webmap-import: Add option to generate Mapbox Vector Tiles (MVT).

1 files changed, 0 insertions, 775 deletions

diff --git a/webmap-publish b/webmap-publish
deleted file mode 100755
index 8db920b..0000000
--- a/webmap-publish
+++ /dev/null
@@ -1,775 +0,0 @@
-#!/usr/bin/python3
-
-#----------------------------------------------------------------------
-# Backend utilities for the Klimatanalys Norr project (create MVT tiles)
-# Copyright © 2024 Guilhem Moulin <info@guilhem.se>
-#
-# This program is free software: you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation, either version 3 of the License, or
-# (at your option) any later version.
-#
-# This program 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 this program.  If not, see <https://www.gnu.org/licenses/>.
-#----------------------------------------------------------------------
-
-from os import O_RDONLY, O_WRONLY, O_CREAT, O_EXCL, O_TRUNC, O_CLOEXEC, O_DIRECTORY
-import os
-import errno
-from fcntl import flock, LOCK_EX, LOCK_UN
-from string import hexdigits
-import json
-import logging
-import math
-import random
-import argparse
-import tempfile
-from pathlib import Path
-import numpy
-
-from osgeo import gdal, ogr, osr
-from osgeo.gdalconst import (
-    OF_VECTOR as GDAL_OF_VECTOR,
-    OF_READONLY as GDAL_OF_READONLY,
-    OF_UPDATE as GDAL_OF_UPDATE,
-    OF_VERBOSE_ERROR as GDAL_OF_VERBOSE_ERROR,
-)
-gdal.UseExceptions()
-
-import common
-from common import (
-    format_bytes,
-    gdalSetOpenExArgs,
-    escape_identifier
-)
-
-# Open and return source dataset.
-def openSourceDS(def_dict):
-    # merge 'open-options-publish' into 'open-options'
-    opts2 = def_dict.get('open-options-publish', None)
-    if opts2 is not None:
-        if 'open-options' not in def_dict:
-            def_dict['open-options'] = {}
-        def_dict['open-options'] |= opts2
-    kwargs, _ = gdalSetOpenExArgs(gdal, def_dict, flags=GDAL_OF_READONLY|GDAL_OF_VERBOSE_ERROR)
-
-    path = def_dict['path']
-    logging.debug('OpenEx(%s, %s)', path, str(kwargs))
-    return gdal.OpenEx(path, **kwargs)
-
-# Choose a random subdirectory under the given base directory and create
-# a new MVT dataset there.
-# The old MVT dataset (if any) remains, which is intentional we don't
-# want to disrupt browser caches and tile → attribute mapping.
-def createMVT(basedir, options=None):
-    drv = gdal.GetDriverByName('MVT')
-    if drv is None:
-        raise Exception('Unknown GDAL driver for format "MVT"')
-
-    # choose an available name at random from 00 to FF
-    weights = [1] * 256
-    for path in basedir.iterdir():
-        filename = path.name
-        if len(filename) == 2 and filename[0] in hexdigits and filename[1] in hexdigits:
-            n = int(filename, 16)
-            weights[n] = 0
-    if all([w == 0 for w in weights]):
-        raise Exception(f'No name available in {args.destdir} (forgot to cleanup?)')
-
-    name = random.choices(range(256), weights=weights, k=1)[0]
-    name = f'{name:0>2x}'
-    path = str(basedir.joinpath(name))
-
-    # merge options from the config file
-    defaults_map = {
-        'min-zoom': 'MINZOOM',
-        'max-zoom': 'MAXZOOM',
-        'max-size': 'MAX_SIZE',
-        'max-features': 'MAX_FEATURES',
-        'simplification': 'SIMPLIFICATION',
-        'simplification-max-zoom': 'SIMPLIFICATION_MAX_ZOOM',
-        'tiling-scheme': 'TILING_SCHEME',
-        'extent': 'EXTENT',
-    }
-    defaults_options = config.get('vector-tiles', None)
-    if defaults_options is not None:
-        for k, v in defaults_options.items():
-            if k not in defaults_map:
-                logging.warning('No default map for key "%s", ignoring', k)
-                continue
-            if options is None:
-                options = {}
-            k = defaults_map[k]
-            if k not in options:
-                if isinstance(v, list):
-                    v = ','.join(map(str, v))
-                options[k] = v
-
-    kwargs = { 'eType': gdal.GDT_Unknown }
-    if options is not None:
-        kwargs['options'] = opts = []
-        for k, v in options.items():
-            opts.append(k + '=' + str(v))
-
-    logging.debug('Create(%s, %s, eType=%s%s)', drv.ShortName, path, kwargs['eType'],
-        ', options=' + str(kwargs['options']) if 'options' in kwargs else '')
-    ds = drv.Create(path, 0, 0, 0, **kwargs)
-    return ds, name
-
-# Filter a source layer.
