RFC: use some in-memory state in SimpleCache to quickly cache-miss some CantConditionalize cases

net/http/http_cache.h

Index: net/http/http_cache.h
diff --git a/net/http/http_cache.h b/net/http/http_cache.h
index 6aa26ae225f7dc2c2d60b0a4b08593fb40a853bf..0420f1a21f739f48166e14e89fd6cd61b852a424 100644
--- a/net/http/http_cache.h
+++ b/net/http/http_cache.h
@@ -31,6 +31,7 @@
 #include "net/base/load_states.h"
 #include "net/base/net_export.h"
 #include "net/base/request_priority.h"
+#include "net/disk_cache/disk_cache.h"
 #include "net/http/http_network_session.h"
 #include "net/http/http_transaction_factory.h"
 
@@ -44,7 +45,6 @@ class ProcessMemoryDump;
 }  // namespace base
 
 namespace disk_cache {
-class Backend;
 class Entry;
 }  // namespace disk_cache
 
@@ -68,6 +68,13 @@ class NET_EXPORT HttpCache : public HttpTransactionFactory {
     DISABLE
   };
 
+  // ### probably don't need to blow all of 3 bits on this.
+  enum OracleByteHints {
+    OBH_ZERO_LIFETIME = 1,
+    OBH_RESPONSE_CANT_CONDITIONALIZE = 2,
+    OBH_UNUSED_SINCE_PREFETCH = 4

[pasko, 2017/06/13 14:30:05]

it's nice to have it here in general, but probably belongs to a separate change,
as it is an orthogonal optimization (and somewhat orthogonal to
cantconditionalize).

[Maks Orlovich, 2017/06/13 14:58:47]

We have to deal with it, since it affects
HttpCache::Transaction::RequiresValidation:

  if (response_.unused_since_prefetch &&
      !(effective_load_flags_ & LOAD_PREFETCH) &&
      response_.headers->GetCurrentAge(
          response_.request_time, response_.response_time,
          cache_->clock_->Now()) < TimeDelta::FromMinutes(kPrefetchReuseMins)) {
    // The first use of a resource after prefetch within a short window skips
    // validation.
    return false;
  }

e.g. things that have unused_since_prefetch set have to be opened since if
they're recent they may be used regardless caching headers. Could probably make
CanRejectBasedOnMemoryEntryData pay attention to 
(effective_load_flags_ & LOAD_PREFETCH) though.

+  };
+
   // A BackendFactory creates a backend object to be used by the HttpCache.
   class NET_EXPORT BackendFactory {
    public:
@@ -329,7 +336,8 @@ class NET_EXPORT HttpCache : public HttpTransactionFactory {
   // Opens the disk cache entry associated with |key|, returning an ActiveEntry
   // in |*entry|. |trans| will be notified via its IO callback if this method
   // returns ERR_IO_PENDING.
-  int OpenEntry(const std::string& key, ActiveEntry** entry,
+  int OpenEntry(const std::string& key,
+                ActiveEntry** entry,
                 Transaction* trans);
 
   // Creates the disk cache entry associated with |key|, returning an