panic in switchPhase

In 6.6.3-9805 there is a panic in the query logs:

_time=2021-08-05T12:04:49.055-07:00 _level=SEVERE _msg=panic: runtime error: invalid memory address or nil pointer dereference 

_time=2021-08-05T12:04:49.055-07:00 _level=SEVERE _msg=request text: <ud>PREPARE SELECT * FROM `bucket-1` USE KEYS[$1];</ud>

This panic is also present in 6.6.3-9806. In addition in 6.6.3-9806, in the ns_server error log there are errors indicating connection refused by query admin/stats endpoint. This results in the reported query throughput number being lower but in reality, there is no performance regression.

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=iris_663-9805_access_af0e&snapshot=iris_663-9806_access_c76e

9805 - http://perf.jenkins.couchbase.com/job/iris-multi-client/10740/ - 126314.0

9806 - http://perf.jenkins.couchbase.com/job/iris-multi-client/10737/ - 83959.0

Not sure how long the query panic has been present, but can try to find the initial offending build. Looks like there is no relevant commit in 9806 to cause this issue. Last god run was 9804 so probably caused by this commit in 9805 and the admin/stats issue is spurious side effect:

http://changelog.build.couchbase.com/?product=couchbase-server&fromVersion=6.6.3&fromBuild=9804&toVersion=6.6.3&toBuild=9806&f_backup=on&f_cbas-core=on&f_go-couchbase=on

https://github.com/couchbase/go-couchbase/commit/4ed93c0ce9e9db8b97f184dccbcd724d46bfde02

https://issues.couchbase.com/browse/MB-47722