Add PPDProvider barebones implementation and associated cache skeleton.

chromeos/printing/ppd_cache.h

Index: chromeos/printing/ppd_cache.h
diff --git a/chromeos/printing/ppd_cache.h b/chromeos/printing/ppd_cache.h
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+++ b/chromeos/printing/ppd_cache.h
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+// 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 "base/optional.h"
+#include "chromeos/chromeos_export.h"
+#include "chromeos/printing/printer_configuration.h"
+
+namespace chromeos {
+namespace printing {
+
+// PpdCache manages a cache of locally-stored PPD files.  At its core, it
+// operates like a persistent hash from PpdReference to files.  If you give the
+// same PpdReference to Find() that was previously given to store, you should
+// get the same FilePath back out (unless the previous entry has timed out of
+// the cache).  However, changing *any* field in PpdReference will make the
+// previous cache entry invalid.  This is the intentional behavior -- we want to
+// re-run the resolution logic if we have new meta-information about a printer.
+class CHROMEOS_EXPORT PpdCache {
+ public:
+  // Create and return a Ppdcache that uses cache_dir to store state.
+  static std::unique_ptr<PpdCache> Create(const base::FilePath& cache_base_dir);
+  virtual ~PpdCache() {}
+
+  // Find a PPD that was previously cached with the given reference.  Note that
+  // all fields of the reference must be the same, otherwise we'll miss in the
+  // cache and re-run resolution for the PPD.
+  //
+  // 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> Find(
+      const Printer::PpdReference& reference) const = 0;
+
+  // Take the contents of a PPD file, store it to the cache, and return the
+  // path to the stored file keyed on reference.
+  //
+  // 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;
+};
+
+}  // namespace printing
+}  // namespace chromeos
+
+#endif  // CHROMEOS_PRINTING_PPD_CACHE_H_