Supporting Solr 9 in 7.x version of Search API Solr module

Here is a diff between solr 7.x XML config and solr 9.x XML config:

diff solr-conf/7.x solr-conf/9.x
diff solr-conf/7.x/schema.xml solr-conf/9.x/schema.xml
13c13
< <schema name="drupal-4.4-solr-7.x" version="1.5">
---
> <schema name="drupal-4.4-solr-9.x" version="1.5">
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<     <fieldType name="location" class="solr.LatLonType" subFieldType="tdouble"/>
---
>     <fieldType name="location" class="solr.LatLonPointSpatialField" docValues="true"/>
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<     <!-- A Geohash is a compact representation of a latitude longitude pair in a single field.
<          See http://wiki.apache.org/solr/SpatialSearch
<      -->
<     <fieldtype name="geohash" class="solr.GeoHashField"/>
< 
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<     <dynamicField name="geos_*" type="geohash" indexed="true"  stored="true" multiValued="false"/>
<     <dynamicField name="geom_*" type="geohash" indexed="true"  stored="true" multiValued="true"/>
---
>     <!-- GeoHash fields are deprecated. LatLonPointSpatial fields solve all needs. But we keep the dedicated field names for backward compatibility. -->
>     <dynamicField name="geos_*" type="location" indexed="true"  stored="true" multiValued="false"/>
>     <dynamicField name="geom_*" type="location" indexed="true"  stored="true" multiValued="true"/>
diff solr-conf/7.x/solrconfig.xml solr-conf/9.x/solrconfig.xml
23c23
< <config name="drupal-4.4-solr-7.x" >
---
> <config name="drupal-4.4-solr-9.x" >
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<   <luceneMatchVersion>${solr.luceneMatchVersion:LUCENE_60}</luceneMatchVersion>
---
>   <luceneMatchVersion>${solr.luceneMatchVersion:LUCENE_80}</luceneMatchVersion>
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>   <!--
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>   -->
>   <lib dir="/opt/solr/contrib/extraction/lib" regex=".*\.jar" />
>   <lib dir="/opt/solr/contrib/langid/lib/" regex=".*\.jar" />
>   <lib dir="/opt/solr/contrib/scripting/lib/" regex=".*\.jar" />
>   <lib dir="/opt/solr/contrib/clustering/lib/" regex=".*\.jar" />
>   <lib dir="/opt/solr/contrib/analysis-extras/lib" regex=".*\.jar" />
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<     <filterCache class="solr.FastLRUCache"
<                  size="512"
---
>     <filterCache size="512"
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<     <queryResultCache class="solr.LRUCache"
<                      size="512"
---
>     <queryResultCache size="512"
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<     <documentCache class="solr.LRUCache"
<                    size="512"
---
>     <documentCache size="512"
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<   <queryResponseWriter name="xslt" class="solr.XSLTResponseWriter">
---
>   <queryResponseWriter name="xslt" class="solr.scripting.xslt.XSLTResponseWriter">
diff solr-conf/7.x/solrcore.properties solr-conf/9.x/solrcore.properties
10c10
< solr.luceneMatchVersion=6.0
---
> solr.luceneMatchVersion=8.0

Attached is the patch to add the Solr 9 XML config.

Note, moving to Solr 9 definitely need a re-index. I set the luceneMatchVersion to LUCENE_80 to make it maybe easier to upgrade to.

Thanks a lot for sharing this!
If anyone else has had a chance to test this, please let me know whether it worked for you.

At a quick test, it mostly seemed to work for me after a few modifications: You hard-coded the absolute path of the contrib directory, so this didn’t work for me. Also, the installation I downloaded had that directory renamed to modules, compared to contrib used in Solr 8 and before. Is this directory named contrib still in your Solr 9 installation?

The only problem still remaining in the version uploaded here is that highlighting doesn’t work: For some reason, even though the unified highligher is used, Solr returns an error with this message: field 'tm_body$value' was indexed without offsets, cannot highlight. According to the documentation, however, the unified highlighter shouldn’t require offsets in the indexed data, so I don’t quite understand this. It works fine with Solr 8 and earlier, with the exact same request.
Adding hl.method=original solves the problem, but I’d prefer to find the source of this discrepancy first before settling on a fix/workaround for it.

Thanks @drunken-monkey for updating the patch. I was using the Solr 9 docker image that happened to symlink the contrib folder hence why I kept it but forget about when I was sharing the code.

Interesting, I hadn't tested that. I can give it a try.