rptest: add test_validate_cloud_storage_cache_size_config

tests/rptest/tests/cluster_config_test.py

@@ -1614,6 +1614,36 @@ def test_validate_cloud_storage_api_endpoint(self, value, valid):
             #Valid api endpoint
             assert valid
 
+    # None for pct_value is std::nullopt, which defaults to 0.0 in the cloud cache.
+    @cluster(num_nodes=3)
+    @parametrize(size_value=0, pct_value=None, valid=False)
+    @parametrize(size_value=0, pct_value=0.0, valid=False)
+    @parametrize(size_value=0, pct_value=-1.0, valid=False)
+    @parametrize(size_value=0, pct_value=101.0, valid=False)
+    @parametrize(size_value=-1, pct_value=None, valid=False)
+    @parametrize(size_value=1024, pct_value=None, valid=True)
+    @parametrize(size_value=10, pct_value=50.0, valid=True)
+    @parametrize(size_value=0, pct_value=0.1, valid=True)
+    def test_validate_cloud_storage_cache_size_config(self, size_value,
+                                                      pct_value, valid):
+        upsert = {}
+        upsert["cloud_storage_cache_size"] = size_value
+        upsert["cloud_storage_cache_size_percent"] = pct_value
+
+        if valid:
+            patch_result = self.admin.patch_cluster_config(upsert=upsert)
+            new_version = patch_result['config_version']
+            wait_for_version_status_sync(self.admin, self.redpanda,
+                                         new_version)
+            updated_config = self.admin.get_cluster_config()
+            assert updated_config["cloud_storage_cache_size"] == size_value
+            assert updated_config[
+                "cloud_storage_cache_size_percent"] == pct_value
+        else:
+            with expect_exception(requests.exceptions.HTTPError,
+                                  lambda e: e.response.status_code == 400):
+                self.admin.patch_cluster_config(upsert=upsert)
+
 
 """
 PropertyAliasData: