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

net/http/http_cache.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.
 
 // This file declares a HttpTransactionFactory implementation that can be
 // layered on top of another HttpTransactionFactory to add HTTP caching.  The
 // caching logic follows RFC 7234 (any exceptions are called out in the code).
 //
 // The HttpCache takes a disk_cache::Backend as a parameter, and uses that for
 // the cache storage.
@@ skipping 13 matching lines @@
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/threading/thread_checker.h"
 #include "base/time/clock.h"
 #include "base/time/time.h"
 #include "net/base/cache_type.h"
 #include "net/base/completion_callback.h"
 #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"
 
 class GURL;
 
 namespace base {
 class SingleThreadTaskRunner;
 namespace trace_event {
 class ProcessMemoryDump;
 }
 }  // namespace base
 
 namespace disk_cache {
-class Backend;
 class Entry;
 }  // namespace disk_cache
 
 namespace net {
 
 class HttpNetworkSession;
 class HttpResponseInfo;
 class IOBuffer;
 class NetLog;
 class ViewCacheHelper;
 struct HttpRequestInfo;
 
 class NET_EXPORT HttpCache : public HttpTransactionFactory {
  public:
   // The cache mode of operation.
   enum Mode {
     // Normal mode just behaves like a standard web cache.
     NORMAL = 0,
     // Disables reads and writes from the cache.
     // Equivalent to setting LOAD_DISABLE_CACHE on every request.
     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:
     virtual ~BackendFactory() {}
 
     // The actual method to build the backend. Returns a net error code. If
     // ERR_IO_PENDING is returned, the |callback| will be notified when the
     // operation completes, and |backend| must remain valid until the
     // notification arrives.
     // The implementation must not access the factory object after invoking the
@@ skipping 241 matching lines @@
   // Returns the PendingOp for the desired |key|. If an entry is not under
   // construction already, a new PendingOp structure is created.
   PendingOp* GetPendingOp(const std::string& key);
 
   // Deletes a PendingOp.
   void DeletePendingOp(PendingOp* pending_op);
 
   // 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
   // ActiveEntry in |*entry|. |trans| will be notified via its IO callback if
   // this method returns ERR_IO_PENDING.
   int CreateEntry(const std::string& key, ActiveEntry** entry,
                   Transaction* trans);
 
   // Destroys an ActiveEntry (active or doomed).
   void DestroyEntry(ActiveEntry* entry);
@@ skipping 93 matching lines @@
   THREAD_CHECKER(thread_checker_);
 
   base::WeakPtrFactory<HttpCache> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(HttpCache);
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_CACHE_H_