Allow message payload to be NULL.

NULL message payloads are used with compaction topics introduced in 0.8.1
which use the NULL payload to indicate the corresponding key should be deleted.

The Kafka protocol guide also notes that the Value (payload) may be NULL.
https://cwiki.apache.org/confluence/display/KAFKA/A+Guide+To+The+Kafka+Protocol#AGuideToTheKafkaProtocol-Messagesets

This commit makes these changes:

- prevents NULL payloads from being copied when RD_KAFKA_MSG_F_COPY is set.
- prevents payload bytes being copied into rkbuf when NULL.
- sets rkm_len to -1 when payload is NULL (just like key).
- Allows NULL payloads to be received in a fetch.

src/rdkafka_broker.c

@@ -2127,6 +2127,10 @@ static int rd_kafka_broker_produce_toppar (rd_kafka_broker_t *rkb,
 
 
 		/* Value(payload) length */
+		if (rkm->rkm_payload == NULL) {
+			/* Set length to -1 to indicate a NULL value. */
+			rkm->rkm_len = -1;
+		}
 		msghdr->part4.Value_len = htonl(rkm->rkm_len);
 		msghdr->part3.Crc =
 			crc32(msghdr->part3.Crc,
@@ -2138,12 +2142,15 @@ static int rd_kafka_broker_produce_toppar (rd_kafka_broker_t *rkb,
 		rd_kafka_buf_push(rkbuf, &msghdr->part4, sizeof(msghdr->part4));
 
 
-		/* Payload */
-		msghdr->part3.Crc =
-			crc32(msghdr->part3.Crc,
-					rkm->rkm_payload,
-					rkm->rkm_len);
-		rd_kafka_buf_push(rkbuf, rkm->rkm_payload, rkm->rkm_len);
+
+		/* Add Payload if not NULL */
+		if (rkm->rkm_payload != NULL) {
+			msghdr->part3.Crc =
+				crc32(msghdr->part3.Crc,
+						rkm->rkm_payload,
+						rkm->rkm_len);
+			rd_kafka_buf_push(rkbuf, rkm->rkm_payload, rkm->rkm_len);
+		}
 
 
 		/* Finalize Crc */
@@ -2801,14 +2808,18 @@ static rd_kafka_resp_err_t rd_kafka_messageset_handle (rd_kafka_broker_t *rkb,
 			/* Create op and push on temporary queue. */
 			rko = rd_kafka_op_new(RD_KAFKA_OP_FETCH);
 
+			rko->rko_rkmessage.key = NULL;
 			if (!RD_KAFKAP_BYTES_IS_NULL(Key)) {
 				rko->rko_rkmessage.key = Key->data;
 				rko->rko_rkmessage.key_len =
 					RD_KAFKAP_BYTES_LEN(Key);
 			}
 
-			rko->rko_rkmessage.payload   = Value->data;
-			rko->rko_rkmessage.len       = Value_len;
+			rko->rko_rkmessage.payload = NULL;
+			if (!RD_KAFKAP_BYTES_IS_NULL(Value)) {
+				rko->rko_rkmessage.payload = Value->data;
+				rko->rko_rkmessage.len = RD_KAFKAP_BYTES_LEN(Value);
+			}
 
 			rko->rko_rkmessage.offset    = hdr->Offset;
 			rko->rko_rkmessage.rkt       = rktp->rktp_rkt;

src/rdkafka_msg.c

@@ -93,6 +93,13 @@ static rd_kafka_msg_t *rd_kafka_msg_new0 (rd_kafka_topic_t *rkt,
 			rkt->rkt_conf.message_timeout_ms * 1000;
 	}
 
+        /* Check for a NULL payload */
+        if (payload == NULL) {
+            rkm->rkm_payload = NULL;
+            rkm->rkm_len = 0;
+            return rkm;
+        }
+
 	if (msgflags & RD_KAFKA_MSG_F_COPY) {
 		/* Copy payload to space following the ..msg_t */
 		rkm->rkm_payload = (void *)(rkm+1);