MINOR: Fix potential bug in LogConfig.getConfigValue and improve test coverage (#7159)

LogConfig.getConfigValue would throw a NoSuchElementException if any log
config was defined without a server default mapping.

Added a unit test for `getConfigValue` and a sanity test for
`toHtml`/`toRst`/`toEnrichedRst`, which were previously not exercised during
the test suite.

Reviewers: Jason Gustafson <jason@confluent.io>, José Armando García Sancio <jsancio@users.noreply.github.com>

core/src/main/scala/kafka/log/LogConfig.scala

@@ -181,9 +181,17 @@ object LogConfig {
     "[PartitionId]:[BrokerId],[PartitionId]:[BrokerId]:... or alternatively the wildcard '*' can be used to throttle " +
     "all replicas for this topic."
 
-  private class LogConfigDef extends ConfigDef {
+  private[log] val ServerDefaultHeaderName = "Server Default Property"
+
+  // Package private for testing
+  private[log] class LogConfigDef(base: ConfigDef) extends ConfigDef(base) {
+    def this() = this(new ConfigDef)
 
     private final val serverDefaultConfigNames = mutable.Map[String, String]()
+    base match {
+      case b: LogConfigDef => serverDefaultConfigNames ++= b.serverDefaultConfigNames
+      case _ =>
+    }
 
     def define(name: String, defType: ConfigDef.Type, defaultValue: Any, validator: Validator,
                importance: ConfigDef.Importance, doc: String, serverDefaultConfigName: String): LogConfigDef = {
@@ -206,11 +214,12 @@ object LogConfig {
       this
     }
 
-    override def headers = List("Name", "Description", "Type", "Default", "Valid Values", "Server Default Property", "Importance").asJava
+    override def headers = List("Name", "Description", "Type", "Default", "Valid Values", ServerDefaultHeaderName,
+      "Importance").asJava
 
     override def getConfigValue(key: ConfigKey, headerName: String): String = {
       headerName match {
-        case "Server Default Property" => serverDefaultConfigNames.get(key.name).get
+        case ServerDefaultHeaderName => serverDefaultConfigNames.getOrElse(key.name, null)
         case _ => super.getConfigValue(key, headerName)
       }
     }
@@ -218,6 +227,9 @@ object LogConfig {
     def serverConfigName(configName: String): Option[String] = serverDefaultConfigNames.get(configName)
   }
 
+  // Package private for testing, return a copy since it's a mutable global variable
+  private[log] def configDefCopy: LogConfigDef = new LogConfigDef(configDef)
+
   private val configDef: LogConfigDef = {
     import org.apache.kafka.common.config.ConfigDef.Importance._
     import org.apache.kafka.common.config.ConfigDef.Range._

core/src/test/scala/unit/kafka/log/LogConfigTest.scala

@@ -19,9 +19,11 @@ package kafka.log
 
 import java.util.Properties
 
-import kafka.server.{ThrottledReplicaListValidator, KafkaConfig, KafkaServer}
+import kafka.server.{KafkaConfig, KafkaServer, ThrottledReplicaListValidator}
 import kafka.utils.TestUtils
-import org.apache.kafka.common.config.ConfigException
+import org.apache.kafka.common.config.ConfigDef.Importance.MEDIUM
+import org.apache.kafka.common.config.ConfigDef.Type.INT
+import org.apache.kafka.common.config.{ConfigException, TopicConfig}
 import org.junit.{Assert, Test}
 import org.junit.Assert._
 import org.scalatest.Assertions._
@@ -113,6 +115,46 @@ class LogConfigTest {
     assertFalse(isValid("100:0,10 :   "))
   }
 
+  /* Sanity check that toHtml produces one of the expected configs */
+  @Test
+  def testToHtml(): Unit = {
+    val html = LogConfig.configDefCopy.toHtmlTable
+    val expectedConfig = "<td>file.delete.delay.ms</td>"
+    assertTrue(s"Could not find `$expectedConfig` in:\n $html", html.contains(expectedConfig))
+  }
+
+  /* Sanity check that toEnrichedRst produces one of the expected configs */
+  @Test
+  def testToEnrichedRst(): Unit = {
+    val rst = LogConfig.configDefCopy.toEnrichedRst
+    val expectedConfig = "``file.delete.delay.ms``"
+    assertTrue(s"Could not find `$expectedConfig` in:\n $rst", rst.contains(expectedConfig))
+  }
+
+  /* Sanity check that toEnrichedRst produces one of the expected configs */
+  @Test
+  def testToRst(): Unit = {
+    val rst = LogConfig.configDefCopy.toRst
+    val expectedConfig = "``file.delete.delay.ms``"
+    assertTrue(s"Could not find `$expectedConfig` in:\n $rst", rst.contains(expectedConfig))
+  }
+
+  @Test
+  def testGetConfigValue(): Unit = {
+    // Add a config that doesn't set the `serverDefaultConfigName`
+    val configDef = LogConfig.configDefCopy
+    val configNameWithNoServerMapping = "log.foo"
+    configDef.define(configNameWithNoServerMapping, INT, 1, MEDIUM, s"$configNameWithNoServerMapping doc")
+
+    val deleteDelayKey = configDef.configKeys.get(TopicConfig.FILE_DELETE_DELAY_MS_CONFIG)
+    val deleteDelayServerDefault = configDef.getConfigValue(deleteDelayKey, LogConfig.ServerDefaultHeaderName)
+    assertEquals(KafkaConfig.LogDeleteDelayMsProp, deleteDelayServerDefault)
+
+    val keyWithNoServerMapping = configDef.configKeys.get(configNameWithNoServerMapping)
+    val nullServerDefault = configDef.getConfigValue(keyWithNoServerMapping, LogConfig.ServerDefaultHeaderName)
+    assertNull(nullServerDefault)
+  }
+
   private def isValid(configValue: String): Boolean = {
     try {
       ThrottledReplicaListValidator.ensureValidString("", configValue)