change updates to handle delete/insert workflow

This makes Nominatim compatible with osm2pgsql's default update
modus operandi of deleting and reinserting data. Deletes are diverted
into a TODO table instead of executing them. When data is reinserted,
the corresponding entry in the TODO table is deleted. After updates are
finished, the remaining entries in the TODO table are executed, doing
the same work as the delete trigger did before.

The new behaviour also works against the gazetteer output with its
insert-only mechanism.

nominatim/tools/replication.py

@@ -130,10 +130,7 @@ def update(conn: Connection, options: MutableMapping[str, Any],
         if endseq is None:
             return UpdateState.NO_CHANGES
 
-        # Consume updates with osm2pgsql.
-        options['append'] = True
-        options['disable_jit'] = conn.server_version_tuple() >= (11, 0)
-        run_osm2pgsql(options)
+        run_osm2pgsql_updates(conn, options)
 
         # Write the current status to the file
         endstate = repl.get_state_info(endseq)
@@ -143,6 +140,25 @@ def update(conn: Connection, options: MutableMapping[str, Any],
     return UpdateState.UP_TO_DATE
 
 
+def run_osm2pgsql_updates(conn: Connection, options: MutableMapping[str, Any]) -> None:
+    """ Run osm2pgsql in append mode.
+    """
+    # Remove any stale deletion marks.
+    with conn.cursor() as cur:
+        cur.execute('TRUNCATE place_to_be_deleted')
+    conn.commit()
+
+    # Consume updates with osm2pgsql.
+    options['append'] = True
+    options['disable_jit'] = conn.server_version_tuple() >= (11, 0)
+    run_osm2pgsql(options)
+
+    # Handle deletions
+    with conn.cursor() as cur:
+        cur.execute('SELECT flush_deleted_places()')
+    conn.commit()
+
+
 def _make_replication_server(url: str, timeout: int) -> ContextManager[ReplicationServer]:
     """ Returns a ReplicationServer in form of a context manager.