`MergeInputReader` drops objects when IDs collide across OSM types

[clarisma]

OSM IDs are only unique within the same object type, but
MergeInputReader.apply_to_reader() appears to treat them as globally unique
when applying changes.

Example:

<osm version="0.6">
    <node id="1" version="1" lon="-90" lat="45"/>
    <node id="2" version="1" lon="90" lat="45"/>
    <way id="1" version="1">
        <nd ref="1"/>
        <nd ref="2"/>
    </way>
    <relation id="1" version="1">
        <member type="way" ref="1"/>
    </relation>
</osm>

<osmChange version="0.6">
    <modify>
        <relation id="1" version="2">
            <member type="way" ref="1" role=""/>
            <tag k="hello" v="world"/>
        </relation>
    </modify>
</osmChange>

Produces:

<?xml version='1.0' encoding='UTF-8'?>
<osm version="0.6" generator="libosmium/2.23.1">
  <node id="1" version="1" lat="45" lon="-90"/>
  <node id="2" version="1" lat="45" lon="90"/>
  <way id="1" version="1">
    <nd ref="1"/>
    <nd ref="2"/>
  </way>
</osm>

The updated file should include relation/1.

If I change the relation's ID to 10 (so it doesn't collide with way/1 -- or node/1), it works as expected:

<?xml version='1.0' encoding='UTF-8'?>
<osm version="0.6" generator="libosmium/2.23.1">
  <node id="1" version="1" lat="45" lon="-90"/>
  <node id="2" version="1" lat="45" lon="90"/>
  <way id="1" version="1">
    <nd ref="1"/>
    <nd ref="2"/>
  </way>
  <relation id="10" version="2">
    <member type="way" ref="1" role=""/>
    <tag k="hello" v="world"/>
  </relation>
</osm>

I discovered this in PyOsmium 4.3.1 using:

def apply_changes(source, changes_file, target):
    changes = osmium.MergeInputReader()
    changes.add_file(str(changes_file))
    reader = osmium.io.Reader(source)
    writer = osmium.io.Writer(str(target), reader.header())
    try:
        changes.apply_to_reader(reader, writer)
    finally:
        reader.close()
        writer.close()

The problematic code appearas to be in lib/merge_input_reader.cc:

class copy_first_with_id {
    osmium::io::Writer* writer;
    osmium::object_id_type id = 0;

public:
    // ...

    void push_back(const osmium::OSMObject& obj) {
        if (obj.id() != id) {
            if (obj.visible()) {
                (*writer)(obj);
            }
            id = obj.id();
        }
    }
};

osmium-tool appears to use similar code (though I haven't verified its output yet), so it may be affected by the same bug.

[lonvia]

Oops. And I can confirm that osmium-tool has the same bug. No wonder that's where that code comes from.