Avoid doing concurrent DNS resolves of the same hostname

net/base/host_cache.cc

+// Copyright (c) 2009 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.
+
+#include "net/base/host_cache.h"
+
+#include "base/logging.h"
+#include "net/base/net_errors.h"
+
+namespace net {
+
+//-----------------------------------------------------------------------------
+
+HostCache::Entry::Entry(int error,
+                        const AddressList& addrlist,
+                        base::TimeTicks expiration)
+    : error(error), addrlist(addrlist), expiration(expiration) {
+}
+
+HostCache::Entry::~Entry() {
+}
+
+//-----------------------------------------------------------------------------
+
+HostCache::HostCache(size_t max_entries, size_t cache_duration_ms)
+    : max_entries_(max_entries), cache_duration_ms_(cache_duration_ms) {
+}
+
+HostCache::~HostCache() {
+}
+
+const HostCache::Entry* HostCache::Lookup(const std::string& hostname,
+                                          base::TimeTicks now) const {
+  if (caching_is_disabled())
+    return NULL;
+
+  EntryMap::const_iterator it = entries_.find(hostname);
+  if (it == entries_.end())
+    return NULL;  // Not found.
+
+  Entry* entry = it->second.get();
+  if (CanUseEntry(entry, now))
+    return entry;
+
+  return NULL;
+}
+
+HostCache::Entry* HostCache::Set(const std::string& hostname,
+                                 int error,
+                                 const AddressList addrlist,
+                                 base::TimeTicks now) {
+  if (caching_is_disabled())
+    return NULL;
+
+  base::TimeTicks expiration = now +
+      base::TimeDelta::FromMilliseconds(cache_duration_ms_);
+
+  scoped_refptr<Entry>& entry = entries_[hostname];
+  if (!entry) {
+    // Entry didn't exist, creating one now.
+    Entry* ptr = new Entry(error, addrlist, expiration);
+    entry = ptr;
+
+    // Compact the cache if we grew it beyond limit -- exclude |entry| from
+    // being pruned though!
+    if (entries_.size() > max_entries_)
+      Compact(now, ptr);
+    return ptr;
+  } else {
+    // Update an existing cache entry.
+    entry->error = error;
+    entry->addrlist = addrlist;
+    entry->expiration = expiration;
+    return entry.get();
+  }
+}
+
+// static
+bool HostCache::CanUseEntry(const Entry* entry, const base::TimeTicks now) {
+  return entry->error == OK && entry->expiration > now;
+}
+
+void HostCache::Compact(base::TimeTicks now, const Entry* pinned_entry) {
+  // Clear out expired entries.
+  for (EntryMap::iterator it = entries_.begin(); it != entries_.end(); ) {
+    Entry* entry = (it->second).get();
+    if (entry != pinned_entry && !CanUseEntry(entry, now)) {
+      entries_.erase(it++);
+    } else {
+      ++it;
+    }
+  }
+
+  if (entries_.size() <= max_entries_)
+    return;
+
+  // If we still have too many entries, start removing unexpired entries
+  // at random.
+  // TODO(eroman): this eviction policy could be better (access count FIFO
+  // or whatever).
+  for (EntryMap::iterator it = entries_.begin();
+       it != entries_.end() && entries_.size() > max_entries_; ) {
+    Entry* entry = (it->second).get();
+    if (entry != pinned_entry) {
+      entries_.erase(it++);
+    } else {
+      ++it;
+    }
+  }
+
+  if (entries_.size() > max_entries_)
+    DLOG(WARNING) << "Still above max entries limit";
+}
+
+}  // namespace net