cst/cache: allow admin api trim only if cache is initialized

Fixes UBSAN assertion caused by invoking a function call on
uninitialized cache object.

include/c++/v1/vector:1394:10: runtime error: reference binding to null pointer of type 'seastar::sharded<cloud_storage::cache>::entry'

src/v/redpanda/admin/server.cc

@@ -103,6 +103,7 @@
 #include <seastar/core/map_reduce.hh>
 #include <seastar/core/prometheus.hh>
 #include <seastar/core/reactor.hh>
+#include <seastar/core/shard_id.hh>
 #include <seastar/core/sharded.hh>
 #include <seastar/core/shared_ptr.hh>
 #include <seastar/core/smp.hh>
@@ -4347,6 +4348,14 @@ admin_server::delete_cloud_storage_lifecycle(
 ss::future<ss::json::json_return_type>
 admin_server::post_cloud_storage_cache_trim(
   std::unique_ptr<ss::http::request> req) {
+    co_await ss::smp::submit_to(ss::shard_id{0}, [this] {
+        if (!_cloud_storage_cache.local_is_initialized()) {
+            throw ss::httpd::bad_request_exception(
+              "Cloud Storage Cache is not available. Is cloud storage "
+              "enabled?");
+        }
+    });
+
     auto max_objects = get_integer_query_param(*req, "objects");
     auto max_bytes = static_cast<std::optional<size_t>>(
       get_integer_query_param(*req, "bytes"));

tests/rptest/tests/cloud_storage_cache_test.py

@@ -0,0 +1,26 @@
+import pytest
+from requests import HTTPError
+
+from rptest.services.admin import Admin
+from rptest.services.cluster import cluster
+from rptest.tests.redpanda_test import RedpandaTest
+
+
+class CloudStorageCacheAdminApisNoCacheTest(RedpandaTest):
+    """
+    Test the Cloud Storage Cache Admin APIs when tiered storage is not configured.
+    """
+    def __init__(self, test_context):
+        super().__init__(test_context, num_brokers=1)
+        self.admin = Admin(self.redpanda)
+
+    @cluster(num_nodes=1)
+    def test_admin_apis(self):
+        for node in self.redpanda.nodes:
+            with pytest.raises(HTTPError) as excinfo:
+                self.admin.cloud_storage_trim(byte_limit=None,
+                                              object_limit=None,
+                                              node=node)
+
+            assert "Cloud Storage Cache is not available. Is cloud storage enabled?" == excinfo.value.response.json(
+            )['message']