[RFC] Replace deprecated_target_new_objfile_hook by observer?

[Joel Brobecker <brobecker@adacore.com>]

Hello,

While working on the problem described in

        http://www.sourceware.org/ml/gdb/2006-09/msg00065.html

one of the things I noticed was that we're using a hook where
each client that wants to be notified inserts himself. The way
it is done is through cooperation, like so:

  1. Declare a static variable:
  
        static void (*target_new_objfile_chain)(struct objfile *);

  2. During the module initialization, store the current value
     of that hook into our static variable, and replace its
     value with our own callback.

        /* Notice when object files get loaded and unloaded.  */
          target_new_objfile_chain = deprecated_target_new_objfile_hook;
          deprecated_target_new_objfile_hook = new_objfile;

  3. At the end of our callback code, check our static variable
     and call the previous client:

        /* deprecated_target_new_objfile_hook callback.
           [snip]  */
        
        static void
        new_objfile (struct objfile *objfile)
        {
          [snip]

          if (target_new_objfile_chain)
            target_new_objfile_chain (objfile);
        }

I propose to replace this with an observer. Would that be OK?

Assuming that it is, there are several platforms that use that
mechanism. It's going to be hard to test all of them. But the changes
themselves should be pretty mechanical. So what I can propose is
to make all the necessary changes to replace that hook model with
an approach using observers and test on x86-linux. And then rely
on the mechanical aspect of the change together with the review of
another pair of eyes. Would that be OK?

The alternative is to have the two approaches coexist for a while.
I just don't think this is necessary, because I don't think the change
is that risky.

-- 
Joel