Fix #153 - details below

Somehow nodenum was getting reset to zero (and saved to flash - which is
bad because it makes the failure permanent).  So I've changed nodenum
selection to occur after we load the saved preferences (and we try to keep
nodenum stable in that case).

I'm puzzled as to how it ever got set to zero (unless there *shudder*
is some errant pointer that clobbered it).  But next week I'm turning
4 byte nodenums back on, which will make this moot - because they
will always be based on macaddr and the current process where nodes
haggle with the mesh to pick a unique one-byte nodenum will be gone.

src/mesh/NodeDB.cpp

@@ -136,16 +136,8 @@ void NodeDB::init()
     memcpy(owner.macaddr, ourMacAddr, sizeof(owner.macaddr));
     sprintf(owner.long_name, "Unknown %02x%02x", ourMacAddr[4], ourMacAddr[5]);
 
-    // Crummy guess at our nodenum (but we will check against the nodedb to avoid conflicts)
-    pickNewNodeNum();
-
     sprintf(owner.short_name, "?%02X", myNodeInfo.my_node_num & 0xff);
 
-    // Include our owner in the node db under our nodenum
-    NodeInfo *info = getOrCreateNode(getNodeNum());
-    info->user = owner;
-    info->has_user = true;
-
     if (!FSBegin()) // FIXME - do this in main?
     {
         DEBUG_MSG("ERROR filesystem mount Failed\n");
@@ -156,6 +148,15 @@ void NodeDB::init()
     loadFromDisk();
     // saveToDisk();
 
+    // Note! We do this after loading saved settings, so that if somehow an invalid nodenum was stored in preferences we won't
+    // keep using that nodenum forever. Crummy guess at our nodenum (but we will check against the nodedb to avoid conflicts)
+    pickNewNodeNum();
+
+    // Include our owner in the node db under our nodenum
+    NodeInfo *info = getOrCreateNode(getNodeNum());
+    info->user = owner;
+    info->has_user = true;
+
     // We set these _after_ loading from disk - because they come from the build and are more trusted than
     // what is stored in flash
     strncpy(myNodeInfo.region, optstr(HW_VERSION), sizeof(myNodeInfo.region));
@@ -175,9 +176,12 @@ void NodeDB::init()
  */
 void NodeDB::pickNewNodeNum()
 {
-    // Pick an initial nodenum based on the macaddr
-    NodeNum r = sizeof(NodeNum) == 1 ? ourMacAddr[5]
-                                     : ((ourMacAddr[2] << 24) | (ourMacAddr[3] << 16) | (ourMacAddr[4] << 8) | ourMacAddr[5]);
+    NodeNum r = myNodeInfo.my_node_num;
+
+    // If we don't have a nodenum at app - pick an initial nodenum based on the macaddr
+    if (r == 0)
+        r = sizeof(NodeNum) == 1 ? ourMacAddr[5]
+                                 : ((ourMacAddr[2] << 24) | (ourMacAddr[3] << 16) | (ourMacAddr[4] << 8) | ourMacAddr[5]);
 
     if (r == NODENUM_BROADCAST || r < NUM_RESERVED)
         r = NUM_RESERVED; // don't pick a reserved node number
@@ -246,15 +250,18 @@ void NodeDB::saveToDisk()
         if (!pb_encode(&stream, DeviceState_fields, &devicestate)) {
             DEBUG_MSG("Error: can't write protobuf %s\n", PB_GET_ERROR(&stream));
             // FIXME - report failure to phone
+
+            f.close();
+        } else {
+            // Success - replace the old file
+            f.close();
+
+            // brief window of risk here ;-)
+            if (!FS.remove(preffile))
+                DEBUG_MSG("Warning: Can't remove old pref file\n");
+            if (!FS.rename(preftmp, preffile))
+                DEBUG_MSG("Error: can't rename new pref file\n");
         }
-
-        f.close();
-
-        // brief window of risk here ;-)
-        if (!FS.remove(preffile))
-            DEBUG_MSG("Warning: Can't remove old pref file\n");
-        if (!FS.rename(preftmp, preffile))
-            DEBUG_MSG("Error: can't rename new pref file\n");
     } else {
         DEBUG_MSG("ERROR: can't write prefs\n"); // FIXME report to app
     }