common/cloud/external_policy_data_updater.cc

// Copyright (c) 2013 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 "components/policy/core/common/cloud/external_policy_data_updater.h"

#include "base/bind.h"
#include "base/bind_helpers.h"
#include "base/callback.h"
#include "base/location.h"
#include "base/logging.h"
#include "base/sequenced_task_runner.h"
#include "base/stl_util.h"
#include "components/policy/core/common/cloud/external_policy_data_fetcher.h"
#include "crypto/sha2.h"
#include "net/base/backoff_entry.h"
#include "url/gurl.h"

namespace policy {

namespace {

// Policies for exponential backoff of failed requests. There are 3 policies for
// different classes of errors.

// For temporary errors (HTTP 500, RST, etc).
const net::BackoffEntry::Policy kRetrySoonPolicy = {
  // Number of initial errors to ignore before starting to back off.
  0,

  // Initial delay in ms: 60 seconds.
  1000 * 60,

  // Factor by which the waiting time is multiplied.
  2,

  // Fuzzing percentage; this spreads delays randomly between 80% and 100%
  // of the calculated time.
  0.20,

  // Maximum delay in ms: 12 hours.
  1000 * 60 * 60 * 12,

  // When to discard an entry: never.
  -1,

  // |always_use_initial_delay|; false means that the initial delay is
  // applied after the first error, and starts backing off from there.
  false,
};

// For other errors (request failed, server errors).
const net::BackoffEntry::Policy kRetryLaterPolicy = {
  // Number of initial errors to ignore before starting to back off.
  0,

  // Initial delay in ms: 1 hour.
  1000 * 60 * 60,

  // Factor by which the waiting time is multiplied.
  2,

  // Fuzzing percentage; this spreads delays randomly between 80% and 100%
  // of the calculated time.
  0.20,

  // Maximum delay in ms: 12 hours.
  1000 * 60 * 60 * 12,

  // When to discard an entry: never.
  -1,

  // |always_use_initial_delay|; false means that the initial delay is
  // applied after the first error, and starts backing off from there.
  false,
};

// When the data fails validation (maybe because the policy URL and the data
// served at that URL are out of sync). This essentially retries every 12 hours,
// with some random jitter.
const net::BackoffEntry::Policy kRetryMuchLaterPolicy = {
  // Number of initial errors to ignore before starting to back off.
  0,

  // Initial delay in ms: 12 hours.
  1000 * 60 * 60 * 12,

  // Factor by which the waiting time is multiplied.
  2,

  // Fuzzing percentage; this spreads delays randomly between 80% and 100%
  // of the calculated time.
  0.20,

  // Maximum delay in ms: 12 hours.
  1000 * 60 * 60 * 12,

  // When to discard an entry: never.
  -1,

  // |always_use_initial_delay|; false means that the initial delay is
  // applied after the first error, and starts backing off from there.
  false,
};

// Maximum number of retries for requests that aren't likely to get a
// different response (e.g. HTTP 4xx replies).
const int kMaxLimitedRetries = 3;

}  // namespace

class ExternalPolicyDataUpdater::FetchJob
    : public base::SupportsWeakPtr<FetchJob> {
 public:
  FetchJob(ExternalPolicyDataUpdater* updater,
           const std::string& key,
           const ExternalPolicyDataUpdater::Request& request,
           const ExternalPolicyDataUpdater::FetchSuccessCallback& callback);
  virtual ~FetchJob();

  const std::string& key() const;
  const ExternalPolicyDataUpdater::Request& request() const;

  void Start();

  void OnFetchFinished(ExternalPolicyDataFetcher::Result result,
                       scoped_ptr<std::string> data);

 private:
  void OnFailed(net::BackoffEntry* backoff_entry);
  void Reschedule();

  // Always valid as long as |this| is alive.
  ExternalPolicyDataUpdater* updater_;

  const std::string key_;
  const ExternalPolicyDataUpdater::Request request_;
  ExternalPolicyDataUpdater::FetchSuccessCallback callback_;

  // If the job is currently running, a corresponding |fetch_job_| exists in the
  // |external_policy_data_fetcher_|. The job must eventually call back to the
  // |updater_|'s OnJobSucceeded() or OnJobFailed() method in this case.
  // If the job is currently not running, |fetch_job_| is NULL and no callbacks
  // should be invoked.
  ExternalPolicyDataFetcher::Job* fetch_job_;  // Not owned.

