mqtt: improve the state machine

To handle PUBLISH before SUBACK and more.

Updated the existing tests and added three new ones.

Reported-by: Christoph Krey
Bug: https://curl.haxx.se/mail/lib-2020-04/0021.html
Closes #5246

1 files changed, 17 insertions, 7 deletions

diff --git a/lib/mqtt.h b/lib/mqtt.h
index b5e447be5..155fbd60a 100644
--- a/lib/mqtt.h
+++ b/lib/mqtt.h
@@ -26,13 +26,22 @@
 extern const struct Curl_handler Curl_handler_mqtt;
 #endif
 
+enum mqttstate {
+  MQTT_FIRST,             /* 0 */
+  MQTT_REMAINING_LENGTH,  /* 1 */
+  MQTT_CONNACK,           /* 2 */
+  MQTT_SUBACK,            /* 3 */
+  MQTT_SUBACK_COMING,     /* 4 - the SUBACK remainder */
+  MQTT_PUBWAIT,    /* 5 - wait for publish */
+  MQTT_PUB_REMAIN,  /* 6 - wait for the remainder of the publish */
+
+  MQTT_NOSTATE = 99 /* never an actual state */
+};
+
 struct mqtt_conn {
-  enum {
-    MQTT_CONNACK,
-    MQTT_SUBACK,
-    MQTT_SUBWAIT,    /* wait for subscribe response */
-    MQTT_SUB_REMAIN  /* wait for the remainder of the subscribe response */
-  } state;
+  enum mqttstate state;
+  enum mqttstate nextstate; /* switch to this after remaining length is
+                               done */
   unsigned int packetid;
 };
 
@@ -41,9 +50,10 @@ struct MQTT {
   char *sendleftovers;
   size_t nsend; /* size of sendleftovers */
 
-  /* when receving a PUBLISH */
+  /* when receving */
   size_t npacket; /* byte counter */
   unsigned char firstbyte;
+  size_t remaining_length;
 };
 
 #endif /* HEADER_CURL_MQTT_H */