Merge pull request #2232 from ferd/tsloughter-lock-file-writes

only write lock file if locks have changed

src/rebar_config.erl

@@ -31,6 +31,7 @@
         ,consult_app_file/1
         ,consult_file/1
         ,consult_lock_file/1
+        ,maybe_write_lock_file/3
         ,write_lock_file/2
         ,verify_config_format/1
         ,format_error/1
@@ -110,6 +111,21 @@ warn_vsn_once() ->
                   "upgrade Rebar3.", [])
     end.
 
+%% Only call `write_lock_file/2' if the locks have changed.
+maybe_write_lock_file(LockFile, Locks, OldLocks) when Locks =/= OldLocks ->
+    write_lock_file(LockFile, Locks);
+maybe_write_lock_file(LockFile, Locks, Locks) ->
+    %% rewrite if the configured format would have changed
+    Terms = consult_file_(LockFile),
+    case Terms of
+        [] ->
+            ok;
+        [{?CONFIG_VERSION, FileLocks}|_] when is_list(FileLocks) ->
+            ok;
+        _ ->
+            write_lock_file(LockFile, Locks)
+    end.
+
 %% @doc Converts the internal format for locks into the multi-version
 %% compatible one used within rebar3 lock files.
 %% @end

src/rebar_prv_lock.erl

@@ -35,7 +35,7 @@ do(State) ->
             OldLocks = rebar_state:get(State, {locks, default}, []),
             Locks = lists:keysort(1, build_locks(State)),
             Dir = rebar_state:dir(State),
-            rebar_config:write_lock_file(filename:join(Dir, ?LOCK_FILE), Locks),
+            rebar_config:maybe_write_lock_file(filename:join(Dir, ?LOCK_FILE), Locks, OldLocks),
             State1 = rebar_state:set(State, {locks, default}, Locks),
 
             OldLockNames = [element(1,L) || L <- OldLocks],