-def getSourceLayer(ds_src, layerdef, extent=None):
-    layername = layerdef['__layername-src__']
-    lyr_src = ds_src.GetLayerByName(layername)
-    if lyr_src is None:
-        raise Exception(f'Source dataset has no layer named "{layername}"')
-
-    count0 = -1
-    if lyr_src.TestCapability(ogr.OLCFastFeatureCount):
-        count0 = lyr_src.GetFeatureCount(force=0)
-
-    srs_src = lyr_src.GetSpatialRef()
-    if srs_src is None:
-        raise Exception('Source layer "{}" has no SRS'.format(layername))
-    elif not srs_src.IsProjected():
-        raise Exception('Source layer "{}" SRS is not projected: {}'.format(
-            layername, srs_src.ExportToPrettyWkt()))
-    # "the value to multiply by linear distances to transform them to meters"
-    linearUnits = srs_src.GetLinearUnits()
-    logging.debug('Source layer "%s" SRS ("%s") linear units: %f', layername,
-                  srs_src.GetName(), linearUnits)
-
-    # get geometry field of the source layer
-    defn = lyr_src.GetLayerDefn()
-    geomFieldCount = defn.GetGeomFieldCount()
-    if geomFieldCount != 1:
-        logging.warning('Source layer "%s" has %d != 1 geometry fields', lyr_src.GetName(), geomFieldCount)
-    geomField = defn.GetGeomFieldDefn(0)
-    geomType = geomField.GetType()
-    geomFieldName_esc = escape_identifier(geomField.GetName())
-
-    # transform extent to source SRS
-    if extent is None or extent.GetSpatialReference().IsSame(srs_src):
-        extent_src = extent
-    else:
-        extent_src = extent.Clone()
-        if extent_src.TransformTo(srs_src) != ogr.OGRERR_NONE:
-            raise Exception('Could not transform extent {} to {}'.format(
-                extent.ExportToWkt(), srs_src.GetName()))
-
-
-    reserved_fields = set(map(lambda x: x.lower(), [
-        'geom',
-        'GeomArea',
-        'GeomPerimeter',
-        'GeomLength',
-    ]))
-
-    # build SQL query
-    drv = ds_src.GetDriver()
-    columns = []
-    for i in range(defn.GetFieldCount()):
-        fld = defn.GetFieldDefn(i)
-        fldType = fld.GetType()
-        if fld.GetName().lower() in reserved_fields:
-            raise Exception(f'Invalid column name "{fld.GetName()}"')
-        fldName_esc = escape_identifier(fld.GetName())
-        column = 'm.' + fldName_esc
-        # for date/time/datetime fields, we let the RDBMS engine do the formatting if possible
-        if fldType == ogr.OFTDate:
-            if drv.ShortName == 'PostgreSQL':
-                column = 'to_char(' + column + ', \'YYYY-MM-DD\') AS '
-            elif drv.ShortName == 'GPKG':
-                column = 'strftime(\'%F\', ' + column + ') AS '
-        elif fldType == ogr.OFTTime:
-            if drv.ShortName == 'PostgreSQL':
-                column = 'to_char(' + column + ', \'HH24:MI:SSOF\') AS '
-            elif drv.ShortName == 'GPKG':
-                # XXX SQLite's strftime() lacks support for time zones
-                column = 'strftime(\'%T\', ' + column + ') AS '
-        elif fldType == ogr.OFTDateTime:
-            if drv.ShortName == 'PostgreSQL':
-                column = 'to_char(' + column + ', \'YYYY-MM-DD"T"HH24:MI:SSOF\') AS '
-            elif drv.ShortName == 'GPKG':
-                # XXX SQLite's strftime() lacks support for time zones
-                column = 'strftime(\'%FT%T\', ' + column + ') AS '
-        if column.endswith(' AS '):
-            column += fldName_esc
-        columns.append(column)
-
-    # add columns with computed info about the geometry (before clipping and rounded to 0.01m²/0.01m)
-    if ogr.GT_IsSubClassOf(geomType, ogr.wkbSurface) or ogr.GT_IsSubClassOf(geomType, ogr.wkbMultiSurface):
-        columns.append('ROUND(CAST(ST_Area(m.' + geomFieldName_esc + ') * '
-            + str(linearUnits) + '*' + str(linearUnits) + ' AS numeric), 2) AS "GeomArea"')
-        columns.append('ROUND(CAST(ST_Perimeter(m.' + geomFieldName_esc + ') * '
-            + str(linearUnits) + ' AS numeric), 2) AS "GeomPerimeter"')