  // Some errors should trigger a limited number of retries, even with backoff.
  // This counts down the number of such retries to stop retrying once the limit
  // is reached.
  int limited_retries_remaining_;

  // Various delays to retry a failed download, depending on the failure reason.
  net::BackoffEntry retry_soon_entry_;
  net::BackoffEntry retry_later_entry_;
  net::BackoffEntry retry_much_later_entry_;

  DISALLOW_COPY_AND_ASSIGN(FetchJob);
};

ExternalPolicyDataUpdater::Request::Request() {
}

ExternalPolicyDataUpdater::Request::Request(const std::string& url,
                                            const std::string& hash,
                                            int64 max_size)
    : url(url), hash(hash), max_size(max_size) {
}

bool ExternalPolicyDataUpdater::Request::operator==(
    const Request& other) const {
  return url == other.url && hash == other.hash && max_size == other.max_size;
}

ExternalPolicyDataUpdater::FetchJob::FetchJob(
    ExternalPolicyDataUpdater* updater,
    const std::string& key,
    const ExternalPolicyDataUpdater::Request& request,
    const ExternalPolicyDataUpdater::FetchSuccessCallback& callback)
    : updater_(updater),
      key_(key),
      request_(request),
      callback_(callback),
      fetch_job_(NULL),
      limited_retries_remaining_(kMaxLimitedRetries),
      retry_soon_entry_(&kRetrySoonPolicy),
      retry_later_entry_(&kRetryLaterPolicy),
      retry_much_later_entry_(&kRetryMuchLaterPolicy) {
}

ExternalPolicyDataUpdater::FetchJob::~FetchJob() {
  if (fetch_job_) {
    // Cancel the fetch job in the |external_policy_data_fetcher_|.
    updater_->external_policy_data_fetcher_->CancelJob(fetch_job_);
    // Inform the |updater_| that the job was canceled.
    updater_->OnJobFailed(this);
  }
}

const std::string& ExternalPolicyDataUpdater::FetchJob::key() const {
  return key_;
}

const ExternalPolicyDataUpdater::Request&
    ExternalPolicyDataUpdater::FetchJob::request() const {
  return request_;
}

void ExternalPolicyDataUpdater::FetchJob::Start() {
  DCHECK(!fetch_job_);
  // Start a fetch job in the |external_policy_data_fetcher_|. This will
  // eventually call back to OnFetchFinished() with the result.
  fetch_job_ = updater_->external_policy_data_fetcher_->StartJob(
      GURL(request_.url), request_.max_size,
      base::Bind(&ExternalPolicyDataUpdater::FetchJob::OnFetchFinished,
                 base::Unretained(this)));
}

void ExternalPolicyDataUpdater::FetchJob::OnFetchFinished(
    ExternalPolicyDataFetcher::Result result,
    scoped_ptr<std::string> data) {
  // The fetch job in the |external_policy_data_fetcher_| is finished.
  fetch_job_ = NULL;

  switch (result) {
    case ExternalPolicyDataFetcher::CONNECTION_INTERRUPTED:
      // The connection was interrupted. Try again soon.
      OnFailed(&retry_soon_entry_);
      return;
    case ExternalPolicyDataFetcher::NETWORK_ERROR:
      // Another network error occurred. Try again later.
      OnFailed(&retry_later_entry_);
      return;
    case ExternalPolicyDataFetcher::SERVER_ERROR:
      // Problem at the server. Try again soon.
      OnFailed(&retry_soon_entry_);
      return;
    case ExternalPolicyDataFetcher::CLIENT_ERROR:
      // Client error. This is unlikely to go away. Try again later, and give up
      // retrying after 3 attempts.
      OnFailed(limited_retries_remaining_ ? &retry_later_entry_ : NULL);
      if (limited_retries_remaining_)
        --limited_retries_remaining_;
      return;
    case ExternalPolicyDataFetcher::HTTP_ERROR:
      // Any other type of HTTP failure. Try again later.
      OnFailed(&retry_later_entry_);
      return;
    case ExternalPolicyDataFetcher::MAX_SIZE_EXCEEDED:
      // Received |data| exceeds maximum allowed size. This may be because the
      // data being served is stale. Try again much later.
      OnFailed(&retry_much_later_entry_);
      return;
    case ExternalPolicyDataFetcher::SUCCESS:
      break;
  }

  if (crypto::SHA256HashString(*data) != request_.hash) {
    // Received |data| does not match expected hash. This may be because the
    // data being served is stale. Try again much later.
    OnFailed(&retry_much_later_entry_);
    return;
  }

