Interface for associating in-memory hint bytes with disk cache entries.

net/disk_cache/disk_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.
 
 // Defines the public interface of the disk cache. For more details see
 // http://dev.chromium.org/developers/design-documents/network-stack/disk-cache
 
 #ifndef NET_DISK_CACHE_DISK_CACHE_H_
 #define NET_DISK_CACHE_DISK_CACHE_H_
 
@@ skipping 166 matching lines @@
   virtual void GetStats(base::StringPairs* stats) = 0;
 
   // Called whenever an external cache in the system reuses the resource
   // referred to by |key|.
   virtual void OnExternalCacheHit(const std::string& key) = 0;
 
   // Returns the estimate of dynamically allocated memory in bytes.
   virtual size_t DumpMemoryStats(
       base::trace_event::ProcessMemoryDump* pmd,
       const std::string& parent_absolute_name) const = 0;
+
+  // Backends can optionally permit one to store, probabilistically, up to a
+  // byte associated with a key of an existing entry in memory.
+
+  // GetMemoryEntryData has the following behavior:
+  // - If the feature is not supported by the backend, returns 0.
+  // - If the entry is not in the cache, returns 0.

[Randy Smith (Not in Mondays), 2017/06/27 21:33:56]

Definitively isn't in the cache, or isn't in the in-memory index associated with
the cache?  I.e. is this case a "Maybe there's an entry with this key--I don't
know" or "There is no entry with this key in the cache"?

The reason I ask is that the ability to be sure that there isn't an entry with
this key in the cache without checking disk, even in a best effort fashion, is a
simple cache feature.  If we want to leave the door open to caches that keep
only a partial index in memory in the future, we might want to have the ability
to respond with a shrug/I dunno.

[Maks Orlovich, 2017/06/28 13:54:31]

Hmm, good point. It's actually sort of a mixture even for SimpleCache: it can be
the first while we are initializing, then becomes the second.

[gavinp, 2017/08/04 17:33:14]

This is subtle; because of course yes we can operate without an index, and the
non-presence of an entry doesn't indicate that we don't have data.

Perhaps move this subtlety out of the context of this comment by rephrasing it
to something more like:

"If this data is not available for this particular key, for any reason, returns
0."

Sound reasonable?

[Maks Orlovich, 2017/08/23 18:52:05]

Rephrased, but added "at this time".

+  // - Otherwise, returns a value that was with very high probability
+  //   given to SetMemoryEntryData(|key|) (and with a very low probability
+  //   to a different key that collides in the in-memory index).

[Randy Smith (Not in Mondays), 2017/06/27 21:33:55]

Hmmm.  I'd rather have an interface that distinguished clearly between
supporting, not-in-cache, and an actual value.  Maybe I'm being silly, but I
could imagine consumers making use of those distinctions.  

How about:

* SetMemoryEntryData() returns false if the feature isn't supported.

* GetMemoryEntryData has an out argument for the value, and return false if the
entry isn't in the cache or the feature isn't supported?

[Maks Orlovich, 2017/06/28 13:54:31]

Not sure how you would use that.
Hmm, there is also the issue of entry being written by an older version, which
didn't use to have the feature. 

+  //
+  // Due to the probability of collisions, including those that can be induced
+  // by hostile 3rd parties, this interface should not be used to make decisions
+  // that affect correctness (especially security).
+  virtual uint8_t GetMemoryEntryData(const std::string& key);
+  virtual void SetMemoryEntryData(const std::string& key, uint8_t data);
 };
 
 // This interface represents an entry in the disk cache.
 class NET_EXPORT Entry {
  public:
   typedef net::CompletionCallback CompletionCallback;
   typedef net::IOBuffer IOBuffer;
 
   // Marks this cache entry for deletion.
   virtual void Doom() = 0;
@@ skipping 156 matching lines @@
     entry->Close();
   }
 };
 
 // Automatically closes an entry when it goes out of scope.
 typedef std::unique_ptr<Entry, EntryDeleter> ScopedEntryPtr;
 
 }  // namespace disk_cache
 
 #endif  // NET_DISK_CACHE_DISK_CACHE_H_