chrome/browser/net/dns_master.cc

// Copyright (c) 2006-2008 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.

// See header file for description of class

#include "chrome/browser/net/dns_master.h"

#include <sstream>

#include "base/histogram.h"
#include "base/stats_counters.h"
#include "base/thread.h"
#include "base/win_util.h"
#include "chrome/browser/net/dns_slave.h"

using base::TimeDelta;

namespace chrome_browser_net {

//------------------------------------------------------------------------------
// This section contains methods for the DnsMaster class.
//------------------------------------------------------------------------------
DnsMaster::DnsMaster(TimeDelta shutdown_wait_time)
  : slave_count_(0),
    shutdown_(false),
    running_slave_count_(0),
    kShutdownWaitTime_(shutdown_wait_time),
    slaves_have_work_(&lock_) {
  for ( int i = 0; i < kSlaveCountMax; i++ ) {
    thread_ids_[i] = 0;
    thread_handles_[i] = 0;
    slaves_[i] = NULL;
  }
}

// Overloaded Resolve() to take a vector of names.
void DnsMaster::ResolveList(const NameList& hostnames) {
  {
    AutoLock auto_lock(lock_);
    if (shutdown_) return;
    if (slave_count_ < kSlaveCountMin) {
      for (int target_count = std::min(static_cast<int>(hostnames.size()),
                                       kSlaveCountMin);
           target_count > 0;
           target_count--)
        PreLockedCreateNewSlaveIfNeeded();
    } else {
      PreLockedCreateNewSlaveIfNeeded();  // Allocate one per list call.
    }

    for (NameList::const_iterator it = hostnames.begin();
      it < hostnames.end();
         it++) {
      PreLockedResolve(*it);
    }
  }
  slaves_have_work_.Signal();
}

// Basic Resolve() takes an invidual name, and adds it
// to the queue.
void DnsMaster::Resolve(const std::string& hostname) {
  if (0 == hostname.length())
    return;
  {
    AutoLock auto_lock(lock_);
    if (shutdown_) return;
    PreLockedCreateNewSlaveIfNeeded();  // Allocate one at a time.
    PreLockedResolve(hostname);
  }
  slaves_have_work_.Signal();
}

bool DnsMaster::AcruePrefetchBenefits(DnsHostInfo* host_info) {
  std::string hostname = host_info->hostname();
  DnsBenefit benefit;
  DnsHostInfo prefetched_host_info;
  {
    AutoLock auto_lock(lock_);
    if (results_.find(hostname) == results_.end()) {
      // Remain under lock to assure static HISTOGRAM constructor is safely run.
      // Use UMA histogram to quantify potential future gains here.
      UMA_HISTOGRAM_LONG_TIMES(L"DNS.UnexpectedResolutionL",
                          host_info->resolve_duration());
      SIMPLE_STATS_COUNTER(L"DNS.PrefetchCacheOblivious");
      host_info->DLogResultsStats("DNS PrefetchCacheOblivious");
      return false;
    }
    benefit = results_[hostname].AcruePrefetchBenefits(host_info);
    switch (benefit) {
      case PREFETCH_NAME_FOUND:
      case PREFETCH_NAME_NONEXISTANT:
        // Remain under lock to push data.
        cache_hits_.push_back(*host_info);
        return true;

      case PREFETCH_CACHE_EVICTION:
        // Remain under lock to push data.
        cache_eviction_map_[hostname] = *host_info;
        return false;
      case PREFETCH_NO_BENEFIT:
        // Prefetch never hit the network. Name was pre-cached.
        return false;

      default:
        DCHECK(false);
        return false;
    }
  }
}

static char* PluralOptionalHostname(size_t count) {
  if (count == 1)
    return "hostname";
  return "hostnames";
}

// Provide sort order so all .com's are together, etc.
struct RightToLeftStringSorter {
  bool operator()(const std::string& left, const std::string& right) const {
    size_t left_length = left.length();
    size_t right_length = right.length();
    const char* left_data = left.data();
    const char* right_data = right.data();
    while (true) {
      if (0 == right_length)
        return false;
      if (0 == left_length)
        return true;
      left_length--;
      right_length--;
      int difference = left_data[left_length] - right_data[right_length];
      if (difference)
        return difference < 0;
    }
  }
};

void DnsMaster::GetHtmlInfo(std::string* output) {
  // Local lists for calling DnsHostInfo
  DnsHostInfo::DnsInfoTable cache_hits;
  DnsHostInfo::DnsInfoTable cache_evictions;
  DnsHostInfo::DnsInfoTable name_not_found;
  DnsHostInfo::DnsInfoTable network_hits;
  DnsHostInfo::DnsInfoTable already_cached;

