fix(consul): tolerate null Service.Meta in fetch_services_from_server

[nic-6443]

What this does

Fix a crash in the Consul service discovery worker when a registered service has no metadata.

Why

When consul services register is used without meta, the agent's catalog API returns "Meta": null. cjson decodes JSON null as a userdata sentinel (cjson.null), which is truthy but not a table. The current guard at apisix/discovery/consul/client.lua:415:

if preserve_metadata and node.Service.Meta
        and next(node.Service.Meta) then

passes the truthiness check, then calls next() on userdata and aborts the entire catalog scan with:

bad argument #1 to 'next' (table expected, got userdata)

The discovery worker never recovers — no nodes are produced for any service in that response, so all upstreams resolved through Consul end up empty until the worker is restarted (and even then it crashes again on the next scrape).

This was introduced by #13230 which extracted client.lua and added the new preserve_metadata path; older code didn't iterate Service.Meta.

Fix

Use type(node.Service.Meta) == "table" instead of relying on truthiness, so cjson.null is treated as "no metadata" (same as the field being absent).

Test

Added a regression test (t/discovery/consul.t TEST 14) that:

1. registers a real consul service without Meta
2. calls consul_client.fetch_services_from_server directly with preserve_metadata=true
3. asserts no error and metadata == nil

The test fails on master with the userdata error and passes with the fix.

[Copilot]

Pull request overview

Fixes a crash in the Consul discovery client when Consul returns "Meta": null (decoded by cjson as cjson.null userdata) by guarding metadata iteration with a type(...) == "table" check, and adds a regression test for this scenario.

Changes:

• Guard next(node.Service.Meta) behind type(node.Service.Meta) == "table" when preserve_metadata=true.
• Add a new test that registers a real Consul service without Meta and verifies fetch_services_from_server does not error and returns metadata == nil.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
apisix/discovery/consul/client.lua	Prevents next() from being called on cjson.null userdata when Service.Meta is null.
t/discovery/consul.t	Adds a regression test covering Consul services registered without Meta.

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