KV-engine stuck in crash loop after <6.6.1 to 6.6.1+ upgrade and SyncWrites in use

NOTE: 7.0+ is not affected by this issue

During upgrade from <6.6.1 to 6.6.2+, a new stat is added and tracked in _local/vbstate to count the bytes in on-disk Prepares - see MB-42306.

There is a bug in how the updates to this stat are handled during compaction - the value is not clamped to zero and hence can underflow - see https://github.com/couchbase/kv_engine/blob/v6.6.1/engines/ep/src/couch-kvstore/couch-kvstore.cc#L1325-L1327 :

        auto prepareBytes = json.find("on_disk_prepare_bytes");

        if (prepareBytes != json.end()) {

            auto onDiskPrepareBytes =

                    std::stoull(prepareBytes->get<std::string>()) -

                    ctx.stats.prepareBytesPurged;

            *prepareBytes = std::to_string(onDiskPrepareBytes);

            updateVbState = true;

        }

Note that if ctx.stats.prepareBytesPurged is greater than prepareBytes read from disk, the value written will underflow to a negative number - or very large positive if represented as a unsigned value.

This issue was fixed in 7.0.0 upwards as part of http://review.couchbase.org/c/kv_engine/+/140801/4/engines/ep/src/couch-kvstore/couch-kvstore.cc#1585 when this code was merged into the master branch at the time. Unfortunately it wasn't flagged as a customer-impacting bug and hence was not tracked as affecting v.6.6.x.