  // Get copies of all useful data under protection of a lock.
  typedef std::map<std::string, DnsHostInfo, RightToLeftStringSorter> Snapshot;
  Snapshot snapshot;
  {
    AutoLock auto_lock(lock_);
    // DnsHostInfo supports value semantics, so we can do a shallow copy.
    for (Results::iterator it(results_.begin()); it != results_.end(); it++) {
      snapshot[it->first] = it->second;
    }
    for (Results::iterator it(cache_eviction_map_.begin());
         it != cache_eviction_map_.end();
         it++) {
      cache_evictions.push_back(it->second);
    }
    // Reverse list as we copy cache hits, so that new hits are at the top.
    size_t index = cache_hits_.size();
    while (index > 0) {
      index--;
      cache_hits.push_back(cache_hits_[index]);
    }
  }

  // Partition the DnsHostInfo's into categories.
  for (Snapshot::iterator it(snapshot.begin()); it != snapshot.end(); it++) {
    if (it->second.was_nonexistant()) {
      name_not_found.push_back(it->second);
      continue;
    }
    if (!it->second.was_found())
      continue;  // Still being processed.
    if (TimeDelta() != it->second.benefits_remaining()) {
      network_hits.push_back(it->second);  // With no benefit yet.
      continue;
    }
    if (DnsHostInfo::kMaxNonNetworkDnsLookupDuration >
      it->second.resolve_duration()) {
      already_cached.push_back(it->second);
      continue;
    }
    // Remaining case is where prefetch benefit was significant, and was used.
    // Since we shot those cases as historical hits, we won't bother here.
  }

  bool brief = false;
#ifdef NDEBUG
  brief = true;
#endif  // NDEBUG

  // Call for display of each table, along with title.
  DnsHostInfo::GetHtmlTable(cache_hits,
      "Prefetching DNS records produced benefits for ", false, output);
  DnsHostInfo::GetHtmlTable(cache_evictions,
      "Cache evictions negated DNS prefetching benefits for ", brief, output);
  DnsHostInfo::GetHtmlTable(network_hits,
      "Prefetching DNS records was not yet beneficial for ", brief, output);
  DnsHostInfo::GetHtmlTable(already_cached,
      "Previously cached resolutions were found for ", brief, output);
  DnsHostInfo::GetHtmlTable(name_not_found,
      "Prefetching DNS records revealed non-existance for ", brief, output);
}

void DnsMaster::PreLockedResolve(const std::string& hostname) {
  // DCHECK(We have the lock);
  DCHECK(0 != slave_count_);
  DCHECK(0 != hostname.length());

  DnsHostInfo* info = &results_[hostname];
  info->SetHostname(hostname);  // Initialize or DCHECK.
  // TODO(jar):  I need to discard names that have long since expired.
  // Currently we only add to the domain map :-/

  DCHECK(info->HasHostname(hostname));

  static StatsCounter count(L"DNS.PrefetchContemplated");
  count.Increment();

  if (!info->NeedsDnsUpdate(hostname)) {
    info->DLogResultsStats("DNS PrefetchNotUpdated");
    return;
  }

  info->SetQueuedState();
  name_buffer_.push(hostname);
}

// GetNextAssignment() is executed on the thread associated with
// with a prefetch slave instance.
// Return value of false indicates slave thread termination is needed.
// Return value of true means info argument was populated
// with a pointer to the assigned DnsHostInfo instance.
bool DnsMaster::GetNextAssignment(std::string* hostname) {
  bool ask_for_help = false;
  {
    AutoLock auto_lock(lock_);  // For map and buffer access
    while (0 == name_buffer_.size() && !shutdown_) {
      // No work pending, so just wait.
      // wait on condition variable while releasing lock temporarilly.
      slaves_have_work_.Wait();
    }
    if (shutdown_)
      return false;  // Tell slaves to terminate also.
    *hostname = name_buffer_.front();
    name_buffer_.pop();

    DnsHostInfo* info = &results_[*hostname];
    DCHECK(info->HasHostname(*hostname));
    info->SetAssignedState();

    ask_for_help = name_buffer_.size() > 0;
  }  // Release lock_
  if (ask_for_help)
    slaves_have_work_.Signal();
  return true;
}

void DnsMaster::SetFoundState(const std::string hostname) {
  AutoLock auto_lock(lock_);  // For map access (changing info values).
  DnsHostInfo* info = &results_[hostname];
  DCHECK(info->HasHostname(hostname));
  if (info->is_marked_to_delete())
    results_.erase(hostname);
  else
    info->SetFoundState();
}

void DnsMaster::SetNoSuchNameState(const std::string hostname) {
  AutoLock auto_lock(lock_);  // For map access (changing info values).
  DnsHostInfo* info = &results_[hostname];
  DCHECK(info->HasHostname(hostname));
  if (info->is_marked_to_delete())
    results_.erase(hostname);
  else
    info->SetNoSuchNameState();
}

bool DnsMaster::PreLockedCreateNewSlaveIfNeeded() {
  // Don't create more then max.
  if (kSlaveCountMax <= slave_count_  || shutdown_)
    return false;

