[laeubi]

Add a Delegation Hook to the Core Framework

Adds org.osgi.framework.hooks.delegation 1.0 with DelegationHook.
The framework consults it as a last step of the Overall Search Order, after nothing was found.
Hooks can only add classes and resources, never replace or hide what the wiring provides.

Motivation is osgi/osgi#970: extensions such as Java SPI mediation (osgi/osgi#372) need to contribute classes and resources across bundles without weaving or framework internals.

Commits

1. .design/requirements-970.md
2. .design/design-970.md
3. API (DelegationHook), new specification chapter, and the search order paragraph in the module layer chapter

Key points

• findClass and findResource are used only if the search order found nothing; findResources results are always appended.
• Not consulted for java.* or for packages the bundle imports, requires or exports.
• Hooks run in ranking order, without framework locks, and a throwing hook leads to a FrameworkEvent.ERROR.
• A class taken from another bundle makes the asking bundle part of the provider's refresh dependency closure.
• Registration requires ServicePermission[DelegationHook, REGISTER]; extension bundles can register it from their ExtensionBundle-Activator.

Open points

• Plain dependency versus a real wire for provider classes.
• Chapter number (58 is a placeholder).
• No TCK yet.

Implementation

Prototypes in both major open source frameworks show the feasibility:

• Equinox: Prototype: support the OSGi DelegationHook eclipse-equinox/equinox#1395
  It shows that the hook fits into BundleLoader and the dependency closure with small changes.
  It has not been exercised by tests yet.
• Felix: Prototype: support the OSGi DelegationHook apache/felix-dev#591
  It consults the hook at the end of the search order in BundleWiringImpl and tracks the provider as a plain, non-wire refresh dependency.
  It has a few focused tests for classes, resources, the dependency closure and failing hooks.