Truncate 'base' log file on rotation

Prior to this change, lager would simply rename all the files and leave
the 'base' filename empty until a log event came into create it. This
caused confusion with users and tooling.

The fix, as suggested by Shamis Shukoor, is to not delete/rename the
base file, but simply try to open it in write only mode. This will
create the file if it does not exist and truncate it if it does.

Co-authored-by: Shamis Shukoor <shamis657@gmail.com>

src/lager_rotator_default.erl

@@ -72,13 +72,19 @@ ensure_logfile(Name, FD, Inode, Buffer) ->
 
 %% renames failing are OK
 rotate_logfile(File, 0) ->
-    file:delete(File);
+    %% open the file in write-only mode to truncate/create it
+    case file:open(File, [write]) of
+        {ok, _FD} ->
+            ok;
+        Error ->
+            Error
+    end;
 rotate_logfile(File, 1) ->
     case file:rename(File, File++".0") of
         ok ->
-            ok;
-        _ ->
-            rotate_logfile(File, 0)
+            rotate_logfile(File, 0);
+        Error ->
+            Error
     end;
 rotate_logfile(File, Count) ->
     _ = file:rename(File ++ "." ++ integer_to_list(Count - 2), File ++ "." ++ integer_to_list(Count - 1)),
@@ -110,6 +116,23 @@ rotate_file_test() ->
     lists:foreach(Outer, lists:seq(0, (RotCount * 2))),
     lager_util:delete_test_dir(TestDir).
 
+rotate_file_zero_count_test() ->
+    %% Test that a rotation count of 0 simply truncates the file
+    TestDir = lager_util:create_test_dir(),
+    TestLog = filename:join(TestDir, "rotation.log"),
+    ?assertMatch(ok, rotate_logfile(TestLog, 0)),
+    ?assertNot(filelib:is_regular(TestLog ++ ".0")),
+    ?assertEqual(true, filelib:is_regular(TestLog)),
+    ?assertEqual(1, length(filelib:wildcard(TestLog++"*"))),
+    %% assert the new file is 0 size:
+    case file:read_file_info(TestLog) of
+        {ok, FInfo} ->
+            ?assertEqual(0, FInfo#file_info.size);
+        _ ->
+            ?assert(false)
+    end,
+    lager_util:delete_test_dir(TestDir).
+
 rotate_file_fail_test() ->
     TestDir = lager_util:create_test_dir(),
     TestLog = filename:join(TestDir, "rotation.log"),
@@ -118,14 +141,26 @@ rotate_file_fail_test() ->
     %% write a file
     file:write_file(TestLog, "hello"),
     %% hose up the permissions
-    os:cmd("chmod u-w " ++ TestDir),
+    os:cmd("chmod -R u-w " ++ TestDir),
     ?assertMatch({error, _}, rotate_logfile(TestLog, 10)),
+    %% check we still only have one file, rotation.log
+    ?assertEqual([TestLog], filelib:wildcard(TestLog++"*")),
     ?assert(filelib:is_regular(TestLog)),
     %% fix the permissions
-    os:cmd("chmod u+w " ++ TestDir),
+    os:cmd("chmod -R u+w " ++ TestDir),
     ?assertMatch(ok, rotate_logfile(TestLog, 10)),
     ?assert(filelib:is_regular(TestLog ++ ".0")),
-    ?assertEqual(false, filelib:is_regular(TestLog)),
+    ?assertEqual(true, filelib:is_regular(TestLog)),
+    ?assertEqual(2, length(filelib:wildcard(TestLog++"*"))),
+    %% assert the new file is 0 size:
+    case file:read_file_info(TestLog) of
+        {ok, FInfo} ->
+            ?assertEqual(0, FInfo#file_info.size);
+        _ ->
+            ?assert(false)
+    end,
+    %% check that the .0 file now has the contents "hello"
+    ?assertEqual({ok, <<"hello">>}, file:read_file(TestLog++".0")),
     lager_util:delete_test_dir(TestDir).
 
 -endif.