Issue rendering tiles in non Spherical Mercator projection

[samfranklin]

I'm attempting to generate a slippy map tiles in a specific projection system (EPSG:27700, British National Grid), for a specific subset of OSM data covering Wales, using this pbf file.

I am using the switch2osm stack, and am running their docker container on Ubuntu 20.04 just on localhost, following these instructions.

To attempt to generate tiles in EPSG:27700, I've made a minor edit to my mapnik xml stylesheet, specifying the epsg:27700 map projection string:
<Map srs="+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs" background-color="#f2efe9">

After editing this style sheet, I've checked that mapnik can generate a BNG tile and correctly transform the osm postgis source data (in EPSG:3857), this is the python snippet:

import mapnik
m = mapnik.Map(256,256)
mapnik.load_map(m, '/home/renderer/src/openstreetmap-carto/mapnik-bng.xml')
extent_uk_tile_s_bng = mapnik.Box2d(0, 0, 700000, 700000)
m.zoom_to_box(extent_uk_tile_s_bng)
mapnik.render_to_file(m, 'output.png')

The output draws a tile in EPSG:27700 and correctly displays my data (in EPSG:3857), so this tells me my mapnik stylesheet appears to be correctly configured.

This is my renderd.conf (only changing the XML path from the default shipped with switch2osm:

$ cat /usr/local/etc/renderd.conf 
[renderd]
num_threads=4
tile_dir=/var/lib/mod_tile
stats_file=/var/run/renderd/renderd.stats

[mapnik]
plugins_dir=/usr/lib/mapnik/3.0/input
font_dir=/usr/share/fonts/truetype
font_dir_recurse=1

[ajt]
URI=/tile/
TILEDIR=/var/lib/mod_tile
XML=/home/renderer/src/openstreetmap-carto/mapnik-bng.xml
HOST=localhost
TILESIZE=256
MAXZOOM=20

The following log entries are generated from request http://localhost/tile/0/0/0.png.

	renderd[2313]: Using bng projection settings
	renderd[2313]: Using bng projection settings
	renderd[2313]: Using bng projection settings
	renderd[2313]: Using bng projection settings
	renderd[2313]: DEBUG: Got incoming connection, fd 7, number 1
	renderd[2313]: DEBUG: Got incoming request with protocol version 2
	renderd[2313]: DEBUG: Got command RenderPrio fd(7) xml(ajt), z(0), x(0), y(0), mime(image/png), options()
	renderd[2313]: DEBUG: START TILE ajt 0 0-0 0-0, new metatile
	renderd[2313]: Rendering projected coordinates 0 0 0 -> 0.000000|0.000000 700000.000000|1400000.000000 to a 1 x 2 tile
	renderd[2313]: DEBUG: DONE TILE ajt 0 0-0 0-0 in 1.705 seconds
	debug: Creating and writing a metatile to /var/lib/mod_tile/ajt/0/0/0/0/0/0.meta
	renderd[2313]: DEBUG: Sending render cmd(3 ajt 0/0/0) with protocol version 2 to fd 7
	renderd[2313]: DEBUG: Connection 0, fd 7 closed, now 0 left

The log entry Rendering projected coordinates 0 0 0 -> 0.000000|0.000000 700000.000000|1400000.000000 to a 1 x 2 tile.
Since the bounding box is not a square, which would explain why mod_tile is rendering two tiles instead the single tile. I can see that gen_tile.cpp is responsible for setting this non-square bounding box on this line

mod_tile/src/gen_tile.cpp

Line 148 in f28cda9

prj->bound_x1 = 700000;

My problem.
When requesting http://localhost/tile/0/0/0.png this only delivers the "northernmost" of the two tiles ie bbox(0 700000,700000 14000000). However, my actual data extent (Wales) is located in the southernmost of the two tiles within bbox(0 0, 700000 700000).

Forgive me, I'm new to map tiling, with webmercator, the bbox is a square, ie x axis and y axis are equal, but the BNG extent is not a square, therefore, two queries:

1. Is there anything in the configuration somewhere that I can change to deliver a 1x1 tile when http://localhost/tile/0/0/0.png is made?

2. If there's no configuration option, would editing the gen_tile.cpp source be enough to deliver a square tile, see below example. Or am I missing something?

		prj->bound_x0 = 0;
		prj->bound_y0 = 0;
		prj->bound_x1 = 1400000;
		prj->bound_y1 = 1400000;
		prj->aspect_x = 1;
		prj->aspect_y = 1;

Thanks for any tips on where to go next.