kafka: Add topic_prefix and escape topic names

These are especially useful when paired with `topic_variable`.
Since the variable is being extracted from a field, we can't guarantee
that the characters being used for the topic name are valid, so
we implement a slightly modified version of stdlib's `QueryEncode`
which sanitizes the name to the range of valid topic chars.

Example:

```toml
[KafkaOutput]
topic_prefix = "heka-"
topic_variable = "Fields[ContainerName]"
```

plugins/kafka/kafka_output.go

@@ -48,6 +48,7 @@ type KafkaOutputConfig struct {
 	HashVariable  string `toml:"hash_variable"`  // HashPartitioner key is extracted from a message variable
 	TopicVariable string `toml:"topic_variable"` // Topic extracted from a message variable
 	Topic         string // Static topic
+	TopicPrefix   string `toml:"topic_prefix"` // String to be prepended to the topic
 
 	RequiredAcks               string `toml:"required_acks"` // NoResponse, WaitForLocal, WaitForAll
 	Timeout                    uint32
@@ -317,9 +318,10 @@ func (k *KafkaOutput) Run(or pipeline.OutputRunner, h pipeline.PluginHelper) (er
 	go k.processKafkaErrors(or, errChan, &wg)
 
 	var (
-		pack  *pipeline.PipelinePack
-		topic = k.config.Topic
-		key   sarama.Encoder
+		pack   *pipeline.PipelinePack
+		topic  = k.config.Topic
+		prefix = k.config.TopicPrefix
+		key    sarama.Encoder
 	)
 
 	for pack = range inChan {
@@ -334,7 +336,7 @@ func (k *KafkaOutput) Run(or pipeline.OutputRunner, h pipeline.PluginHelper) (er
 
 		if msgBytes, err := or.Encode(pack); err == nil {
 			if msgBytes != nil {
-				err = k.producer.QueueMessage(topic, key, sarama.ByteEncoder(msgBytes))
+				err = k.producer.QueueMessage(topicEscape(prefix+topic), key, sarama.ByteEncoder(msgBytes))
 				if err != nil {
 					atomic.AddInt64(&k.processMessageFailures, 1)
 					or.LogError(err)
@@ -371,6 +373,53 @@ func (k *KafkaOutput) CleanupForRestart() {
 	return
 }
 
+// Escapes a topic name by encoding any non-valid kafka topic name
+func topicEscape(s string) string {
+	hexCount := 0
+	for i := 0; i < len(s); i++ {
+		if shouldEscape(s[i]) {
+			hexCount++
+		}
+	}
+
+	if hexCount == 0 {
+		return s
+	}
+
+	t := make([]byte, len(s)+2*hexCount)
+	j := 0
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if shouldEscape(c) {
+			t[j] = '-'
+			t[j+1] = "0123456789ABCDEF"[c>>4]
+			t[j+2] = "0123456789ABCDEF"[c&15]
+			j += 3
+		} else {
+			t[j] = s[i]
+			j++
+		}
+	}
+	return string(t)
+}
+
+func shouldEscape(c byte) bool {
+	// List of valid kafka topic characters can be found at:
+	// https://github.com/apache/kafka/blob/43b92f8b1ce8140c432edf11b0c842f5fbe04120/core/src/main/scala/kafka/common/Topic.scala#L25
+	//   val legalChars = "[a-zA-Z0-9\\._\\-]"
+	if 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' {
+		return false
+	}
+
+	switch c {
+	case '.', '_', '-':
+		return false
+	}
+
+	// Everything else must be escaped.
+	return true
+}
+
 func init() {
 	pipeline.RegisterPlugin("KafkaOutput", func() interface{} {
 		return new(KafkaOutput)