-    elif ogr.GT_IsSubClassOf(geomType, ogr.wkbCurve) or ogr.GT_IsSubClassOf(geomType, ogr.wkbMultiCurve):
-        columns.append('ROUND(CAST(ST_Length(m.' + geomFieldName_esc + ') * '
-            + str(linearUnits) + ' AS numeric), 2) AS "GeomLength"')
-
-    transform_geometry = layerdef.get('transform-geometry', None)
-    if transform_geometry is None:
-        columns.append('m.' + geomFieldName_esc)
-    elif transform_geometry == 'centroid':
-        columns.append('ST_Centroid(m.' + geomFieldName_esc + ') AS ' + geomFieldName_esc)
-    else:
-        raise Exception(f'Unsupported geometry transformation: {transform_geometry}')
-
-    query = 'SELECT ' + ', '.join(columns) + ' FROM ' + escape_identifier(lyr_src.GetName()) + ' m'
-
-    # add WHERE clauses and spatial filter; for GPKG/SQlite3, the query
-    # is too complex and the driver can't use ExecuteSQL()'s spatialFilter
-    # option so we add the ST_Intersects() condition
-    if drv.ShortName == 'PostgreSQL':
-        spatialFilter = extent_src
-    else:
-        spatialFilter = None
-
-    cond = layerdef.get('where', None)
-    if (extent_src is not None and spatialFilter is None) or cond is not None:
-        query += ' WHERE '
-        if extent_src is not None and spatialFilter is None:
-            query += 'ST_Intersects(m.' + geomFieldName_esc + ', \'' + extent_src.ExportToWkt() + '\')'
-            if cond is not None:
-                query += ' AND '
-        if cond is not None:
-            query += '(' + cond.strip() + ')'
-
-    logging.debug('SQL query: %s', query)
-    lyr_src = ds_src.ExecuteSQL(query, spatialFilter=spatialFilter)
-
-    count1 = -1
-    if lyr_src.TestCapability(ogr.OLCFastFeatureCount):
-        count1 = lyr_src.GetFeatureCount(force=0)
-    if count0 >= 0 and count1 >= 0:
-        logging.debug('Source layer "%s" has %d features, of which %d are to be exported',
-                      layername, count0, count1)
-    return lyr_src
-
-# Export a layer from the source dataset, returning the next FID availble.
-# TODO bump metadata_maxsize util the compressed version doesn't exceed 64k
-def exportLayer(lyr_dst, lyr_src, srs=None, clip=None, layername=None,
-        coordinate_precision=None, nextfid=0,
-        # rotate after the feature which bring the file to the 256kiB mark
-        metadata_maxsize=256*1024, metadata=None):
-    if srs is None:
-        raise Exception('Missing target SRS')
-    if clip is not None:
-        clip_srs = clip.GetSpatialReference()
-        if not clip_srs.IsSame(srs):
-            raise Exception('Clipping geometry {} has SRS {},\nexpected {}'.format(
-                clip.ExportToWkt(), clip_srs.ExportToPrettyWkt(), srs.ExportToPrettyWkt()))
-    if metadata is None:
-        metadata_fp = None
-    else:
-        metadata_fp = metadata.get('fp', None)
-        exportToJson_options = []
-        if coordinate_precision is not None:
-            exportToJson_options = ['COORDINATE_PRECISION=' + str(coordinate_precision)]
-        metadata_size = metadata.get('cursize', 0)
-
-    srs_src = lyr_src.GetLayerDefn().GetGeomFieldDefn(0).GetSpatialRef()
-    if srs.IsSame(srs_src):
-        logging.debug('Both source and destination have the same SRS (%s), skipping coordinate transformation',
-            srs.GetName())
-        ct = None
-    else:
-        # TODO Use GetSupportedSRSList() and SetActiveSRS() with GDAL ≥3.7.0
-        # when possible, see apps/ogr2ogr_lib.cpp
-        logging.debug('Creating transforming from source SRS (%s) to destination SRS (%s)',
-            srs_src.GetName(), srs.GetName())
-        ct = osr.CoordinateTransformation(srs_src, srs)
-        if ct is None:
-            raise Exception(f'Could not create transformation from source SRS ({srs_src.GetName()}) '
-                + f'to destination SRS ({srs.GetName()})')
-
-    # extract metadata index and filename from nextfid (an exception is
-    # raised if nextfid doesn't fit in 4 bytes)
-    bstr = nextfid.to_bytes(4, byteorder='big')
