Kamon Logback logs empty values for context entries that are not in the current context

[yiminyuan]

• Scala 2.12.12
• Kamon 2.1.9

We currently use a JSON logger. If I add this config: kamon.instrumentation.logback.mdc.copy.entries = [ foo ], then foo would be logged as an empty value even though foo is not in the current context:

{
  "foo" : ""
}

So the workaround is adding the following code to logback.xml and remove kamon.instrumentation.logback.mdc.copy.entries = [ foo ]:

<conversionRule conversionWord="contextEntry" converterClass="kamon.instrumentation.logback.tools.ContextEntryConverter" />

<pattern>
  <omitEmptyFields>true</omitEmptyFields>
  <pattern>
    {
      "foo":"%contextEntry{foo}"
    }
  </pattern>
</pattern>

Temporary fix
In https://github.com/kamon-io/Kamon/blob/master/instrumentation/kamon-logback/src/main/scala/kamon/instrumentation/logback/LogbackInstrumentation.scala, it seems only context tags that are in the current context are reported.

currentContext.tags.iterator().foreach(t => {
  MDC.put(t.key, Tag.unwrapValue(t).toString)
})

We have a very simple use case that all K-V pairs have the string type. So we use context tags instead of context entries.

Expected behavior
If a context entry is not in the current context, then it should not be logged.

Proposed fix
In https://github.com/kamon-io/Kamon/blob/master/instrumentation/kamon-logback/src/main/scala/kamon/instrumentation/logback/LogbackInstrumentation.scala, because of Context.key[Any](key, ""), which means "" is returned by default if key is not in the current context. Hence, we can add extra if-check like this to eliminate the issue:

settings.mdcCopyKeys.foreach { key =>
  currentContext.get(Context.key[Any](key, "")) match {
    case Some(value) =>
      val s = value.toString
      if(s.nonEmpty)
        MDC.put(key, s)
    case keyValue if keyValue != null =>
      val s = keyValue.toString
      if(s.nonEmpty)
        MDC.put(key, s)
    case _ => // Just ignore the nulls.
  }
}

[SimunKaracic]

Fixed in 2.1.13

[yiminyuan]

Awesome! This commit 85717aa looks good. Thank you for fixing it!