DNS: Expose stale results through HostResolverImpl.

net/dns/host_resolver.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_DNS_HOST_RESOLVER_H_
 #define NET_DNS_HOST_RESOLVER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "net/base/address_family.h"
 #include "net/base/completion_callback.h"
 #include "net/base/host_port_pair.h"
 #include "net/base/net_export.h"
 #include "net/base/prioritized_dispatcher.h"
 #include "net/base/request_priority.h"
+#include "net/dns/host_cache.h"
 
 namespace base {
 class Value;
 }
 
 namespace net {
 
 class AddressList;
 class BoundNetLog;
-class HostCache;
 class HostResolverProc;
 class NetLog;
 
 // This class represents the task of resolving hostnames (or IP address
 // literal) to an AddressList object.
 //
 // HostResolver can handle multiple requests at a time, so when cancelling a
 // request the RequestHandle that was returned by Resolve() needs to be
 // given.  A simpler alternative for consumers that only have 1 outstanding
 // request at a time is to create a SingleRequestHostResolver wrapper around
@@ skipping 115 matching lines @@
                       const BoundNetLog& net_log) = 0;
 
   // Resolves the given hostname (or IP address literal) out of cache or HOSTS
   // file (if enabled) only. This is guaranteed to complete synchronously.
   // This acts like |Resolve()| if the hostname is IP literal, or cached value
   // or HOSTS entry exists. Otherwise, ERR_DNS_CACHE_MISS is returned.
   virtual int ResolveFromCache(const RequestInfo& info,
                                AddressList* addresses,
                                const BoundNetLog& net_log) = 0;
 
+  // Like |ResolveFromCache()|, but can return a stale response. If it does,
+  // fills in |stale_info| with details on why the response is not valid.
+  // (Defaults to calling |ResolveFromCache()| if not overridden.)

[Randy Smith (Not in Mondays), 2016/04/28 16:29:55]

This implies that |stale_info| is not filled in if there isn't a stale response,
in which case how can we reliably tell that there isn't a stale response? 
Requiring the caller to fill in stale_info to say "not stale" before calling
doesn't seem reasonable.  Maybe have this contract always fill in stale_info so
there's a reliable way to check?

[Julia Tuttle, 2016/04/29 17:34:47]

Oh, it's supposed to (and does) always fill in stale_info *if* there's a
response (which you can tell from the return code). I'll document that.

+  virtual int ResolveStaleFromCache(const RequestInfo& info,
+                                    AddressList* addresses,
+                                    HostCache::StaleEntryInfo* stale_info,
+                                    const BoundNetLog& net_log);
+
+  // Uses |ResolveStaleFromCache| and |Resolve| to get fresh data (either from
+  // the cache or from the network) but also return stale data if any is
+  // available.
+  //
+  // If fresh data is available in cache, populates |addresses| and returns
+  // synchronously. Sets |*stale_error| to ERR_UNEXPECTED, since it should not
+  // be used.
+  //
+  // If stale data is available in cache, starts a network request like
+  // |Resolve()|, but also returns the stale data through |stale_error|,
+  // |stale_addresses|, and |stale_info|.
+  //
+  // If no data is available in cache, starts a network request like
+  // |Resolve()| and sets |*stale_error| to ERR_DNS_CACHE_MISS.

[Randy Smith (Not in Mondays), 2016/04/28 16:29:55]

Confirming: All cases that start a network request return ERR_IO_PENDING?

[Julia Tuttle, 2016/04/29 17:34:47]

Yup. Should be clearer in the new version of the comment.

+  int ResolveStale(const RequestInfo& info,
+                   RequestPriority priority,
+                   AddressList* addresses,
+                   const CompletionCallback& callback,
+                   RequestHandle* out_req,
+                   int* stale_error,
+                   AddressList* stale_addresses,
+                   HostCache::StaleEntryInfo* stale_info,
+                   const BoundNetLog& net_log);
+
   // Cancels the specified request. |req| is the handle returned by Resolve().
   // After a request is canceled, its completion callback will not be called.
   // CancelRequest must NOT be called after the request's completion callback
   // has already run or the request was canceled.
   virtual void CancelRequest(RequestHandle req) = 0;
 
   // Enable or disable the built-in asynchronous DnsClient.
   virtual void SetDnsClientEnabled(bool enabled);
 
   // Returns the HostResolverCache |this| uses, or NULL if there isn't one.
@@ skipping 17 matching lines @@
  protected:
   HostResolver();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(HostResolver);
 };
 
 }  // namespace net
 
 #endif  // NET_DNS_HOST_RESOLVER_H_