Description Laurent Goubet 2011-05-11 09:04:29 EDT

I don't really know if this is a CVS issue, or a more generic platform/Compare problem. Here is the gist of it :

I have two files "A" and "B" which are part of the same logical resource. Thus, "A" cannot be compared alone, and neither can "B". Comparing either one of them should open the synchronize perspective and show the synchronization state of both. That, I can do.

Basically, I have defined my own model provider through the org.eclipse.core.resources.modelProviders extension point. Its enablement value looks like this :
<enablement>
  <and>
    <adapt type="org.eclipse.core.resources.IFile"/>
    <test
           property="contentTypeId"
           value="my.content.type.id">
    </test>
  </and>
</enablement>

So it applies on elements that can be adapted to "IFile" and which have a set content type (both "A" and "B" have that content type). With this, calling the action "right click on "A" > Compare With > Latest From HEAD" forces me to open the synchronize view, and this view displays both of my files for comparison. So far, so good. (for the record, my model provider then is called from "org.eclipse.team.internal.ccvs.ui.actions.SyncAction#isOKToShowSingleFile()")

However, if I right-click either one of the files in the synchronize view, and use the action "Open In Compare Editor" (or if I simply double clic the files), my model provider is _not_ called ... and thus the compare editor that gets open is opened on a single file instead of being opened on my ResourceMapping.

Trying to track down why, it turns out that
  org.eclipse.team.internal.ui.synchronize.actions.OpenInCompareAction()
calls
  ModelSynchronizeParticipant msp = (ModelSynchronizeParticipant) participant;
  ICompareInput input = msp.asCompareInput(object);
this "asCompareInput" implementation correctly tries to find the compare adapter corresponding to the given object (the IFile "A" in this case) :
  Utils.getCompareAdapter(object)
Which in turn calls
  org.eclipse.team.internal.ui.Utils.getModelProvider(Object)
in order to determine the proper ModelProvider for that IFile.

However, this implementation of "getModelProvider" adapts the IFile into a ResourceMapping directly (o.getAdapter(ResourceMapping.class)) to retrieve the corresponding model provider (mapping.getModelProvider()). And there lies the problem : the platform already defines an adapter factory in order to adapt IFile into ResourceMapping. A third party plugin cannot override this adapter factory safely (there are simply too many things that rely on this adaptation, and an adapter factory cannot be provided with an "enablement" condition). As such, the "ModelProvider" that gets used for the "open in compare editor" action is _always_ the "Workspace model provider", and the corresponding implementation of "asCompareInput", "org.eclipse.team.internal.ui.mapping.ResourceModelPersistenceAdapter#asCompareInput".

This feels like a half-provided API : third party plugins are allowed to make files part of a logical model spanning more than one file ... but they cannot make it so that these files cannot be compared alone. As I see it, the implementation of "org.eclipse.team.internal.ui.Utils.getModelProvider(Object)" is too restrictive (adapting directly to ResourceMapping) and should instead make use of "ModelProvider#getMatchingResources()" if the "object" passed as parameter can be adapted to IResource.

I have used something like this successfuly without notable problems (though I haven't tested much, this is only meant to give you an idea of where the problem is. I can transform this into a patch if you'd rather) :

public static ModelProvider getModelProvider(Object o) {
  if (o instanceof ModelProvider) {
    return (ModelProvider)o;
  }

  // FORK Starts here
  // Can "o" be adapted to an IResource?
  IResource resource = getResource(o);
  if (resource != null) {
    IModelProviderDescriptor[] descriptors = ModelProvider.getModelProviderDescriptors();
    for (int i = 0; i < descriptors.length; i++) {
      IModelProviderDescriptor descriptor = descriptors[i];
      try {
        IResource[] resources = descriptor.getMatchingResources(new IResource[] {resource});
        if (resources.length > 0) {
          return descriptor.getModelProvider();
        }
      } catch (CoreException e) {
        TeamUIPlugin.log(e);
      }
    }
  }
  // FORK Ends here

  ResourceMapping mapping = getResourceMapping(o);
  if (mapping != null) {
    return mapping.getModelProvider();
  }
  return null;
}