Implement fetching, caching and retrieval of external policy data

chrome/browser/policy/external_data_manager.h

Index: chrome/browser/policy/external_data_manager.h
diff --git a/chrome/browser/policy/external_data_manager.h b/chrome/browser/policy/external_data_manager.h
index 4c97f868e96f0ba905d33703bbbc6a112a360bdd..155a511051e1f850e48c224b30d1d29e4907adff 100644
--- a/chrome/browser/policy/external_data_manager.h
+++ b/chrome/browser/policy/external_data_manager.h
@@ -18,6 +18,14 @@ namespace policy {
 // references.
 class ExternalDataManager {
  public:
+  // Retrieves the external data referenced by |policy| and invokes |callback|
+  // with the result. If |policy| does not reference any external data, the
+  // |callback| is invoked with a NULL pointer. Otherwise, the |callback| is
+  // invoked with the referenced data once it has been successfully retrieved.
+  // If retrieval is temporarily impossible (e.g. no network connectivity), the
+  // |callback| will be invoked when the temporary hindrance is resolved. If
+  // retrieval is permanently impossible (e.g. |policy| references data that
+  // does not exist on the server), the |callback| will never be invoked.

[Joao da Silva, 2013/07/19 11:15:15]

Document what happens when Fetch is called multiple times for the same policy:

- does it fetch the external data twice?
- does it reuse previously fetched data?
- do all the callbacks get eventually called back?

[bartfab (slow), 2013/07/19 13:06:30]

I would not want to document this in ExternalDataManager as this is a generic
interface for which we may have platform-specific implementations in the future.
On some platforms, we may want to fetch the data every time it is requested
(e.g. Windows, if data is referenced via UNC paths), on others, we may want to
cache it (e.g. cloud policy, where data is referenced via URLs).

I added a comment to CloudExternalDataManagerBase, which is the cloud-specific
implementation with well-defined caching behavior.

   virtual void Fetch(const std::string& policy,
                      const ExternalDataFetcher::FetchCallback& callback) = 0;
 };