KAFKA-3219: Fix long topic name validation

core/src/main/scala/kafka/common/Topic.scala

@@ -22,7 +22,7 @@ import kafka.coordinator.GroupCoordinator
 
 object Topic {
   val legalChars = "[a-zA-Z0-9\\._\\-]"
-  private val maxNameLength = 255
+  private val maxNameLength = 249
   private val rgx = new Regex(legalChars + "+")
 
   val InternalTopics = Set(GroupCoordinator.GroupMetadataTopicName)

core/src/test/scala/unit/kafka/common/TopicTest.scala

@@ -31,6 +31,7 @@ class TopicTest {
     for (i <- 1 to 6)
       longName += longName
     invalidTopicNames += longName
+    invalidTopicNames += longName.drop(6)
     val badChars = Array('/', '\\', ',', '\u0000', ':', "\"", '\'', ';', '*', '?', ' ', '\t', '\r', '\n', '=')
     for (weirdChar <- badChars) {
       invalidTopicNames += "Is" + weirdChar + "illegal"
@@ -47,7 +48,7 @@ class TopicTest {
     }
 
     val validTopicNames = new ArrayBuffer[String]()
-    validTopicNames += ("valid", "TOPIC", "nAmEs", "ar6", "VaL1d", "_0-9_.")
+    validTopicNames += ("valid", "TOPIC", "nAmEs", "ar6", "VaL1d", "_0-9_.", longName.drop(7))
     for (i <- 0 until validTopicNames.size) {
       try {
         Topic.validate(validTopicNames(i))

docs/ops.html

@@ -34,6 +34,8 @@ <h4><a id="basic_ops_add_topic" href="#basic_ops_add_topic">Adding and removing
 <p>
 The partition count controls how many logs the topic will be sharded into. There are several impacts of the partition count. First each partition must fit entirely on a single server. So if you have 20 partitions the full data set (and read and write load) will be handled by no more than 20 servers (no counting replicas). Finally the partition count impacts the maximum parallelism of your consumers. This is discussed in greater detail in the <a href="#intro_consumers">concepts section</a>.
 <p>
+Each sharded partition log is placed into its own folder under the Kafka log directory. The name of such folders consists of the topic name, appended by a dash (-) and the partition id. Since a typical folder name can not be over 255 characters long, there will be a limitation on the length of topic names. We assume the number of partitions will not ever be above 100,000. Therefore, topic names cannot be longer than 249 characters. This leaves just enough room in the folder name for a dash and a potentially 5 digit long partition id.
+<p>
 The configurations added on the command line override the default settings the server has for things like the length of time data should be retained. The complete set of per-topic configurations is documented <a href="#topic-config">here</a>.
 
 <h4><a id="basic_ops_modify_topic" href="#basic_ops_modify_topic">Modifying topics</a></h4>