Add context-aware "time" library wrapper.

go/src/infra/libs/clock/context.go

+// Copyright (c) 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package clock
+
+import (
+        "time"
+
+        "golang.org/x/net/context"
+)
+
+// Context key for clock.
+type clockKeyType int
+
+// Unique value for clock key.
+var clockKey clockKeyType
+
+// Factory is a generator function that produces a Clock instnace.
+type Factory func(context.Context) Clock
+
+// SetClockFactory creates a new Context using the supplied Clock factory.
+func SetClockFactory(ctx context.Context, f Factory) context.Context {
+        return context.WithValue(ctx, clockKey, f)
+}
+
+// SetClock creates a new Context using the supplied Clock.
+func SetClock(ctx context.Context, c Clock) context.Context {
+        return SetClockFactory(ctx, func(ctx context.Context) Clock { return c })
+}
+
+// GetClock returns the Clock set in the supplied Context, defaulting to SystemClock()
+// if none is set.
+func GetClock(ctx context.Context) (clock Clock) {
+        v := ctx.Value(clockKey)
+        if v != nil {
+                f := v.(func(context.Context) Clock)
+                if f != nil {
+                        clock = f(ctx)
+                }
+        }
+        if clock == nil {
+                panic("No Clock instance has been set in this context.")
+        }
+        return
+}
+
+//
+// "Implement" the Clock interface at the package level.
+//
+
+// Now calls Clock.Now on the Clock instance stored in the supplied Context.
+func Now(ctx context.Context) time.Time {
+        return GetClock(ctx).Now()
+}
+
+// Sleep calls Clock.Sleep on the Clock instance stored in the supplied Context.
+func Sleep(ctx context.Context, d time.Duration) {
+        GetClock(ctx).Sleep(d)
+}
+
+// NewTimer calls Clock.NewTimer on the Clock instance stored in the supplied Context.
+func NewTimer(ctx context.Context) Timer {
+        return GetClock(ctx).NewTimer()
+}
+
+// After calls Clock.After on the Clock instance stored in the supplied Context.
+func After(ctx context.Context, d time.Duration) <-chan time.Time {
+        return GetClock(ctx).After(d)
+}