diff --git a/src/time/ClockCache.hxx b/src/time/ClockCache.hxx
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+++ b/src/time/ClockCache.hxx
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+/*
+ * Copyright 2007-2019 Content Management AG
+ * All rights reserved.
+ *
+ * author: Max Kellermann <mk@cm4all.com>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <chrono>
+
+/**
+ * Cache the now() method of a clock.
+ */
+template<typename Clock>
+class ClockCache {
+	using value_type = typename Clock::time_point;
+	mutable value_type value;
+
+public:
+	ClockCache() = default;
+	ClockCache(const ClockCache &) = delete;
+	ClockCache &operator=(const ClockCache &) = delete;
+
+	gcc_pure
+	const auto &now() const noexcept {
+		if (value <= value_type())
+			value = Clock::now();
+		return value;
+	}
+
+	void flush() noexcept {
+		value = {};
+	}
+
+	/**
+	 * Inject a fake value.  This can be helpful for unit tests.
+	 */
+	void Mock(value_type _value) noexcept {
+		value = _value;
+	}
+};