Description

Fixes asymmetric PATCH behavior when converting upstream.nodes between array and hash table formats.

Problem Statement

The Admin API exhibits inconsistent behavior when patching a route's upstream.nodes field:

• Converting from array → hash table works
• Converting from hash table → array fails with validation error:
  {"error_msg":"invalid configuration: property \"upstream\" validation failed: property \"nodes\" validation failed: object matches none of the required"}

This happens despite both formats being officially supported by the schema.

Root Cause

The merge() function in apisix/core/table.lua only checks if the origin value is an array before deciding whether to replace or recursively merge:

if _M.nkeys(origin[k]) ~= #origin[k] then
  merge(origin[k] or {}, extend[k] or {})  -- Merge if origin is hash
else
  origin[k] = v  -- Replace if origin is array
end

When patching from hash table → array:

• Origin is hash table, so it tries to merge
• This creates an invalid mixed structure with both string keys (from hash) and numeric indexes (from array)
• Validation fails because it's neither a valid hash nor a valid array

Solution

Modified the merge logic to check both the origin and incoming values:

• If either is an array → replace (prevents invalid mixed structures)
• If both are hash tables → merge (safe to combine)

Which issue(s) this PR fixes:

Fixes #13045

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)