Add PPDProvider barebones implementation and associated cache skeleton.

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>
+
+#include "base/files/file_path.h"
+#include "chromeos/printing/printer_configuration.h"
+
+namespace chromeos {
+namespace printing {
+
+// PPDCache is manages a cache of locally-stored ppd files.
+class CHROMEOS_EXPORT PPDCache {
+ public:
+  struct Options {
+    // Directory we'll use to do the caching.
+    ::base::FilePath cache_directory;
+  };
+
+  static ::std::unique_ptr<PPDCache> Create();
+  static ::std::unique_ptr<PPDCache> Create(const Options& options);
+  virtual ~PPDCache() {}
+
+  // Return a default set of options that are sane.
+  static Options Defaults();
+
+  // Look for a PPD for the given manufacturer/model in the cache, and return it
+  // if it does exist.  Return a null pointer if it does not exist.
+  virtual std::unique_ptr<Printer::PPDFile> Lookup(

[skau, 2016/09/17 00:59:59]

When specifying PPDFile, my thought was that it would be the input to PPDCache
rather than a result.  Then the PPDCache wouldn't need to store the mapping
between (manufacturer, model) -> file since that relationship can be many ->
one.

[Carlson, 2016/10/06 15:56:05]

Reworked this based on the discussions we had.

+      const ::std::string& manufacturer,
+      const ::std::string& model) const = 0;
+
+  // Take the contents of a ppd file, store it to the cache, and return a handle
+  // to the generated PPDFile.  If this manufacturer-model tuple already exists,
+  // the new data will replace it.  Returns null on failure-to-store.
+  //
+  // On success, the returned PPDFile is guaranteed to be valid until the next
+  // Store() call.
+  //
+  // TODO(justincarlson) -- What guarantees should we provide about consistency
+  // on failure?  Probably we want to guarantee that existing entries are
+  // unchanged on failure.
+  virtual std::unique_ptr<Printer::PPDFile> Store(
+      const ::std::string& manufacturer,
+      const ::std::string& model,
+      int64_t time_added_ms,
+      int64_t last_updated_time_ms,
+      const ::std::string& compressed_ppd_contents) = 0;
+};
+
+}  // namespace printing
+}  // namespace chromeos
+
+#endif  // CHROMEOS_PRINTING_PPD_CACHE_H_