-    nextfid_idx = int.from_bytes(bstr[-2:], byteorder='big')
-    nextfid_p   = int.from_bytes(bstr[:2],  byteorder='big')
-    metadata_maxlen = 1 << (8*2)
-
-    defn_dst = lyr_dst.GetLayerDefn()
-
-    # we roll our own loop rather than using ogr2ogr/GDALVectorTranslate()
-    # since we want to do a custom JSON serialization at the same time,
-    # in order to avoid duplicating properties across tiles or clipping
-    # overlays at tile boundaries
-
-    count = 0
-    feature = lyr_src.GetNextFeature()
-    while feature is not None:
-        geom = feature.GetGeometryRef().Clone()
-        if ct is not None and geom.Transform(ct) != ogr.OGRERR_NONE:
-            raise Exception('Could not apply coordinate transformation')
-        geom.FlattenTo2D()
-
-        if metadata is not None:
-            # serialize the feature to JSON and save it
-            # TODO there is a lot of room for improvement here
-            #  - geometries could be exported to WKB or FlatGeoBuf rather than GeoJSON
-            #  - columns (schema) could be saved in layers.json only, so
-            #  the object would be an array of length n+1 where the
-            #  geometry would be at index 0 or n
-            #  - data could be serialized to binary, cf. BJSON or GeoBuf
-            feature.SetGeometry(geom)
-            obj = feature.ExportToJson(as_object=True, options=exportToJson_options)
-            #props = { key:value for key, value in obj['properties'].items() if value is not None }
-            props = obj['properties']
-            props['geom'] = obj['geometry']
-
-            bstr = json.dumps(props, ensure_ascii=False, separators=(',',':')).encode('utf-8')
-
-            #if nextfid_idx >= metadata_maxlen or nextfid_p * metadata_maxlen + nextfid_idx != nextfid:
-            #    raise Exception(f'impossible: {nextfid_p}*{metadata_maxlen} + {nextfid_idx} != {nextfid}')
-
-            if metadata_fp is None:
-                metadata_fp = openNewMetadaFile(metadata, nextfid_p)
-                metadata_size = 0
-            else:
-                metadata_fp.write(b',')
-
-            metadata_size += 1 + metadata_fp.write(bstr)
-
-        # clip the feature and export it (without fields to MVT)
-        geom2 = geom if clip is None else geom.Intersection(clip)
-        feature2 = ogr.Feature(defn_dst)
-        feature2.SetGeometryDirectly(geom2)
-        feature2.SetFID(nextfid)
-
-        if lyr_dst.CreateFeature(feature2) != ogr.OGRERR_NONE:
-            raise Exception(f'Could not transfer source feature #{feature.GetFID()}')
-
-        nextfid += 1
-        if metadata is not None:
-            nextfid_idx += 1
-            if metadata_size > metadata_maxsize or nextfid_idx >= metadata_maxlen:
-                closeMetadataFile(metadata)
-                metadata_fp = metadata_size = None
-                nextfid_p += 1
-                nextfid_idx = 0
-                nextfid = nextfid_p * metadata_maxlen
-                # will open a new while when processiong the next feature
-
-        count += 1
-        feature = lyr_src.GetNextFeature()
-
-    logging.info('Exported %d features to layer "%s" from "%s"', count,
-                 lyr_dst.GetName(), layername)
-
-    if metadata is not None:
-        # save size of the metadata file currently open for proper accounting
-        metadata['cursize'] = 0 if metadata_fp is None else metadata_size
-    return nextfid, count
-
-# Open a new metadata file.
-def openNewMetadaFile(metadata, nextfid_p):
-    name = nextfid_p.to_bytes(length=2, byteorder='big').hex()
-    name = Path(metadata['tempdir']).joinpath(name).with_suffix('.json')
-    logging.debug('open(%s)', str(name))
-
-    fd = os.open(name, O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, mode=0o0644,
-                 dir_fd=metadata.get('dirfd', None))
-    metadata['fp'] = fp = os.fdopen(fd, mode='wb', closefd=True)
-    fp.write(b'[')
-    return fp
-
-# Close a metadata file.
