fix: don't share datatype across recursive validator calls

lib/jsonschema.lua

@@ -595,9 +595,15 @@ local reg_map = {
 
 generate_validator = function(ctx, schema)
   -- get type informations as they will be necessary anyway
-  local datatype = ctx:localvartab(sformat('%s(%s)',
+  -- These must be real `local`s (not shared table fields) because they hold
+  -- per-call state. Recursive calls into nested schema validators (e.g. via
+  -- `items` or `properties`) would otherwise mutate the outer function's
+  -- `datatype`, leading to wrong-branch execution after the recursion returns
+  -- (e.g. running a string-only check like `maxLength` on a table value and
+  -- crashing inside `utf8_len`).
+  local datatype = ctx:localvar(sformat('%s(%s)',
     ctx:libfunc('type'), ctx:param(1)))
-  local datakind = ctx:localvartab(sformat('%s == "table" and %s(%s)',
+  local datakind = ctx:localvar(sformat('%s == "table" and %s(%s)',
     datatype, ctx:libfunc('lib.tablekind'), ctx:param(1)))
 
   if type(schema) == "table" and schema._org_val ~= nil then

t/default.lua

@@ -233,3 +233,20 @@ local t = {
 local ok, err = validator(t)
 assert(ok~=nil, ("pattern: failed to check pattern with `%%`: %s"):format(err))
 ngx.say("passed: pass check pattern with `%`")
+
+----------------------------------------------------- test case 9
+-- regression: per-call `datatype` must not be shared across recursive calls.
+-- Schemas like `{type=array, maxLength=N, items={type=string}}` used to crash
+-- because the items recursion mutated the outer function's datatype variable,
+-- causing the outer string-only `maxLength` check to run on the array value
+-- and call `utf8_len` on a table.
+local rule = {
+    type = "array",
+    maxLength = 10,
+    items = { type = "string" },
+}
+local validator = jsonschema.generate_validator(rule)
+local pcall_ok, valid, val_err = pcall(validator, { "a", "b", "c" })
+assert(pcall_ok, "fail: validator threw an error: " .. tostring(valid))
+assert(valid, "fail: validator returned false: " .. tostring(val_err))
+ngx.say("passed: recursive datatype is not shared across calls")