HSQLDB is mainly used in gt-epsg-hsql ThreadedHsqlEpsgFactory to resolve EPSG codes from a database stored in, for example, /tmp/Geotools/Databases/HSQL/v188.8.131.52/EPSG.data (where java.io.tmpdir is /tmp).
Upgrading will give us fixes including better synchronisation and better Java 8 support.
Other uses of hsqldb:
GeoTools gt-imagemosaic-jdbc: declares a dependency on hsqldb but does not use it.
GeoTools arcsde-plugin: several import org.hsqldb.Session
GeoServer gpxppio (community): import org.hsqldb.lib.StringInputStream
GeoServer geogig (community): LogStore creates HSQLDB sample files and HSQLLogStoreTest has test coverage
GeoServer gs-web-wps: WPSExecuteTransformer has import org.hsqldb.lib.StringInputStream
GeoServer gs-wps-core: MultiplicityValidator has import org.hsqldb.lib.Collection
Attached hsqldb-2.4.1.jar was rebuilt with debugging information. Replace the jar in your Maven repo with this one to get line numbers in your stack traces and the ability to place breakpoints in hsqldb in Eclipse.
The pull request also increases the VERSION in the EPSG database file path to guard against format changes and fixes the incorrect capitalisation of /tmp/Geotools by changing it to /tmp/GeoTools.
If this change reduces the number of CoverageViewTest failures, it should be backported.
This upgrade fixes GEOS-8802. In tests with maxcpus=2, it reduces the failure rate from 31% to 0%. I will backport this change to 19.x and 18.x.
Cherry-picked on to 19.x and 18.x.