Expose computation of md5 content checksums for files via a file manager private API.

chrome/browser/chromeos/extensions/file_manager/private_api_file_system.cc

 // Copyright 2013 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 "chrome/browser/chromeos/extensions/file_manager/private_api_file_system.h"
 
 #include <sys/statvfs.h>
 
 #include "base/posix/eintr_wrapper.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 #include "base/task_runner_util.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chromeos/drive/drive.pb.h"
 #include "chrome/browser/chromeos/drive/file_system_interface.h"
 #include "chrome/browser/chromeos/drive/file_system_util.h"
 #include "chrome/browser/chromeos/extensions/file_manager/event_router.h"
 #include "chrome/browser/chromeos/extensions/file_manager/event_router_factory.h"
 #include "chrome/browser/chromeos/extensions/file_manager/private_api_util.h"
 #include "chrome/browser/chromeos/file_manager/fileapi_util.h"
 #include "chrome/browser/chromeos/file_manager/volume_manager.h"
 #include "chrome/browser/chromeos/fileapi/file_system_backend.h"
+#include "chrome/browser/drive/drive_api_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/common/extensions/api/file_manager_private.h"
 #include "chrome/common/extensions/api/file_manager_private_internal.h"
 #include "chromeos/disks/disk_mount_manager.h"
 #include "content/public/browser/child_process_security_policy.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/render_view_host.h"
 #include "content/public/common/url_constants.h"
 #include "net/base/escape.h"
@@ skipping 668 matching lines @@
     entry->file_full_path =
         "/" + entry_definition_list->at(i).full_path.AsUTF8Unsafe();
     entry->file_is_directory = entry_definition_list->at(i).is_directory;
     entries.push_back(entry);
   }
 
   results_ = extensions::api::file_manager_private_internal::
       ResolveIsolatedEntries::Results::Create(entries);
   SendResponse(true);
 }
+
+bool FileManagerPrivateComputeChecksumFunction::RunAsync() {
+  using extensions::api::file_manager_private::ComputeChecksum::Params;
+  const scoped_ptr<Params> params(Params::Create(*args_));
+  EXTENSION_FUNCTION_VALIDATE(params);
+
+  if (params->file_url.empty()) {
+    // TODO(kenobi): call SetError()
+    return false;
+  }
+
+  scoped_refptr<storage::FileSystemContext> file_system_context =
+      file_manager::util::GetFileSystemContextForRenderViewHost(
+          GetProfile(), render_view_host());
+
+  storage::FileSystemURL file_url(
+      file_system_context->CrackURL(GURL(params->file_url)));
+  if (!file_url.is_valid()) {
+    // TODO(kenobi): Call SetError()
+    return false;
+  }
+
+  const std::string& md5 = drive::util::GetMd5Digest(file_url.path());

[mtomasz, 2015/01/08 00:38:40]

This utility makes file IO, it must not be called on UI thread. Please do
PostTask to FILE thread *or* use a blocking pool.

See:
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/pro...

[Ben Kwa, 2015/01/08 19:29:14]

Interesting, the doc for AsyncExtensionFunction made it sound like it runs
asynchronously w.r.t. the UI thread, which I took to mean it didn't run on the
UI thread.  So, what does it mean to say these run "asynchronously relative to
the browser's UI thread"?

I changed it to use PostTaskAndReplyWithResult.

[mtomasz, 2015/01/09 05:19:02]

The comment sounds very cryptic. IIUC AsyncExtensionFunction 
*can* involve other threads and return the result asynchronously with
SendResponse. In contrast to it, SyncExtensionFunction requires to set result
synchronously in RunSync(). But both RunSync and RunAsync run on UI thread.

+  SetResult(new base::StringValue(md5));
+  SendResponse(true);
+
+  return true;
+}
 }  // namespace extensions