--- a/src/lib-dict/Makefile.am	Fri Mar 17 12:58:08 2017 +0200
+++ b/src/lib-dict/Makefile.am	Wed Mar 22 10:32:04 2017 +0200
@@ -15,6 +15,7 @@
 	dict-memcached.c \
 	dict-memcached-ascii.c \
 	dict-redis.c \
+	dict-fail.c \
 	dict-transaction-memory.c
 
 libdict_la_SOURCES = \

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/lib-dict/dict-fail.c	Wed Mar 22 10:32:04 2017 +0200
@@ -0,0 +1,123 @@
+/* Copyright (c) 2017 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "dict.h"
+#include "dict-private.h"
+
+struct dict_iterate_context dict_iter_unsupported =
+{
+	.dict = &dict_driver_fail,
+};
+
+struct dict_transaction_context dict_transaction_unsupported =
+{
+        .dict = &dict_driver_fail,
+};
+
+static int dict_fail_init(struct dict *dict_driver ATTR_UNUSED,
+			  const char *uri ATTR_UNUSED,
+			  const struct dict_settings *set ATTR_UNUSED,
+			  struct dict **dict_r ATTR_UNUSED, const char **error_r)
+{
+	*error_r = "Unsupported operation (dict does not support this feature)";
+	return -1;
+}
+
+static void dict_fail_deinit(struct dict *dict ATTR_UNUSED)
+{
+}
+
+static int dict_fail_wait(struct dict *dict ATTR_UNUSED)
+{
+	return -1;
+}
+
+static int dict_fail_lookup(struct dict *dict ATTR_UNUSED, pool_t pool ATTR_UNUSED,
+			    const char *key ATTR_UNUSED, const char **value_r ATTR_UNUSED)
+{
+	return -1;
+}
+
+static struct dict_iterate_context *
+dict_fail_iterate_init(struct dict *dict ATTR_UNUSED, const char *const *paths ATTR_UNUSED,
+		       enum dict_iterate_flags flags ATTR_UNUSED)
+{
+	return &dict_iter_unsupported;
+}
+
+static bool dict_fail_iterate(struct dict_iterate_context *ctx ATTR_UNUSED,
+			      const char **key_r ATTR_UNUSED, const char **value_r ATTR_UNUSED)
+{
+	return FALSE;
+}
+
+static int dict_fail_iterate_deinit(struct dict_iterate_context *ctx ATTR_UNUSED)
+{
+	return -1;
+}
+
+static struct dict_transaction_context *dict_fail_transaction_init(struct dict *dict ATTR_UNUSED)
+{
+	return &dict_transaction_unsupported;
+}
+
+static int dict_fail_transaction_commit(struct dict_transaction_context *ctx ATTR_UNUSED,
+					bool async ATTR_UNUSED,
+					dict_transaction_commit_callback_t *callback,
+					void *context)
+{
+	if (callback != NULL)
+		callback(DICT_COMMIT_RET_FAILED, context);
+	return -1;
+}
+
+static void dict_fail_transaction_rollback(struct dict_transaction_context *ctx ATTR_UNUSED)
+{
+}
+
+static void dict_fail_set(struct dict_transaction_context *ctx ATTR_UNUSED,
+			  const char *key ATTR_UNUSED, const char *value ATTR_UNUSED)
+{
+}
+
+static void dict_fail_unset(struct dict_transaction_context *ctx ATTR_UNUSED,
+			    const char *key ATTR_UNUSED)
+{
+}
+
+static void dict_fail_atomic_inc(struct dict_transaction_context *ctx ATTR_UNUSED,
+				 const char *key ATTR_UNUSED, long long diff ATTR_UNUSED)
+{
+}
+
+static bool dict_fail_switch_ioloop(struct dict *dict ATTR_UNUSED)
+{
+	return TRUE;
+}
+
+static void dict_fail_set_timestamp(struct dict_transaction_context *ctx ATTR_UNUSED,
+				    const struct timespec *ts ATTR_UNUSED)
+{
+}
+
+struct dict dict_driver_fail = {
+	.name = "fail",
+	.v = {
+		.init = dict_fail_init,
+		.deinit = dict_fail_deinit,
+		.wait = dict_fail_wait,
+		.lookup = dict_fail_lookup,
+		.iterate_init = dict_fail_iterate_init,
+		.iterate = dict_fail_iterate,
+		.iterate_deinit = dict_fail_iterate_deinit,
+		.transaction_init = dict_fail_transaction_init,
+		.transaction_commit = dict_fail_transaction_commit,
+		.transaction_rollback = dict_fail_transaction_rollback,
+		.set = dict_fail_set,
+		.unset = dict_fail_unset,
+		.atomic_inc = dict_fail_atomic_inc,
+		.lookup_async = NULL,
+		.switch_ioloop = dict_fail_switch_ioloop,
+		.set_timestamp = dict_fail_set_timestamp
+	},
+};

--- a/src/lib-dict/dict-private.h	Fri Mar 17 12:58:08 2017 +0200
+++ b/src/lib-dict/dict-private.h	Wed Mar 22 10:32:04 2017 +0200
@@ -77,5 +77,9 @@
 extern struct dict dict_driver_memcached_ascii;
 extern struct dict dict_driver_redis;
 extern struct dict dict_driver_cdb;
+extern struct dict dict_driver_fail;
+
+extern struct dict_iterate_context dict_iter_unsupported;
+extern struct dict_transaction_context dict_transaction_unsupported;
 
 #endif