled when the main quit signal is
+	// triggered. The key is the id of the context. The mutex must be held
+	// when accessing this map.
+	cancelFns map[uint32]context.CancelFunc
 }
 
+// NewContextGuard constructs and returns a new instance of ContextGuard.
 func NewContextGuard() *ContextGuard {
 	return &ContextGuard{
-		DefaultTimeout: DefaultTimeout,
-		Quit:           make(chan struct{}),
+		quit:      make(chan struct{}),
+		cancelFns: make(map[uint32]context.CancelFunc),
 	}
 }
 
-// WithCtxQuit is used to create a cancellable context that will be cancelled
-// if the main quit signal is triggered or after the default timeout occurred.
-func (g *ContextGuard) WithCtxQuit() (context.Context, func()) {
-	return g.WithCtxQuitCustomTimeout(g.DefaultTimeout)
-}
+// Quit is used to signal the main quit channel, which will cancel all
+// non-blocking contexts derived from the ContextGuard.
+func (g *ContextGuard) Quit() {
+	g.stopped.Do(func() {
+		g.mu.Lock()
+		defer g.mu.Unlock()
 
-// WithCtxQuitCustomTimeout is used to create a cancellable context that will be
-// cancelled if the main quit signal is triggered or after the given timeout
-// occurred.
-func (g *ContextGuard) WithCtxQuitCustomTimeout(
-	timeout time.Duration) (context.Context, func()) {
+		for _, cancel := range g.cancelFns {
+			cancel()
+		}
 
-	timeoutTimer := time.NewTimer(timeout)
-	ctx, cancel := context.WithCancel(context.Background())
+		close(g.quit)
+	})
+}
 
-	g.Wg.Add(1)
-	go func() {
-		defer timeoutTimer.Stop()
-		defer cancel()
-		defer g.Wg.Done()
+// Done returns a channel that will be closed when the main quit signal is
+// triggered.
+func (g *ContextGuard) Done() <-chan struct{} {
+	return g.quit
+}
 
-		select {
-		case <-g.Quit:
+// WgAdd is used to add delta to the internal wait group of the ContextGuard.
+func (g *ContextGuard) WgAdd(delta int) {
+	g.wg.Add(delta)
+}
 
-		case <-timeoutTimer.C:
+// WgDone is used to decrement the internal wait group of the ContextGuard.
+func (g *ContextGuard) WgDone() {
+	g.wg.Done()
+}
 
-		case <-ctx.Done():
-		}
-	}()
+// WgWait is used to block until the internal wait group of the ContextGuard is
+// empty.
+func (g *ContextGuard) WgWait() {
+	g.wg.Wait()
+}
 
-	return ctx, cancel
+// ctxGuardOptions is used to configure the behaviour of the context derived
+// via the WithCtx method of the ContextGuard.
+type ctxGuardOptions struct {
+	blocking    bool
+	withTimeout bool
+	timeout     time.Duration
 }
 
-// CtxBlocking is used to create a cancellable context that will NOT be
+// ContextGuardOption defines the signature of a functional option that can be
+// used to configure the behaviour of the context derived via the WithCtx method
+// of the ContextGuard.
+type ContextGuardOption func(*ctxGuardOptions)
+
+// WithBlockingCG is used to create a cancellable context that will NOT be
 // cancelled if the main quit signal is triggered, to block shutdown of
-// important tasks. The context will be cancelled if the timeout is reached.
-func (g *ContextGuard) CtxBlocking() (context.Context, func()) {
-	return g.CtxBlockingCustomTimeout(g.DefaultTimeout)
+// important tasks.
+func WithBlockingCG() ContextGuardOption {
+	return func(o *ctxGuardOptions) {
+		o.blocking = true
+	}
+}
+
+// WithCustomTimeoutCG is used to create a cancellable context with a custom
+// timeout. Such a context will be cancelled if either the parent context is
+// cancelled, the timeout is reached or, if the Blocking option is not provided,
+// the main quit signal is triggered.
+func WithCustomTimeoutCG(timeout time.Duration) ContextGuardOption {
+	return func(o *ctxGuardOptions) {
+		o.withTimeout = true
+		o.timeout = timeout
+	}
+}
+
+// WithTimeoutCG is used to create a cancellable context with a default timeout.
+// Such a context will be cancelled if either the parent context is cancelled,
+// the timeout is reached or, if the Blocking option is not provided, the main
+// quit signal is triggered.
+func WithTimeoutCG() ContextGuardOption {
+	return func(o *ctxGuardOptions) {
+		o.withTimeout = true
+		o.timeout = DefaultTimeout
+	}
 }
 
-// CtxBlockingCustomTimeout is used to create a cancellable context with a
-// custom timeout that will NOT be cancelled if the main quit signal is
-// triggered, to block shutdown of important tasks. The context will be
-// cancelled if the timeout is reached.
-func (g *ContextGuard) CtxBlockingCustomTimeout(
-	timeout time.Duration) (context.Context, func()) {
+// Create is used to derive a cancellable context from the parent. Various
+// options can be provided to configure the behaviour of the derived context.
+func (g *ContextGuard) Create(ctx context.Context,
+	options ...ContextGuardOption) (context.Context, context.CancelFunc) {
 
-	timeoutTimer := time.NewTimer(timeout)
-	ctx, cancel := context.WithCancel(context.Background())
+	// Exit early if the parent context has already been cancelled.
+	select {
+	case <-ctx.Done():
+		return ctx, func() {}
+	default:
+	}
+
+	var opts ctxGuardOptions
+	for _, o := range options {
+		o(&opts)
+	}
 
-	g.Wg.Add(1)
+	g.mu.Lock()
+	defer g.mu.Unlock()
+
+	var cancel context.CancelFunc
+	if opts.withTimeout {
+		ctx, cancel = context.WithTimeout(ctx, opts.timeout)
+	} else {
+		ctx, cancel = context.WithCancel(ctx)
+	}
+
+	if opts.blocking {
+		g.ctxBlocking(ctx, cancel)
+
+		return ctx, cancel
+	}
+
+	// If the call is non-blocking, then we can exit early if the main quit
+	// signal has been triggered.
+	select {
+	case <-g.quit:
+		cancel()
+
+		return ctx, cancel
+	default:
+	}
+
+	cancel = g.ctxQuitUnsafe(ctx, cancel)
+
+	return ctx, cancel
+}
+
+// ctxQuitUnsafe spins off a goroutine that will block until the passed context
+// is cancelled or until the quit channel has been signaled after which it will
+// call the passed cancel function and decrement the wait group.
+//
+// NOTE: the caller must hold the ContextGuard's mutex before calling this
+// function.
+func (g *ContextGuard) ctxQuitUnsafe(ctx context.Context,
+	cancel context.CancelFunc) context.CancelFunc {
+
+	cancel = g.addCancelFnUnsafe(cancel)
+
+	g.wg.Add(1)
 	go func() {
-		defer timeoutTimer.Stop()
 		defer cancel()
-		defer g.Wg.Done()
+		defer g.wg.Done()
 
 		select {
-		case <-timeoutTimer.C:
+		case <-g.quit:
 
 		case <-ctx.Done():
 		}
 	}()
 
-	return ctx, cancel
+	return cancel
 }
 
-// WithCtxQuitNoTimeout is used to create a cancellable context that will be
-// cancelled if the main quit signal is triggered.
-func (g *ContextGuard) WithCtxQuitNoTimeout() (context.Context, func()) {
-	ctx, cancel := context.WithCancel(context.Background())
+// ctxBlocking spins off a goroutine that will block until the passed context
+// is cancelled after which it will call the passed cancel function and
+// decrement the wait group.
+func (g *ContextGuard) ctxBlocking(ctx context.Context,
+	cancel context.CancelFunc) {
 
-	g.Wg.Add(1)
+	g.wg.Add(1)
 	go func() {
 		defer cancel()
-		defer g.Wg.Done()
+		defer g.wg.Done()
 
 		select {
-		case <-g.Quit:
-
 		case <-ctx.Done():
 		}
 	}()
+}
 
-	return ctx, cancel
+// addCancelFnUnsafe adds a context cancel function to the manager and returns a
+// call-back which can safely be used to cancel the context.
+//
+// NOTE: the caller must hold the ContextGuard's mutex before calling this
+// function.
+func (g *ContextGuard) addCancelFnUnsafe(
+	cancel context.CancelFunc) context.CancelFunc {
+
+	id := g.id.Add(1)
+	g.cancelFns[id] = cancel
+
+	return g.cancelCtxFn(id)
+}
+
+// cancelCtxFn returns a call-back that can be used to cancel the context
+// associated with the passed id.
+func (g *ContextGuard) cancelCtxFn(id uint32) context.CancelFunc {
+	return func() {
+		g.mu.Lock()
+
+		fn, ok := g.cancelFns[id]
+		if !ok {
+			g.mu.Unlock()
+			return
+		}
+		delete(g.cancelFns, id)
+		g.mu.Unlock()
+
+		fn()
+	}
 }