DNS: Expose stale results through HostResolverImpl.

net/dns/host_resolver.h

Index: net/dns/host_resolver.h
diff --git a/net/dns/host_resolver.h b/net/dns/host_resolver.h
index 4e19e63af3f8311c507d195e3ecb933829995a34..6a3f334ce30acb97757d2d7b20712cce8233fbc1 100644
--- a/net/dns/host_resolver.h
+++ b/net/dns/host_resolver.h
@@ -18,6 +18,7 @@
 #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;
@@ -27,7 +28,6 @@ namespace net {
 
 class AddressList;
 class BoundNetLog;
-class HostCache;
 class HostResolverProc;
 class NetLog;
 
@@ -163,6 +163,38 @@ class NET_EXPORT HostResolver {
                                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