Consider the verification time as well as the expiration time when caching certificate verification…

net/base/expiring_cache.h

 // Copyright (c) 2012 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.
 
 #ifndef NET_BASE_EXPIRING_CACHE_H_
 #define NET_BASE_EXPIRING_CACHE_H_
 #pragma once
 
 #include <map>
 #include <utility>
 
 #include "base/basictypes.h"
 #include "base/gtest_prod_util.h"
 #include "base/time.h"
 
 namespace net {
 
 // Cache implementation where all entries have an explicit expiration policy. As
 // new items are added, expired items will be removed first.
 // The template types have the following requirements:
 //  KeyType must be LessThanComparable, Assignable, and CopyConstructible.
 //  ValueType must be CopyConstructible and Assignable.
-template <typename KeyType, typename ValueType>
+//  ExpirationType must be CopyConstructible and Assignable.
+//  ExpirationCompare is a function class that takes two arguments of the
+//    type ExpirationType and returns a bool. If |comp| is an instance of
+//    ExpirationCompare, then the expression |comp(current, expiration)| shall
+//    return true iff |current| is still valid within |expiration|.
+//
+// A simple use of this class may use base::TimeTicks, which provides a
+// monotonically increasing clock, for the expiration type. Because it's always
+// increasing, std::less<> can be used, which will simply ensure that |now| is
+// sorted before |expiration|:
+//
+//   ExpiringCache<std::string, std::string, base::TimeTicks,
+//                 std::less<base::TimeTicks> > cache(0);
+//   // Add a value that expires in 5 minutes
+//   cache.Put("key1", "value1", base::TimeTicks::Now(),
+//             base::TimeTicks::Now() + base::TimeDelta::FromMinutes(5));
+//   // Add another value that expires in 10 minutes.
+//   cache.Put("key2", "value2", base::TimeTicks::Now(),
+//             base::TimeTicks::Now() + base::TimeDelta::FromMinutes(10));
+//
+// Alternatively, there may be some more complex expiration criteria, at which
+// point a custom functor may be used:
+//
+//   struct ComplexExpirationFunctor {
+//     bool operator()(const ComplexExpiration& now,
+//                     const ComplexExpiration& expiration) const;
+//   };
+//   ExpiringCache<std::string, std::string, ComplexExpiration,
+//                 ComplexExpirationFunctor> cache(15);
+//   // Add a value that expires once the 'sprocket' has 'cog'-ified.
+//   cache.Put("key1", "value1", ComplexExpiration("sprocket"),
+//             ComplexExpiration("cog"));
+template <typename KeyType,
+          typename ValueType,
+          typename ExpirationType,
+          typename ExpirationCompare>
 class ExpiringCache {
  private:
   // Intentionally violate the C++ Style Guide so that EntryMap is known to be
   // a dependent type. Without this, Clang's two-phase lookup complains when
   // using EntryMap::const_iterator, while GCC and MSVC happily resolve the
   // typename.
 
   // Tuple to represent the value and when it expires.
-  typedef std::pair<ValueType, base::TimeTicks> Entry;
+  typedef std::pair<ValueType, ExpirationType> Entry;
   typedef std::map<KeyType, Entry> EntryMap;
 
  public:
   typedef KeyType key_type;
   typedef ValueType value_type;
+  typedef ExpirationType expiration_type;
 
   // This class provides a read-only iterator over items in the ExpiringCache
   class Iterator {
    public:
     explicit Iterator(const ExpiringCache& cache)
         : cache_(cache),
           it_(cache_.entries_.begin()) {
     }
     ~Iterator() {}
 
     bool HasNext() const { return it_ != cache_.entries_.end(); }
     void Advance() { ++it_; }
 
     const KeyType& key() const { return it_->first; }
     const ValueType& value() const { return it_->second.first; }
-    const base::TimeTicks& expiration() const { return it_->second.second; }
+    const ExpirationType& expiration() const { return it_->second.second; }
 
    private:
     const ExpiringCache& cache_;
 
     // Use a second layer of type indirection, as both EntryMap and
     // EntryMap::const_iterator are dependent types.
     typedef typename ExpiringCache::EntryMap EntryMap;
     typename EntryMap::const_iterator it_;
   };
 
 
   // Constructs an ExpiringCache that stores up to |max_entries|.
   explicit ExpiringCache(size_t max_entries) : max_entries_(max_entries) {}
   ~ExpiringCache() {}
 
   // Returns the value matching |key|, which must be valid at the time |now|.
   // Returns NULL if the item is not found or has expired. If the item has
   // expired, it is immediately removed from the cache.
   // Note: The returned pointer remains owned by the ExpiringCache and is
   // invalidated by a call to a non-const method.
-  const ValueType* Get(const KeyType& key, base::TimeTicks now) {
+  const ValueType* Get(const KeyType& key, const ExpirationType& now) {
     typename EntryMap::iterator it = entries_.find(key);
     if (it == entries_.end())
       return NULL;
 
     // Immediately remove expired entries.
-    if (!CanUseEntry(it->second, now)) {
+    if (!expiration_comp_(now, it->second.second)) {
       entries_.erase(it);
       return NULL;
     }
 
     return &it->second.first;
   }
 
   // Updates or replaces the value associated with |key|.
   void Put(const KeyType& key,
            const ValueType& value,
-           base::TimeTicks now,
-           base::TimeDelta ttl) {
-    base::TimeTicks expiration = now + ttl;
+           const ExpirationType& now,
+           const ExpirationType& expiration) {
     typename EntryMap::iterator it = entries_.find(key);
     if (it == entries_.end()) {
       // Compact the cache if it grew beyond the limit.
       if (entries_.size() == max_entries_ )
         Compact(now);
 
       // No existing entry. Creating a new one.
       entries_.insert(std::make_pair(key, Entry(value, expiration)));
     } else {
       // Update an existing cache entry.
@@ skipping 10 matching lines @@
   // Returns the number of entries in the cache.
   size_t size() const { return entries_.size(); }
 
   // Returns the maximum number of entries in the cache.
   size_t max_entries() const { return max_entries_; }
 
   bool empty() const { return entries_.empty(); }
 
  private:
   FRIEND_TEST_ALL_PREFIXES(ExpiringCacheTest, Compact);
-
-  // Returns true if this cache entry's result is valid at time |now|.
-  static bool CanUseEntry(const Entry& entry, const base::TimeTicks now) {
-    return entry.second > now;
-  }
+  FRIEND_TEST_ALL_PREFIXES(ExpiringCacheTest, CustomFunctor);
 
   // Prunes entries from the cache to bring it below |max_entries()|.
-  void Compact(base::TimeTicks now) {
+  void Compact(const ExpirationType& now) {
     // Clear out expired entries.
     typename EntryMap::iterator it;
     for (it = entries_.begin(); it != entries_.end(); ) {
-      if (!CanUseEntry(it->second, now)) {
+      if (!expiration_comp_(now, it->second.second)) {
         entries_.erase(it++);
       } else {
         ++it;
       }
     }
 
     if (entries_.size() < max_entries_)
       return;
 
     // If the cache is still too full, start deleting items 'randomly'.
     for (it = entries_.begin();
          it != entries_.end() && entries_.size() >= max_entries_;) {
       entries_.erase(it++);
     }
   }
 
   // Bound on total size of the cache.
   size_t max_entries_;
 
   EntryMap entries_;
+  ExpirationCompare expiration_comp_;
 
   DISALLOW_COPY_AND_ASSIGN(ExpiringCache);
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_EXPIRING_CACHE_H_