Improve DevAddr generation

CONTRIBUTING.md

@@ -0,0 +1,16 @@
+## Code of Conduct
+
+Be cooperative when you ask for help. We will spend time helping you for free,
+so be please ready to spend your time too:
+ * Read the server documentation and [Troubleshooting](doc/Troubleshooting.md)
+   guide first to see if you can resolve the issue yourself.
+ * Describe thoroughly the issue you are having and include (only the) relevant
+   portions of server logs.
+ * Be ready to repeat the course of actions against latest server binaries to
+   generate more debug information or verify the bugfix.
+ * Be ready to investigate behaviour of your device or gateway. These can be
+   faulty too.
+
+Be respectful to the maintainers and other contributors. Open source contributors
+put long hours into developing projects and doing user support. Those projects
+and user support are available for free. We believe this deserves some respect.

src/lorawan_mac.erl

@@ -147,7 +147,7 @@ handle_join(Gateway, RxQ, AppEUI, DevEUI, DevNonce, AppKey) ->
         NewAddr = if
             D#device.link == undefined;
             byte_size(D#device.link) < 4 ->
-                create_devaddr(NetID);
+                create_devaddr(NetID, 3);
             true ->
                 D#device.link
         end,
@@ -200,10 +200,18 @@ initial_rxwin({A1, A2, _}, {B1, B2, B3}) ->
 apply_default(Value, _Default) when is_number(Value) -> Value;
 apply_default(_Else, Default) -> Default.
 
-create_devaddr(NetID) ->
+create_devaddr(NetID, Attempts) ->
     <<_:17, NwkID:7>> = NetID,
-    %% FIXME: verify uniqueness
-    <<NwkID:7, 0:1, (crypto:strong_rand_bytes(3))/binary>>.
+    <<_:7, NwkAddr:25/bitstring>> = crypto:strong_rand_bytes(4),
+    DevAddr = <<NwkID:7, NwkAddr:25/bitstring>>,
+    % assert uniqueness
+    case mnesia:read(links, DevAddr, read) of
+        [] ->
+            DevAddr;
+        [#link{}] when Attempts > 0 ->
+            create_devaddr(NetID, Attempts-1)
+        %% FIXME: do not crash when Attempts == 0
+    end.
 
 txaccept(TxQ, RX1DROffset, RX2DataRate, AppKey, AppNonce, NetID, DevAddr) ->
     MHDR = <<2#001:3, 0:3, 0:2>>,