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

Index: chrome/browser/chromeos/extensions/file_manager/private_api_file_system.cc
diff --git a/chrome/browser/chromeos/extensions/file_manager/private_api_file_system.cc b/chrome/browser/chromeos/extensions/file_manager/private_api_file_system.cc
index f0c04de646ba64abf48ef92f99cc6d32104f01e3..fcf656cd5ff11a43392c18a8f8f3a2c51dcf5cc6 100644
--- a/chrome/browser/chromeos/extensions/file_manager/private_api_file_system.cc
+++ b/chrome/browser/chromeos/extensions/file_manager/private_api_file_system.cc
@@ -21,6 +21,7 @@
 #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"
@@ -709,4 +710,32 @@ void FileManagerPrivateInternalResolveIsolatedEntriesFunction::
       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