[laeubi]

Prototype: support the OSGi DelegationHook

Implements the proposed org.osgi.framework.hooks.delegation.DelegationHook in the Felix framework.
Spec proposal: osgi/org.osgi.framework#77, background osgi/osgi#970.
Equinox prototype: eclipse-equinox/equinox#1395.

Changes

• Add the org.osgi.framework.hooks.delegation API and export it from the framework and main bundles.
• DelegationHookSupport calls the hooks in ranking order.
  A hook that throws is reported as FrameworkEvent.ERROR and not called again.
• BundleWiringImpl consults the hooks as the last step of the search order, after dynamic imports.
  This covers classes, single resources and resource enumerations.
  Hooks are skipped for java.* and for imported, required or exported packages.
• BundleRevisionDependencies tracks classes taken from another bundle as a non-wire dependency, so the asking bundle is part of the provider's refresh closure.

Status

Prototype to show feasibility and surface spec questions, not a complete implementation.
DelegationHookTest covers class lookup with the dependency closure, the java.* skip, resource handling and a throwing hook.
No permission checks on registration yet, and the skip for imported, required and exported packages is not tested.

Findings for the spec discussion

• Felix tracks the dependency as a plain refresh dependency, not a wire.
  This relates to the open point in the spec PR about a plain dependency versus a wire.
• The dependency affects refresh only and does not stop a provider from being uninstalled.
• Felix's re-entrancy guard is keyed by name only, so it also blocks a class and a resource lookup that share a name.
• The hook runs inside the class-loading call.
  No global framework lock is held there, but the loader's own class-loading lock stays held.