-def closeMetadataFile(metadata):
-    fp = metadata.pop('fp')
-    fp.write(b']')
-    logging.debug('close(%d)', fp.fileno())
-    fp.close()
-
-# Wrapper around os.write() that loops until the entire data is written.
-def write_all(fd, buf):
-    offset = 0
-    while offset < len(buf):
-        try:
-            n = os.write(fd, buf[offset:])
-        except IOError as e:
-            if e.errno != errno.EAGAIN:
-                raise e
-            #logging.debug('Got EAGAIN while trying to write')
-        else:
-            #logging.debug('Wrote %d bytes of %d', n, len(buf) - offset)
-            offset += n
-    return offset
-
-# Compress a file to Brotli.
-# TODO increase chunksize to see if helps with performances
-def compress_brotli(path, suffix='.br', dir_fd=None, chunksize=65536,
-                    oFlags=0, **kwargs):
-    compressor = brotli.Compressor(**kwargs)
-    fd_in = os.open(path, O_RDONLY|O_CLOEXEC, dir_fd=dir_fd)
-    size_in = size_out = 0
-    try:
-        # hard-code the mode to save an fstat(2) call
-        fd_out = os.open(path + suffix, O_WRONLY|O_CREAT|O_CLOEXEC|oFlags,
-                         mode=0o0644, dir_fd=dir_fd)
-        try:
-            buf_in = os.read(fd_in, chunksize)
-            while len(buf_in) > 0:
-                #logging.debug('Read %d bytes of %d', len(buf_in), chunksize)
-                size_in += len(buf_in)
-                size_out += write_all(fd_out, compressor.process(buf_in))
-                buf_in = os.read(fd_in, chunksize)
-            size_out += write_all(fd_out, compressor.finish())
-        finally:
-            os.close(fd_out)
-    finally:
-        os.close(fd_in)
-    return size_in, size_out
-
-
-if __name__ == '__main__':
-    common.init_logger(app=os.path.basename(__file__), level=logging.INFO)
-
-    parser = argparse.ArgumentParser(description='Export GIS layers to Mapbox Vector Tiles (MVT).')
-    parser.add_argument('--debug', action='count', default=0,
-        help=argparse.SUPPRESS)
-    parser.add_argument('--destdir', default=os.curdir,
-        help=f'destination directory for generated files (default: {os.curdir})')
-    parser.add_argument('--lockfile', default=None,
-        help=f'obtain an exclusive lock while exporting features to MVT')
-    parser.add_argument('--name', default=None,
-        help=f'name to use in the layer map (default: last component of DESTDIR)')
-    parser.add_argument('--webroot', default=None,
-        help=f'path to the web root')
-    parser.add_argument('--metadata', default=None,
-        help=f'path to the JSON metadata file (default: DESTDIR/../metadata.json)')
-    parser.add_argument('--metadata-lockfile', default=None,
-        help=f'path to the JSON metadata lock file (default: DESTDIR/../.metadata.json.lck)')
-    parser.add_argument('--compress', action=argparse.BooleanOptionalAction, default=False,
-        help='whether to compress tiles (default: False)')
-    parser.add_argument('groupname', nargs='*', help='group layer name(s) to process')
-    args = parser.parse_args()
-
-    if args.debug > 0:
-        logging.getLogger().setLevel(logging.DEBUG)
-    if args.debug > 1:
-        from http.client import HTTPConnection
-        HTTPConnection.debuglevel = 1
-        requests_log = logging.getLogger('urllib3')
-        requests_log.setLevel(logging.DEBUG)
-        requests_log.propagate = True
-
-    if args.debug > 0:
-        logging.getLogger().setLevel(logging.DEBUG)
-    if args.debug > 1:
-        gdal.ConfigurePythonLogging(enable_debug=True)
-
-    if args.compress:
-        # fail early if the module is missing
-        import brotli
-
-    destdirname = args.name if args.name is not None else Path(args.destdir).name
-    if not destdirname:
-        logging.error('Could not infer --name value')
-        exit(1)
-
-    config = common.get_config(groupnames=None if args.groupname == [] else args.groupname)
-
-    # validate configuration
-    if 'dataset' not in config:
-        raise Exception('Configuration does not specify source dataset')
-
-    export_layers = {}
-    mvtconf = {}
-    for layername, layerdef in config.get('layers', {}).items():
-        exportdef = layerdef.get('publish', None)
-        if exportdef is None:
