Add `webmap-publish` script to export layers to Mapbox Vector Tiles.

1 files changed, 760 insertions, 0 deletions

diff --git a/webmap-publish b/webmap-publish
new file mode 100755
index 0000000..36bed88
--- /dev/null
+++ b/webmap-publish
@@ -0,0 +1,760 @@
+#!/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,
+    gdalVersionMin,
+    gdalGetMetadataItem,
+    escapeIdentifier
+)
+
+# 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(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 = common.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 = escapeIdentifier(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 = escapeIdentifier(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"')
+    columns.append('m.' + geomFieldName_esc)
+
+    query = 'SELECT ' + ', '.join(columns) + ' FROM ' + escapeIdentifier(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')
+
+        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)
+
+    common.load_config(groupnames=None if args.groupname == [] else args.groupname)
+
+    # validate configuration
+    if 'dataset' not in common.config:
+        raise Exception('Configuration does not specify source dataset')
+
+    export_layers = {}
+    for layername, layerdef in common.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 not in export_layers:
+                export_layers[export_layername] = []
+            export_layers[export_layername].append(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(common.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(common.config.get('SRS', None))
+    srs.SetAxisMappingStrategy(osr.OAMS_TRADITIONAL_GIS_ORDER)
+    extent, extent_simple = common.getExtent(common.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',
+                'COMPRESS': 'NO',
+                'TYPE': 'overlay',
+                'BUFFER': 32,
+                'TILE_EXTENSION': tile_extension.removeprefix('.'),
+                'TEMPORARY_DB': Path(tempdir).joinpath('tmp-mvt.db'),
+        })
+
+        # 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, layerdefs 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}"')
+
+                for layerdef in layerdefs:
+                    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)