system_keyspace, view: reduce interdependencies

system_keyspace is an implementation detail for most of its users, not
part of the interface, as it's only used to store internal data. Therefore,
including it in a header file causes unneeded dependencies.

This patch removes a dependency between views and system_keyspace.hh
by moving view_name and view_build_progress into a separate header file,
and using forward declarations where possible. This allows us to
remove an inclusion of system_keyspace.hh from a header file (the last
one), so that further changes to system_keyspace.hh will cause fewer
recompilations.
Message-Id: <20181228215736.11493-1-avi@scylladb.com>

db/system_keyspace.cc

@@ -62,6 +62,7 @@
 #include "query_context.hh"
 #include "partition_slice_builder.hh"
 #include "db/config.hh"
+#include "system_keyspace_view_types.hh"
 #include "schema_builder.hh"
 #include "md5_hasher.hh"
 #include "release.hh"

db/system_keyspace.hh

@@ -127,12 +127,7 @@ struct range_estimates {
 };
 
 using view_name = std::pair<sstring, sstring>;
-struct view_build_progress {
-    view_name view;
-    dht::token first_token;
-    std::optional<dht::token> next_token;
-    shard_id cpu_id;
-};
+struct view_build_progress;
 
 extern schema_ptr hints();
 extern schema_ptr batchlog();

db/system_keyspace_view_types.hh

@@ -0,0 +1,47 @@
+/*
+ * Copyright (C) 2018 ScyllaDB
+ */
+
+/*
+ * This file is part of Scylla.
+ *
+ * Scylla is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Scylla is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Scylla.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <seastar/core/reactor.hh>
+#include <seastar/core/sstring.hh>
+#include <utility>
+#include <optional>
+#include "seastarx.hh"
+
+namespace dht {
+
+class token;
+
+}
+
+namespace db::system_keyspace {
+
+using view_name = std::pair<sstring, sstring>;
+
+struct view_build_progress {
+    view_name view;
+    dht::token first_token;
+    std::optional<dht::token> next_token;
+    shard_id cpu_id;
+};
+
+}

db/view/view.cc

@@ -58,6 +58,8 @@
 #include "cql3/util.hh"
 #include "db/view/view.hh"
 #include "db/view/view_builder.hh"
+#include "db/system_keyspace_view_types.hh"
+#include "db/system_keyspace.hh"
 #include "frozen_mutation.hh"
 #include "gms/inet_address.hh"
 #include "keys.hh"

db/view/view_builder.hh

@@ -22,7 +22,6 @@
 #pragma once
 
 #include "database_fwd.hh"
-#include "db/system_keyspace.hh"
 #include "db/system_distributed_keyspace.hh"
 #include "dht/i_partitioner.hh"
 #include "keys.hh"
@@ -46,6 +45,14 @@
 #include <unordered_map>
 #include <vector>
 
+namespace db::system_keyspace {
+
+using view_name = std::pair<sstring, sstring>;
+class view_build_progress;
+
+}
+
+
 namespace db::view {
 
 /**