-            raise Exception(f'Layer "{layername}" has no publication definition')
-        if isinstance(exportdef, str):
-            exportdef = { exportdef:{} }
-        elif isinstance(exportdef, list):
-            exportdef = { l:{} for l in exportdef }
-        for export_layername, export_layerdef in exportdef.items():
-            if export_layername in export_layers:
-                raise Exception(f'Duplicate definition for {export_layername}')
-            x = {}
-            for k in ('target_name', 'minzoom', 'maxzoom'):
-                if k in export_layerdef:
-                    x[k] = export_layerdef[k]
-            mvtconf[export_layername] = x
-            export_layers[export_layername] = export_layerdef
-            export_layerdef['__layername-src__'] = layername
-
-    if args.destdir != os.curdir:
-        try:
-            os.mkdir(args.destdir, mode=0o0755)
-        except FileExistsError:
-            pass
-
-    # open source DS
-    ds = openSourceDS(config['dataset'])
-
-    # get configured Spatial Reference System and extent; force traditional GIS order
-    # on the target SRS since it's what MVT and JSON export are expecting
-    srs = common.getSRS(osr, config.get('SRS', None))
-    srs.SetAxisMappingStrategy(osr.OAMS_TRADITIONAL_GIS_ORDER)
-    extent, extent_simple = common.getExtent(config.get('extent', None), srs=srs)
-
-    if srs.IsProjected():
-        # will export with centimeter precision
-        linearUnits = srs.GetLinearUnits()
-        coordinate_precision = math.ceil(math.log10(linearUnits) + 2)
-        if coordinate_precision < 0:
-            coordinate_precision = 0
-        logging.debug('Coordinate precision: %d', coordinate_precision)
-
-    if args.lockfile is not None:
-        lock_fd = os.open(args.lockfile, O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, mode=0o644)
-        logging.debug('flock("%s", LOCK_EX)', args.lockfile)
-        flock(lock_fd, LOCK_EX)
-
-    # always lock destdir (until the program exits) to avoid concurrent
-    # generations of the same tileset
-    destdir_fd = os.open(args.destdir, O_RDONLY|O_CLOEXEC|O_DIRECTORY)
-    logging.debug('flock("%s", LOCK_EX)', args.destdir)
-    flock(destdir_fd, LOCK_EX)
-
-    nFeatures = 0
-    tile_extension = '.pbf'
-    basedir = Path(f'/proc/self/fd/{destdir_fd}')
-    with tempfile.TemporaryDirectory() as tempdir:
-        # use a temporary directory for TEMPORARY_DB
-        logging.debug('Using "%s" as temporary directory', tempdir)
-
-        ds_dst, dirname = createMVT(basedir, options={
-                'FORMAT': 'DIRECTORY',
-                # OpenLayers doesn't support compressed tiles, so instead
-                # we use brotli_static to let the browser transparently
-                # uncompress the responses
-                'COMPRESS': 'NO',
-                'TYPE': 'overlay',
-                'BUFFER': 32,
-                'TILE_EXTENSION': tile_extension.removeprefix('.'),
-                'TEMPORARY_DB': Path(tempdir).joinpath('tmp-mvt.db'),
-                'CONF': json.dumps(mvtconf, ensure_ascii=False, separators=(',',':')),
-        })
-
-        # use a temporary directory for the metadata to avoid messing
-        # around with he MVT directory while building is in progress
-        metadata = { 'dirfd': destdir_fd }
-        metadata['tempdir'] = Path(tempfile.mkdtemp(prefix='.' + dirname + '-attr.',
-                                   dir=basedir)).relative_to(basedir)
-        try:
-            logging.debug('Using "%s" as temporary metadata directory', metadata['tempdir'])
-            os.chmod(metadata['tempdir'], 0o0755, dir_fd=metadata.get('dirfd', None),
-                     follow_symlinks=False)
-            nextfid = 0
-            for layername, layerdef in export_layers.items():
-                # create destination layer
-                lyr_dst = ds_dst.CreateLayer(layername, srs=srs, geom_type=ogr.wkbUnknown)
-                if lyr_dst is None:
-                    raise Exception(f'Could not create destination layer "{layername}"')
-
-                lyr_src = getSourceLayer(ds, layerdef, extent=extent)
-                try:
-                    # use the non-dense geometry (a rectangle) for clipping as it's faster
