mandoline filesystem: add a sqlite3 vfs to proxy filesystem usage.

sql/mojo/enable_mojo_fs.h

Index: sql/mojo/enable_mojo_fs.h
diff --git a/sql/mojo/enable_mojo_fs.h b/sql/mojo/enable_mojo_fs.h
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+++ b/sql/mojo/enable_mojo_fs.h
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+// Copyright 2015 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 SQL_MOJO_ENABLE_MOJO_FS_H_
+#define SQL_MOJO_ENABLE_MOJO_FS_H_
+
+#include "base/macros.h"
+#include "components/filesystem/public/interfaces/directory.mojom.h"
+
+typedef struct sqlite3_vfs sqlite3_vfs;
+
+namespace sql {
+
+// Changes the default sqlite3 vfs to a vfs that uses proxies calls to the
+// mojo:filesystem service. Instantiating this object transparently changes how
+// the entire //sql/ subsystem works in the process of the caller; all paths
+// are treated as relative to |directory|.
+class ScopedMojoFilesystemVFS {
+ public:
+  explicit ScopedMojoFilesystemVFS(filesystem::DirectoryPtr directory);
+  ~ScopedMojoFilesystemVFS();
+
+  // Returns the directory of the current VFS.
+  filesystem::DirectoryPtr& GetDirectory();
+
+ private:
+  friend sqlite3_vfs* GetParentVFS(sqlite3_vfs* mojo_vfs);
+  friend filesystem::DirectoryPtr& GetRootDirectory(sqlite3_vfs* mojo_vfs);
+
+  // The default vfs at the time MojoVFS was installed. We use the to pass
+  // through things like randomness requests and per-platform sleep calls.
+  sqlite3_vfs* parent_;
+
+  // When we initialize the subsystem, we are given a filesystem::Directory
+  // object, which is the root directory of a mojo:filesystem. All access to
+  // various files are specified from this root directory.
+  filesystem::DirectoryPtr root_directory_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedMojoFilesystemVFS);
+};
+
+}  // namespace sql
+
+#endif  // SQL_MOJO_ENABLE_MOJO_FS_H_