refactor: extract reusable building blocks from K8s and Nacos discovery

[nic-6443]

Summary

Extract composable building blocks from Kubernetes and Nacos discovery modules so external consumers can reuse core logic without duplicating internal implementation.

Changes

kubernetes/core.lua (new):

• Config parsing (get_apiserver, setup_namespace_selector, etc.)
• Parameterized endpoint callbacks via create_endpoint_callbacks(options) — supports key_prefix for multi-cluster dict isolation and duplicate_port_number for flexible port key storage
• Handle factory (create_handle) and lifecycle (start_fetch) with graceful stop
• Node resolution with LRU cache (resolve_nodes)
• Dict helpers (dump_endpoints_from_dict)

nacos/client.lua (new):

• Stateless HTTP client (request, get_token_param)
• HMAC-SHA1 signing (get_signed_param)
• URL building (build_base_uri) with inline credential parsing
• Instance fetching (fetch_from_host) with configurable key_builder, preserve_metadata, and timeout
• Service scanning (get_nacos_services) with optional filter callback

informer_factory.lua enhancements:

• Graceful stop support via informer.stop flag — checked in watch loops, list pagination, and before post_list cleanup
• Configurable ssl_verify (from apiserver config, default false)
• ssl_server_name support for HTTPS connections

kubernetes/schema.lua:

• Add ssl_verify option to service config (single and multiple mode)

Both init.lua modules are refactored to use the new building blocks. No behavior change for existing users — all public APIs (init_worker, nodes, dump_data, check_discovery_ready) remain identical.

Motivation

This makes the discovery modules more composable. External consumers (e.g., dynamic discovery managers that receive registry configs at runtime) can create custom handles, reuse endpoint processing callbacks, and build on the Nacos HTTP client without duplicating hundreds of lines of internal logic.

Testing

All existing t/discovery/ and t/kubernetes/discovery/ tests should pass without modification.

[Copilot]

Pull request overview

This PR refactors APISIX service discovery by extracting reusable core logic from the Kubernetes and Nacos discovery implementations into standalone modules, enabling external consumers to reuse the underlying primitives while keeping existing discovery module APIs stable.

Changes:

• Added reusable Kubernetes discovery building blocks (apisix/discovery/kubernetes/core.lua) and refactored the Kubernetes discovery init module to use them.
• Added a reusable Nacos HTTP client module (apisix/discovery/nacos/client.lua) and refactored the Nacos discovery init module to use it.
• Enhanced Kubernetes informer factory to support graceful stop and configurable TLS behavior (ssl_verify, ssl_server_name), and extended the Kubernetes discovery schema with ssl_verify.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
apisix/discovery/nacos/init.lua	Refactored to use the extracted Nacos client for service scanning and instance fetching.
apisix/discovery/nacos/client.lua	New reusable Nacos HTTP client and discovery primitives (auth, signing, URL building, fetching).
apisix/discovery/kubernetes/schema.lua	Adds service.ssl_verify to schema (single + multi mode).
apisix/discovery/kubernetes/init.lua	Refactored to use new kubernetes.core helpers for handle creation, fetch lifecycle, and node resolution.
apisix/discovery/kubernetes/informer_factory.lua	Adds stop-flag checks and makes TLS verification + SNI configurable via apiserver settings.
apisix/discovery/kubernetes/core.lua	New reusable Kubernetes discovery core (config parsing, callbacks, handle factory, lifecycle, node resolution, dict helpers).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.