-                    # and there is no need to reproject (the SRSs match)
-                    nextfid, count0 = exportLayer(lyr_dst, lyr_src,
-                        layername=layerdef['__layername-src__'],
-                        srs=srs, clip=extent_simple,
-                        coordinate_precision=coordinate_precision,
-                        nextfid=nextfid, metadata=metadata)
-                    nFeatures += count0
-                finally:
-                    ds.ReleaseResultSet(lyr_src)
-                lyr_dst = None
-
-            # close datasets
-            ds = None
-            ds_dst = None
-
-            # move metadata to the MVT structure (now that it has been closed)
-            logging.debug('rename(%s/%s, %s/%s/attr)',
-                          args.destdir, metadata['tempdir'],
-                          args.destdir, dirname)
-            os.rename(metadata['tempdir'],
-                      Path(dirname).joinpath('attr'),
-                      src_dir_fd=metadata.get('dirfd', None),
-                      dst_dir_fd=destdir_fd)
-            metadata.pop('tempdir', None)
-
-        except Exception as e:
-            # cleanup
-            for root, dirnames, filenames, rootfd in os.fwalk(top=metadata['tempdir'],
-                        dir_fd=metadata.get('dirfd', None), topdown=False,
-                        follow_symlinks=False):
-                for filename in filenames:
-                    #logging.debug('unlink(%s/%s)', root, filename)
-                    os.unlink(filename, dir_fd=rootfd)
-                for dirname in dirnames:
-                    #logging.debug('rmdir(%s/%s)', root, dirname)
-                    os.rmdir(dirname, dir_fd=rootfd)
-            #logging.debug('rmdir(%s/%s)', args.destdir, metadata['tempdir'])
-            os.rmdir(metadata['tempdir'], dir_fd=metadata.get('dirfd', None))
-            raise e
-
-        finally:
-            if 'fp' in metadata:
-                closeMetadataFile(metadata)
-
-            srs = None
-            extent = None
-            extent_simple = None
-
-    try:
-        # OpenLayers doesn't make use of that file so delete it
-        os.unlink(Path(dirname).joinpath('metadata.json'), dir_fd=destdir_fd)
-    except FileNotFoundError:
-        pass
-
-    logging.info('Done creating tiles and attribute files (total %d features)', nFeatures)
-
-    if args.lockfile is not None:
-        # release the shared lock, the remaining shouldn't be as demanding
-        logging.debug('flock("%s", LOCK_UN)', args.lockfile)
-        flock(lock_fd, LOCK_UN)
-
-
-    stats_tiles = []
-    stats_metadata = []
-    if args.compress:
-        compress_mode = brotli.MODE_GENERIC
-
-    # collect stats and optionally compress
-    # TODO: optimize traversal, cf.
-    # https://r3n.hashnode.dev/tech-chronicles-conquer-memory-monsters-with-pythons-yield-in-large-directory-processing
-    # TODO: benchmark brotli quality to trade-off compression ratio vs. execution time
-    # (also, tile and attribute files can have different levels if needs be)
-    for root, _, filenames, rootfd in os.fwalk(top=dirname, dir_fd=destdir_fd,
-                follow_symlinks=False):
-        for filename in filenames:
-            lfilename = filename.lower()
-            if lfilename.endswith(tile_extension):
-                arr = stats_tiles
-            elif lfilename.endswith('.json'):
-                arr = stats_metadata
-            else:
-                continue
-
-            if not args.compress:
-                st = os.stat(filename, dir_fd=rootfd)
-                arr.append(st.st_size)
-            else:
-                #logging.debug("Compressing %s/%s", root, filename)
-                szi, szo = compress_brotli(filename, dir_fd=rootfd,
-                    oFlags=O_EXCL, mode=compress_mode, quality=5)
-                arr.append((szi, szo))
-
-    # display stats
-    if len(stats_tiles) > 0:
-        stats_tiles = numpy.array(stats_tiles, dtype=numpy.int64, copy=False)
-
-        arr = stats_tiles[:,0] if args.compress else stats_tiles
-        tot = numpy.sum(arr)
-        logging.info('Tile count: %d [min=%s, max=%s, sum=%s, avg=%s]',
