Add SetFileSize() IPC for WebSQL.

storage/browser/database/vfs_backend.cc

 // Copyright (c) 2011 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.
 
 #include "storage/browser/database/vfs_backend.h"
 
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
 #include "third_party/sqlite/sqlite3.h"
@@ skipping 137 matching lines @@
 #endif
   return attributes;
 }
 
 // static
 int64 VfsBackend::GetFileSize(const base::FilePath& file_path) {
   int64 size = 0;
   return (base::GetFileSize(file_path, &size) ? size : 0);
 }
 
+// static
+bool VfsBackend::SetFileSize(const base::FilePath& file_path, int64 size) {
+  // TODO(shess): Is there a base::File sitting around to do this?  Otherwise it

[michaeln, 2015/03/19 00:36:33]

There is not, IPC::TakeFileHandleForProcess(file.Pass(),...) is used to xfer the
handle to the renderer.

[michaeln, 2015/04/01 21:04:48]

Except for incognito profiles.

[Scott Hess - ex-Googler, 2015/04/01 22:42:20]

Seems like I should change the comment either way :-).

+  // might make sense to expose truncate() somewhere rather than having to open
+  // a file.
+  int flags = 0;
+  flags |= base::File::FLAG_READ;
+  flags |= base::File::FLAG_WRITE;
+  flags |= base::File::FLAG_OPEN;
+  base::File file = base::File(file_path, flags);

[michaeln, 2015/03/19 00:36:33]

Depending on when/how xTruncate is used, seems like this SetLength function
could fail due to sqlite having write locks, but ftruncate() would succeed if
the open fd were passed directly to it. Is that possible and does sqlite try to
use xTruncate at times when it would get tripped up by that?

[Scott Hess - ex-Googler, 2015/03/25 20:24:23]

On POSIX, locks are advisory only, so the file descriptors can be in both places
just fine.  This kind of problem could occur on Windows, though, I think.  I
think there are a couple options to get this right in the long term, worst case
would be to keep a little platform-specific code to handle the
locking-vs-sandbox distinctions.

+  if (!file.IsValid())
+    return false;
+  return file.SetLength(size);
+}
+
 }  // namespace storage