Network error should not result in a Apache syntax error for a WMTS source

[marisn]

Our national mapping agency struggles to keep their ArcGIS based servers up and running all the time – quite often they reply with an error (e.g. 503). Unfortunately in a such case, it is not possible to operate MapCache, as it fails when a configuration file is parsed. This is bad as a single broken WMTS source means that Apache process will fail to start. It is also impossible to use (undocumented!) fallback source, as each individual source is parsed first.

The GDAL source performs some checks on data source that requires opening of the source thus I don't see how to easily work around the issue.

mapcache/lib/source_gdal.c

Line 712 in 4e7c0e3

if( GDALGetProjectionRef( hDataset ) != NULL

# apachectl configtest
ERROR 1: HTTP error code : 503
ERROR 1: Missing Capabilities.Contents element
AH00526: Syntax error on line 58 of /etc/apache2/sites-enabled/prod.conf:
gdalOpen failed on data /home/conf/source.xml

[tbonfort]

The configuration check is actually only needed to set/cache src->srs_wkt. I believe it would be relatively easy to defer this caching when actually accessing a metatile.
NB: this deferred check/caching should be protected by a mutex, as it will potentially be ran concurrently, in pseudo-code:

if src->srs_wkt == NULL:
  aquire_mutex()
  if src->srs_wkt == NULL:
    call gdal methods to get srs_wkt
    if error: //i.e. your 503
      release_mutext()
      return error
    src->srs_wkt = srs_wkt
  release_mutex()