schema_registry: Reject records if schema id validation without registry

If the schema registry is disabled, but schema id validation is enabled,
reject records and log an error.

Fixes redpanda-data#11116

Signed-off-by: Ben Pope <ben@redpanda.com>

src/v/pandaproxy/schema_registry/validation.cc

@@ -457,9 +457,16 @@ std::optional<schema_id_validator> maybe_make_schema_id_validator(
   const model::topic& topic,
   const cluster::topic_properties& props) {
     auto mode = config::shard_local_cfg().enable_schema_id_validation();
-    return api != nullptr && should_validate_schema_id(props, mode)
-             ? std::make_optional<schema_id_validator>(api, topic, props, mode)
-             : std::nullopt;
+    if (should_validate_schema_id(props, mode)) {
+        if (!api) {
+            vlog(
+              plog.error,
+              "{} requires schema_registry to be enabled in redpanda.yaml",
+              config::shard_local_cfg().enable_schema_id_validation.name());
+        }
+        return std::make_optional<schema_id_validator>(api, topic, props, mode);
+    }
+    return std::nullopt;
 }
 
 ss::future<schema_id_validator::result>

tests/rptest/tests/schema_registry_test.py

@@ -12,6 +12,7 @@
 import json
 from typing import Optional
 import uuid
+import re
 import requests
 import time
 import random
@@ -2176,6 +2177,45 @@ def test_enable_schema_id_validation(self, mode):
             pass
 
 
+class SchemaValidationWithoutSchemaRegistry(RedpandaTest):
+    INVALID_CONFIG_LOG_ALLOW_LIST = DEFAULT_LOG_ALLOW_LIST + [
+        re.compile(
+            r"enable_schema_id_validation requires schema_registry to be enabled in redpanda.yaml"
+        ),
+    ]
+
+    def __init__(self, *args, **kwargs):
+        super(SchemaValidationWithoutSchemaRegistry,
+              self).__init__(*args,
+                             extra_rp_conf={
+                                 'enable_schema_id_validation':
+                                 SchemaIdValidationMode.REDPANDA.value
+                             },
+                             schema_registry_config=None,
+                             **kwargs)
+
+    @cluster(num_nodes=1, log_allow_list=INVALID_CONFIG_LOG_ALLOW_LIST)
+    def test_disabled_schema_registry(self):
+        rpk = RpkTool(self.redpanda)
+        topic = "no_schema_registry"
+        rpk.create_topic(
+            topic,
+            config={
+                TopicSpec.PROPERTY_RECORD_KEY_SCHEMA_ID_VALIDATION: 'true',
+            })
+        try:
+            rpk.produce(topic, "key", "value")
+            assert False, "expected INVALID_RECORD"
+        except RpkException as e:
+            print(e)
+            assert "INVALID_RECORD" in e.stderr
+
+        wait_until(lambda: self.redpanda.search_log_all(
+            "enable_schema_id_validation requires schema_registry to be enabled in redpanda.yaml"
+        ),
+                   timeout_sec=5)
+
+
 class SchemaValidationTopicPropertiesTest(RedpandaTest):
     def __init__(self, *args, **kwargs):
         super(SchemaValidationTopicPropertiesTest,