Add context-aware "time" library wrapper.

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

Index: go/src/infra/libs/clock/context.go
diff --git a/go/src/infra/libs/clock/context.go b/go/src/infra/libs/clock/context.go
new file mode 100644
index 0000000000000000000000000000000000000000..66af5ac5bd589576f526196c027196550c926112
--- /dev/null
+++ b/go/src/infra/libs/clock/context.go
@@ -0,0 +1,71 @@
+// Copyright 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
+
+// SetFactory creates a new Context using the supplied Clock factory.
+func SetFactory(ctx context.Context, f Factory) context.Context {
+	return context.WithValue(ctx, clockKey, f)
+}
+
+// Set creates a new Context using the supplied Clock.
+func Set(ctx context.Context, c Clock) context.Context {
+	return SetFactory(ctx, func(context.Context) Clock { return c })
+}
+
+// Get returns the Clock set in the supplied Context, defaulting to
+// SystemClock() if none is set.
+func Get(ctx context.Context) (clock Clock) {
+	v := ctx.Value(clockKey)
+	if v != nil {
+		f := v.(Factory)
+		if f != nil {
+			clock = f(ctx)
+		}
+	}
+	if clock == nil {
+		clock = GetSystemClock()
+	}
+	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 Get(ctx).Now()
+}
+
+// Sleep calls Clock.Sleep on the Clock instance stored in the supplied Context.
+func Sleep(ctx context.Context, d time.Duration) {
+	Get(ctx).Sleep(d)
+}
+
+// NewTimer calls Clock.NewTimer on the Clock instance stored in the supplied
+// Context.
+func NewTimer(ctx context.Context) Timer {
+	return Get(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 Get(ctx).After(d)
+}