[Spice-devel] [PATCH 1/2] Ensure that plugins cannot bypass version check

[Christophe Fergeau]

On Tue, Apr 17, 2018 at 06:39:05PM +0200, Christophe de Dinechin wrote:
> When does this kind of scenario happen? Imagine we added support for
> loading/unloading plugins. Version 13 adds a new interface to unload
> plugins, and a new “unloadable plugin” entry point. Therefore,
> starting with version 13, the agent would try the new entry point
> first, and if it’s there, assume it can unload the plugin, otherwise
> fallback to the old method and mark the plugin as not-unloadable.
> Therefore, the change in version 13 would still be binary compatible
> with version 3. Years later, in version 21, we decide to drop support
> for non-unloadable plugins. So now the agent refuses to load a plugin
> if it does not have the new entry point. In that case, we’d want to
> bump the “compatible version” to 13, since all plugins after 13 have
> that new entry point. The key win here is that we’d not have to force
> the last compatible version to be 21. 
> 
> This could mean years of differences in terms of binary compatibility.
> We could deprecate things slowly and offer graceful transitions.

This would work if all that is needed to use the new interface is a
recompile, and not active porting, or if we make sure plugins using the
old interface can no longer be built when version 13 comes around.
If we are instead considering a method used in version 10 which is
called 'my_method'. version 13 deprecates 'my_method', and replaces it
with 'my_new_method', but API/ABI compatibility is kept with version 10,
so I can rebuild my plugin calling 'my_method' against version 13. I
assume the plugin ABI version would become 13 after being recompiled?
Then in version 21 I decide to remove 'my_method' because it's been long
obsolete, but my plugin recompiled against version 13 will no longer
work.
Am I missing something, or misusing this?

Christophe
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