Fix delay of first uplink message after join

After a join cancel the next scheduled doWork job and re-schedule it for
direct execution to prevent delay of the first uplink message.

README.md

@@ -216,9 +216,14 @@ To run the job the `doWorkCallback()` function is executed by the LMIC scheduler
 `doWorkCallback()` calls the `processWork()` function where the actual work is performed.  
 The first `doWork` run is started in `setup()`. On completion `doWork` reschedules itself for the next run.
 
+When the node has joined and the `EV_JOINED` event is handled by the event handler, the next scheduled doWork job is cancelled and is re-scheduled for immediate execution. This is done to prevent that any uplink will have to wait until the current doWork interval ends. `processWork()` skips doing any work while the node is still joining. As a result sending the first uplink message may have to wait until the current doWork interval ends (max `DO_WORK_INTERVAL_SECONDS` seconds). Directly running the doWork job after a join prevents the in this case unnecessary and unwanted delay.
+
 ### 3.3 processWork() function
 
-The `processWork()` function contains user code that performs the actual work like reading sensor data and scheduling uplink messages.
+The `processWork()` function contains user code that performs the actual work like reading sensor data and scheduling uplink messages. In LMIC-node `processWork()` will skip doing any work if the node is still joining for two reasons:
+
+1. To prevent unnecessary incrementing of the counter.
+2. Uplink messages cannot yet be sent.
 
 ### 3.4 processDownlink() function
 

src/LMIC-node.cpp

@@ -571,6 +571,14 @@ void onEvent(ev_t ev)
             // during join, but because slow data rates change
             // max TX size, it is not used in this example.                    
             LMIC_setLinkCheckMode(0);
+
+            // The doWork job has probably run already (while
+            // the node was still joining) and have rescheduled itself.
+            // Cancel the next scheduled doWork job and re-schedule
+            // for immediate execution to prevent that any uplink will
+            // have to wait until the current doWork interval ends.
+            os_clearCallback(&doWorkJob);
+            os_setCallback(&doWorkJob, doWorkCallback);
             break;
 
         case EV_TXCOMPLETE:

src/LMIC-node.h

@@ -44,6 +44,7 @@ enum class InitType { Hardware, PostInitSerial };
 enum class PrintTarget { All, Serial, Display };
 
 // Forward declarations
+static void doWorkCallback(osjob_t* job);
 void processWork(ostime_t timestamp);
 void processDownlink(ostime_t eventTimestamp, uint8_t fPort, uint8_t* data, uint8_t dataLength);
 void onLmicEvent(void *pUserData, ev_t ev);