  DnsSlave* slave_instance = new DnsSlave(this, slave_count_);
  DWORD thread_id;
  size_t stack_size = 0;
  unsigned int flags = CREATE_SUSPENDED;
  if (win_util::GetWinVersion() >= win_util::WINVERSION_XP) {
    // 128kb stack size.
    stack_size = 128*1024;
    flags |= STACK_SIZE_PARAM_IS_A_RESERVATION;
  }
  HANDLE handle = CreateThread(NULL,  // security
                               stack_size,
                               DnsSlave::ThreadStart,
                               reinterpret_cast<void*>(slave_instance),
                               flags,
                               &thread_id);
  DCHECK(NULL != handle);
  if (NULL == handle)
    return false;

  // Carlos suggests it is not valuable to do a set priority.
  //  BOOL WINAPI SetThreadPriority(handle,int nPriority);

  thread_ids_[slave_count_] = thread_id;
  thread_handles_[slave_count_] = handle;
  slaves_[slave_count_] = slave_instance;
  slave_count_++;

  ResumeThread(handle);  // WINAPI call.
  running_slave_count_++;

  return true;
}

void DnsMaster::SetSlaveHasTerminated(int slave_index) {
  DCHECK_EQ(GetCurrentThreadId(), thread_ids_[slave_index]);
  AutoLock auto_lock(lock_);
  running_slave_count_--;
  DCHECK(thread_ids_[slave_index]);
  thread_ids_[slave_index] = 0;
}

bool DnsMaster::ShutdownSlaves() {
  int running_slave_count;
  {
    AutoLock auto_lock(lock_);
    shutdown_ = true;  // Block additional resolution requests.
    // Empty the queue gracefully
    while (name_buffer_.size() > 0) {
      std::string hostname = name_buffer_.front();
      name_buffer_.pop();
      DnsHostInfo* info = &results_[hostname];
      DCHECK(info->HasHostname(hostname));
      // We should be in Queued state, so simulate to end of life.
      info->SetAssignedState();  // Simulate slave assignment.
      info->SetNoSuchNameState();  // Simulate failed lookup.
      results_.erase(hostname);
    }
    running_slave_count = running_slave_count_;
    // Release lock, so slaves can finish up.
  }

  if (running_slave_count) {
    slaves_have_work_.Broadcast();  // Slaves need to check for termination.

    DWORD result = WaitForMultipleObjects(
                      slave_count_,
                      thread_handles_,
                      TRUE,  // Wait for all
                      static_cast<DWORD>(kShutdownWaitTime_.InMilliseconds()));

    DCHECK(result != WAIT_TIMEOUT) << "Some slaves didn't stop";
    if (WAIT_TIMEOUT == result)
      return false;
  }
  {
    AutoLock auto_lock(lock_);
    while (0 < slave_count_--) {
      if (0 == thread_ids_[slave_count_]) {  // Thread terminated.
        int result = CloseHandle(thread_handles_[slave_count_]);
        CHECK(0 != result);
        thread_handles_[slave_count_] = 0;
        delete slaves_[slave_count_];
        slaves_[slave_count_] = NULL;
      }
    }
  }
  return true;
}

void DnsMaster::DiscardAllResults() {
  AutoLock auto_lock(lock_);
  // Delete anything listed so far in this session that shows in about:dns.
  cache_eviction_map_.clear();
  cache_hits_.clear();


  // Try to delete anything in our work queue.
  while (!name_buffer_.empty()) {
    // Emulate processing cycle as though host was not found.
    std::string hostname = name_buffer_.front();
    name_buffer_.pop();
    DnsHostInfo* info = &results_[hostname];
    DCHECK(info->HasHostname(hostname));
    info->SetAssignedState();
    info->SetNoSuchNameState();
  }
  // Now every result_ is either resolved, or is being worked on by a slave.

  // Step through result_, recording names of all hosts that can't be erased.
  // We can't erase anything being worked on by a slave.
  Results assignees;
  for (Results::iterator it = results_.begin(); results_.end() != it; ++it) {
    std::string hostname = it->first;
    DnsHostInfo* info = &it->second;
    DCHECK(info->HasHostname(hostname));
    if (info->is_assigned()) {
      info->SetPendingDeleteState();
      assignees[hostname] = *info;
    }
  }
  DCHECK(kSlaveCountMax >= assignees.size());
  results_.clear();
  // Put back in the names being worked on by slaves.
  for (Results::iterator it = assignees.begin(); assignees.end() != it; ++it) {
    DCHECK(it->second.is_marked_to_delete());
    results_[it->first] = it->second;
  }
}

}  // namespace chrome_browser_net