Add in-memory caching to PpdCache available printers.

chromeos/printing/ppd_cache.h

 // Copyright 2016 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 CHROMEOS_PRINTING_PPD_CACHE_H_
 #define CHROMEOS_PRINTING_PPD_CACHE_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 55 matching lines @@
   // If a different PPD was previously Stored for the given reference, it
   // will be replaced.
   //
   // If a FilePath is returned, it is guaranteed to be non-empty and
   // remain valid until the next Store() call.
   virtual base::Optional<base::FilePath> Store(
       const Printer::PpdReference& reference,
       const std::string& ppd_contents) = 0;
 
   // Return a map of available printers, if we have one available and it's
-  // not too stale.
-  virtual base::Optional<PpdProvider::AvailablePrintersMap>
-  FindAvailablePrinters() = 0;
+  // not too stale.  Returns null if no map is available.
+  //
+  // If a map is returned, ownership is retained by the cache.
+  virtual const PpdProvider::AvailablePrintersMap* FindAvailablePrinters() = 0;

[skau, 2016/11/10 20:46:37]

Make a note of the expected lifetime of the map i.e. when, if ever, it's safe to
retain.

[Carlson, 2016/11/11 01:39:27]

Done.

 
   // Store |available_printers|, replacing any existing entry.
   virtual void StoreAvailablePrinters(
-      const PpdProvider::AvailablePrintersMap& available_printers) = 0;
+      std::unique_ptr<PpdProvider::AvailablePrintersMap>
+          available_printers) = 0;
 };
 
 }  // namespace printing
 }  // namespace chromeos
 
 #endif  // CHROMEOS_PRINTING_PPD_CACHE_H_