缓存缓存

// From RFC 2616 section 13.2.4:
//
// The calculation to determine if a response has expired is quite simple:
//
//   response_is_fresh = (freshness_lifetime > current_age)
//
// Of course, there are other factors that can force a response to always be
// validated or re-fetched.
//
// From RFC 5861 section 3, a stale response may be used while revalidation is
// performed in the background if
//
//   freshness_lifetime + stale_while_revalidate > current_age
//
ValidationType HttpResponseHeaders::RequiresValidation(
    const Time& request_time,
    const Time& response_time,
    const Time& current_time) const {
  FreshnessLifetimes lifetimes = GetFreshnessLifetimes(response_time);
  if (lifetimes.freshness.is_zero() && lifetimes.staleness.is_zero())
    return VALIDATION_SYNCHRONOUS;

  TimeDelta age = GetCurrentAge(request_time, response_time, current_time);

  if (lifetimes.freshness > age)
    return VALIDATION_NONE;

  if (lifetimes.freshness + lifetimes.staleness > age)
    return VALIDATION_ASYNCHRONOUS;

  return VALIDATION_SYNCHRONOUS;
}

// From RFC 2616 section 13.2.4:
//
// The max-age directive takes priority over Expires, so if max-age is present
// in a response, the calculation is simply:
//
//   freshness_lifetime = max_age_value
//
// Otherwise, if Expires is present in the response, the calculation is:
//
//   freshness_lifetime = expires_value - date_value
//
// Note that neither of these calculations is vulnerable to clock skew, since
// all of the information comes from the origin server.
//
// Also, if the response does have a Last-Modified time, the heuristic
// expiration value SHOULD be no more than some fraction of the interval since
// that time. A typical setting of this fraction might be 10%:
//
//   freshness_lifetime = (date_value - last_modified_value) * 0.10
//
// If the stale-while-revalidate directive is present, then it is used to set
// the |staleness| time, unless it overridden by another directive.
//
HttpResponseHeaders::FreshnessLifetimes
HttpResponseHeaders::GetFreshnessLifetimes(const Time& response_time) const {
  FreshnessLifetimes lifetimes;
  // Check for headers that force a response to never be fresh.  For backwards
  // compat, we treat "Pragma: no-cache" as a synonym for "Cache-Control:
  // no-cache" even though RFC 2616 does not specify it.
  if (HasHeaderValue("cache-control", "no-cache") ||
      HasHeaderValue("cache-control", "no-store") ||
      HasHeaderValue("pragma", "no-cache")) {
    return lifetimes;
  }

  // Cache-Control directive must_revalidate overrides stale-while-revalidate.
  bool must_revalidate = HasHeaderValue("cache-control", "must-revalidate");

  if (must_revalidate || !GetStaleWhileRevalidateValue(&lifetimes.staleness)) {
    DCHECK_EQ(TimeDelta(), lifetimes.staleness);
  }

  // NOTE: "Cache-Control: max-age" overrides Expires, so we only check the
  // Expires header after checking for max-age in GetFreshnessLifetimes.  This
  // is important since "Expires: <date in the past>" means not fresh, but
  // it should not trump a max-age value.
  if (GetMaxAgeValue(&lifetimes.freshness))
    return lifetimes;

  // If there is no Date header, then assume that the server response was
  // generated at the time when we received the response.
  Time date_value;
  if (!GetDateValue(&date_value))
    date_value = response_time;

  Time expires_value;
  if (GetExpiresValue(&expires_value)) {
    // The expires value can be a date in the past!
    if (expires_value > date_value) {
      lifetimes.freshness = expires_value - date_value;
      return lifetimes;
    }

    DCHECK_EQ(TimeDelta(), lifetimes.freshness);
    return lifetimes;
  }
  
  ...
  
  if ((response_code_ == 200 || response_code_ == 203 ||
       response_code_ == 206) &&
      !must_revalidate) {
    // TODO(darin): Implement a smarter heuristic.
    Time last_modified_value;
    if (GetLastModifiedValue(&last_modified_value)) {
      // The last-modified value can be a date in the future!
      if (last_modified_value <= date_value) {
        lifetimes.freshness = (date_value - last_modified_value) / 10;
        return lifetimes;
      }
    }
  }

  // These responses are implicitly fresh (unless otherwise overruled):
  if (response_code_ == 300 || response_code_ == 301 || response_code_ == 308 ||
      response_code_ == 410) {
    lifetimes.freshness = TimeDelta::Max();
    lifetimes.staleness = TimeDelta();  // It should never be stale.
    return lifetimes;
  }

  // Our heuristic freshness estimate for this resource is 0 seconds, in
  // accordance with common browser behaviour. However, stale-while-revalidate
  // may still apply.
  DCHECK_EQ(TimeDelta(), lifetimes.freshness);
  return lifetimes;
}