[Tile-serving] [openstreetmap/osm2pgsql] Add --middle-propagate-changes option (#1310)

[Sarah Hoffmann]

@lonvia commented on this pull request.

The change looks okay. Just some nitpicking wrt naming: internally, we have called the process 'dependency management'. So the previous terminology of forward-dependency might fit better in general than propagate-changes.

> @@ -193,11 +194,7 @@ class multithreaded_processor
             auto copy_thread = std::make_shared<db_copy_thread_t>(conninfo);
 
             for (auto const &out : m_outputs) {
-                if (out->need_forward_dependencies()) {
-                    m_clones[i].push_back(out->clone(midq, copy_thread));
-                } else {
-                    m_clones[i].emplace_back(nullptr);
-                }
+                m_clones[i].push_back(out->clone(midq, copy_thread));

Given that we no longer push back nullptr here, I'd expect some ifs to be unnecessary somewhere else. Possibly line 249?

> @@ -72,13 +72,7 @@ middle_pgsql_t::table_desc::table_desc(options_t const &options,
     m_copy_target->schema = options.middle_dbschema;
     m_copy_target->id = "id"; // XXX hardcoded column name
 
-    // Gazetteer doesn't use mark-pending processing and consequently
-    // needs no way-node index.
-    // TODO Currently, set here to keep the impact on the code small.
-    // We actually should have the output plugins report their needs
-    // and pass that via the constructor to middle_t, so that middle_t
-    // itself doesn't need to know about details of the output.
-    if (options.output_backend != "gazetteer") {
+    if (options.propagate_changes) {
         m_prepare_intarray = build_sql(options, ts.prepare_mark);
         m_array_indexes = build_sql(options, ts.create_index);

May I suggest to rename `prepare_mark` and `create_index` to reflect that this is the SQL needed for propagate_changes? The names were never particularly good to start with.

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