-            arr.size,
-            format_bytes(numpy.min(arr)), format_bytes(numpy.max(arr)),
-            format_bytes(tot), format_bytes(round(tot/arr.size)))
-
-        if args.compress:
-            arr = stats_tiles[:,1]
-            ztot = numpy.sum(arr)
-            logging.info('After compression: min=%s, max=%s, sum=%s, avg=%s (ratio %.1f%%)',
-                format_bytes(numpy.min(arr)), format_bytes(numpy.max(arr)),
-                format_bytes(ztot), format_bytes(round(ztot/arr.size)),
-                100. * (1. - ztot/tot))
-
-    if len(stats_metadata) > 0:
-        stats_metadata = numpy.array(stats_metadata, dtype=numpy.int64, copy=False)
-
-        arr = stats_metadata[:,0] if args.compress else stats_metadata
-        tot = numpy.sum(arr)
-        logging.info('Metadata file count: %d [min=%s, max=%s, sum=%s, avg=%s]',
-            arr.size,
-            format_bytes(numpy.min(arr)), format_bytes(numpy.max(arr)),
-            format_bytes(tot), format_bytes(round(tot/arr.size)))
-
-        if args.compress:
-            arr = stats_metadata[:,1]
-            ztot = numpy.sum(arr)
-            logging.info('After compression: min=%s, max=%s, sum=%s, avg=%s (ratio %.1f%%)',
-                format_bytes(numpy.min(arr)), format_bytes(numpy.max(arr)),
-                format_bytes(ztot), format_bytes(round(ztot/arr.size)),
-                100. * (1. - ztot/tot))
-
-
-    # update metadata.json
-    if args.metadata is None:
-        mapfile = Path(args.destdir).parent.joinpath('metadata.json')
-    else:
-        mapfile = Path(args.metadata)
-
-    # first, place an exclusive lock (it is important that all
-    # invocations of the program with the same --metadata file, even on
-    # different layer groups, share the same lock file to avoid
-    # concurrent updates of the medata.json)
-    if args.metadata_lockfile is None:
-        mapfile_lock = mapfile.with_name('.' + mapfile.name + '.lck')
-    else:
-        mapfile_lock = Path(args.metadata_lockfile)
-    mapfile_lock_fd = os.open(mapfile_lock, O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, mode=0o644)
-    logging.debug('flock("%s", LOCK_EX)', mapfile_lock)
-    flock(mapfile_lock_fd, LOCK_EX)
-
-    # load the file
-    try:
-        with open(mapfile, mode='rb') as fp:
-            mapobj = json.load(fp)
-    except FileNotFoundError:
-        mapobj = {}
-
-    # remove the compressed version as we are prepare to update the
-    # uncompressed one
-    mapfile_compressed = mapfile.with_name(mapfile.name + '.br')
-    try:
-        logging.debug('unlink(%s)', mapfile_compressed)
-        os.unlink(mapfile_compressed)
-    except FileNotFoundError:
-        pass
-
-    k = 'layermap'
-    if k not in mapobj:
-        layermap = mapobj[k] = {}
-    else:
-        layermap = mapobj[k]
-    destpath = Path(args.destdir)
-    if args.webroot is not None:
-        destpath = destpath.relative_to(args.webroot)
-    destpath = '/' + destpath.as_posix().removeprefix('/') + '/' + dirname
-    layermap[destdirname] = destpath
-
-    # dump the new object to the temporary file
-    bstr = json.dumps(mapobj, ensure_ascii=False, separators=(',',':')).encode('utf-8')
-    mapfile_tmp = mapfile.with_stem('.' + mapfile.stem + '.new')
-    fd = os.open(mapfile_tmp, O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, mode=0o644)
-    with os.fdopen(fd, mode='wb', closefd=True) as fp:
-        fp.write(bstr)
-
-    # compress the temporary file
-    if args.compress:
-        mapfile_tmp_compressed = mapfile_tmp.with_name(mapfile_tmp.name + '.br')
-        compressor = brotli.Compressor()
-        fd = os.open(mapfile_tmp_compressed, O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, mode=0o644)
-        try:
-            write_all(fd, compressor.process(bstr))
-            write_all(fd, compressor.finish())
-        finally:
-            os.close(fd)
-
-    # rename temporary files to production (starting with the uncompressed one)
-    logging.debug('rename(%s, %s)', mapfile_tmp, mapfile)
-    os.rename(mapfile_tmp, mapfile)
-
-    if args.compress:
-        logging.debug('rename(%s, %s)', mapfile_tmp_compressed, mapfile_compressed)
-        os.rename(mapfile_tmp_compressed, mapfile_compressed)