  // If the callback rejects the data, try again much later.
  if (!callback_.Run(*data)) {
    OnFailed(&retry_much_later_entry_);
    return;
  }

  // Signal success.
  updater_->OnJobSucceeded(this);
}

void ExternalPolicyDataUpdater::FetchJob::OnFailed(net::BackoffEntry* entry) {
  if (entry) {
    entry->InformOfRequest(false);

    // This function may have been invoked because the job was obsoleted and is
    // in the process of being deleted. If this is the case, the WeakPtr will
    // become invalid and the delayed task will never run.
    updater_->task_runner_->PostDelayedTask(
        FROM_HERE,
        base::Bind(&FetchJob::Reschedule, AsWeakPtr()),
        entry->GetTimeUntilRelease());
  }

  updater_->OnJobFailed(this);
}

void ExternalPolicyDataUpdater::FetchJob::Reschedule() {
  updater_->ScheduleJob(this);
}

ExternalPolicyDataUpdater::ExternalPolicyDataUpdater(
    scoped_refptr<base::SequencedTaskRunner> task_runner,
    scoped_ptr<ExternalPolicyDataFetcher> external_policy_data_fetcher,
    size_t max_parallel_fetches)
    : task_runner_(task_runner),
      external_policy_data_fetcher_(external_policy_data_fetcher.release()),
      max_parallel_jobs_(max_parallel_fetches),
      running_jobs_(0),
      shutting_down_(false) {
  DCHECK(task_runner_->RunsTasksOnCurrentThread());
}

ExternalPolicyDataUpdater::~ExternalPolicyDataUpdater() {
  DCHECK(task_runner_->RunsTasksOnCurrentThread());
  shutting_down_ = true;
  STLDeleteValues(&job_map_);
}

void ExternalPolicyDataUpdater::FetchExternalData(
    const std::string key,
    const Request& request,
    const FetchSuccessCallback& callback) {
  DCHECK(task_runner_->RunsTasksOnCurrentThread());

  // Check whether a job exists for this |key| already.
  FetchJob* job = job_map_[key];
  if (job) {
    // If the current |job| is handling the given |request| already, nothing
    // needs to be done.
    if (job->request() == request)
      return;

    // Otherwise, the current |job| is obsolete. If the |job| is on the queue,
    // its WeakPtr will be invalidated and skipped by StartNextJobs(). If |job|
    // is currently running, it will call OnJobFailed() immediately.
    delete job;
    job_map_.erase(key);
  }

  // Start a new job to handle |request|.
  job = new FetchJob(this, key, request, callback);
  job_map_[key] = job;
  ScheduleJob(job);
}

void ExternalPolicyDataUpdater::CancelExternalDataFetch(
    const std::string& key) {
  DCHECK(task_runner_->RunsTasksOnCurrentThread());

  // If a |job| exists for this |key|, delete it. If the |job| is on the queue,
  // its WeakPtr will be invalidated and skipped by StartNextJobs(). If |job| is
  // currently running, it will call OnJobFailed() immediately.
  std::map<std::string, FetchJob*>::iterator job = job_map_.find(key);
  if (job != job_map_.end()) {
    delete job->second;
    job_map_.erase(job);
  }
}

void ExternalPolicyDataUpdater::StartNextJobs() {
  if (shutting_down_)
    return;

  while (running_jobs_ < max_parallel_jobs_ && !job_queue_.empty()) {
    FetchJob* job = job_queue_.front().get();
    job_queue_.pop();

    // Some of the jobs may have been invalidated, and have to be skipped.
    if (job) {
      ++running_jobs_;
      // A started job will always call OnJobSucceeded() or OnJobFailed().
      job->Start();
    }
  }
}

void ExternalPolicyDataUpdater::ScheduleJob(FetchJob* job) {
  DCHECK_EQ(job_map_[job->key()], job);

  job_queue_.push(job->AsWeakPtr());

  StartNextJobs();
}

void ExternalPolicyDataUpdater::OnJobSucceeded(FetchJob* job) {
  DCHECK(running_jobs_);
  DCHECK_EQ(job_map_[job->key()], job);

  --running_jobs_;
  job_map_.erase(job->key());
  delete job;

  StartNextJobs();
}

void ExternalPolicyDataUpdater::OnJobFailed(FetchJob* job) {
  DCHECK(running_jobs_);
  DCHECK_EQ(job_map_[job->key()], job);

  --running_jobs_;

  // The job is not deleted when it fails because a retry attempt may have been
  // scheduled.
  StartNextJobs();
}

}  // namespace policy