Server requires auth switch, but no auth switch handler provided

As per http://stackoverflow.com/questions/41941663/server-requires-auth-switch-but-no-auth-switch-handler-provided and https://github.com/mysqljs/mysql/issues/1396

This error did not occur on our test mysql server, but it does occur on the production one.

When adding the authSwitchHandler as suggested the only thing that gets printed is { pluginName: '', pluginData: <Buffer > }
Doesn't seem very helpful.
debug: true added a whole load of stuff, some of which follows (redacted in case hex strings contains credentials:

Add command: Connection
Add command: Connection
 raw: <redacted>
2 undefined ==> Connection#handshakeInit(0,,60)
Server hello packet: capability flags:41516=(long flag, connect with db, compress, protocol 41, transactions, secure connection)
Sending handshake packet: flags:8582095=(long password, found rows, long flag, connect with db, odbc, local files, ignore space, protocol 41, ignore sigpipe, transactions, reserved, secure connection, multi results, session track)
2 59003828 <== Connection#handshakeInit(1,,79)
2 59003828 <== <redacted>
 raw: <redacted>
1 undefined ==> Connection#handshakeInit(0,,60)
Server hello packet: capability flags:41516=(long flag, connect with db, compress, protocol 41, transactions, secure connection)
Sending handshake packet: flags:8582095=(long password, found rows, long flag, connect with db, odbc, local files, ignore space, protocol 41, ignore sigpipe, transactions, reserved, secure connection, multi results, session track)
1 59003827 <== Connection#handshakeInit(1,,79)
1 59003827 <== <redacted>
Add command: Connection
 raw: fe
2 59003828 ==> Connection#handshakeResult(2,EOF,5)
Request ended
 raw: fe
1 59003827 ==> Connection#handshakeResult(2,EOF,5)
 raw: <redacted>
3 undefined ==> Connection#handshakeInit(0,,60)
Server hello packet: capability flags:41516=(long flag, connect with db, compress, protocol 41, transactions, secure connection)
Sending handshake packet: flags:8582095=(long password, found rows, long flag, connect with db, odbc, local files, ignore space, protocol 41, ignore sigpipe, transactions, reserved, secure connection, multi results, session track)
3 59003829 <== Connection#handshakeInit(1,,79)
3 59003829 <== <redacted>
 raw: fe
3 59003829 ==> Connection#handshakeResult(2,EOF,5)

Start by reading the linked Stack Overflow question and mysqljs/mysql issue 1396, then trace the connection handshake and authSwitchHandler behavior discussed in this report. No repository file or test is named, so locate the relevant handshake code and add a reproducible test for the server response. Done means the production authentication switch is handled or reported with useful plugin information.