MINOR: Improve PlainSaslServer error message for empty tokens

[ijuma]

Empty username or password would result in the "expected 3 tokens"
error instead of "username not specified" or "password not specified".

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[ijuma]

I'm using assertThrows so this depends on #6248

[ijuma]

cc @apovzner

[rajinisivaram]

@ijuma Thanks for the PR, looks good. We should probably change the exception from SaslException to SaslAuthenticationException for the missing username/password cases. I have opened a related PR to fix a regression: #6252.

[rajinisivaram]

Do we still need this?

[ijuma]

Good point. We can add code to split (and maybe rename it) to verify if two utf8nul are not present and return an error like this one. Would it be SaslException still?

[rajinisivaram]

I think this can also be SaslAuthenticationException since it is only returning error based on the information the client passed in, so it wont be leaking any server state.

[ijuma]

Sounds good.

[ijuma]

Thanks for the review, I did wonder SaslException versus SaslAuthenticationException, but was not sure if it had been intentional. Will change.

[rajinisivaram]

@ijuma Thanks for the updates, LGTM

[ijuma]

JDK 11 build passed, JDK 8 had an unrelated failure:

org.scalatest.junit.JUnitTestFailedError: Should have received an class org.apache.kafka.common.errors.GroupMaxSizeReachedException during the cluster roll
at org.scalatest.junit.AssertionsForJUnit$class.newAssertionFailedException(AssertionsForJUnit.scala:100)
at org.scalatest.junit.JUnitSuite.newAssertionFailedException(JUnitSuite.scala:71)
at org.scalatest.Assertions$class.fail(Assertions.scala:1089)
at org.scalatest.junit.JUnitSuite.fail(JUnitSuite.scala:71)
at kafka.api.ConsumerBounceTest.testRollingBrokerRestartsWithSmallerMaxGroupSizeConfigDisruptsBigGroup(ConsumerBounceTest.scala:376)

Merging to trunk.