fix(dsrepl): properly handle error conditions in generation mgmt

Also update few outdated typespecs. Also make error reasons easier
to comprehend.

apps/emqx_durable_storage/src/emqx_ds_storage_layer.erl

@@ -410,12 +410,14 @@ delete_next(
             {ok, end_of_stream}
     end.
 
--spec update_config(shard_id(), emqx_ds:time(), emqx_ds:create_db_opts()) -> ok.
+-spec update_config(shard_id(), emqx_ds:time(), emqx_ds:create_db_opts()) ->
+    ok | {error, overlaps_existing_generations}.
 update_config(ShardId, Since, Options) ->
     Call = #call_update_config{since = Since, options = Options},
     gen_server:call(?REF(ShardId), Call, infinity).
 
--spec add_generation(shard_id(), emqx_ds:time()) -> ok.
+-spec add_generation(shard_id(), emqx_ds:time()) ->
+    ok | {error, overlaps_existing_generations}.
 add_generation(ShardId, Since) ->
     gen_server:call(?REF(ShardId), #call_add_generation{since = Since}, infinity).
 
@@ -481,13 +483,21 @@ init({ShardId, Options}) ->
     {ok, S}.
 
 handle_call(#call_update_config{since = Since, options = Options}, _From, S0) ->
-    S = #s{} = handle_update_config(S0, Since, Options),
+    case handle_update_config(S0, Since, Options) of
+        S = #s{} ->
             commit_metadata(S),
             {reply, ok, S};
+        Error = {error, _} ->
+            {reply, Error, S0}
+    end;
 handle_call(#call_add_generation{since = Since}, _From, S0) ->
-    S = #s{} = handle_add_generation(S0, Since),
+    case handle_add_generation(S0, Since) of
+        S = #s{} ->
             commit_metadata(S),
             {reply, ok, S};
+        Error = {error, _} ->
+            {reply, Error, S0}
+    end;
 handle_call(#call_list_generations_with_lifetimes{}, _From, S) ->
     Generations = handle_list_generations_with_lifetimes(S),
     {reply, Generations, S};
@@ -531,7 +541,7 @@ open_shard(ShardId, DB, CFRefs, ShardSchema) ->
     ).
 
 -spec handle_add_generation(server_state(), emqx_ds:time()) ->
-    server_state() | {error, nonmonotonic}.
+    server_state() | {error, overlaps_existing_generations}.
 handle_add_generation(S0, Since) ->
     #s{shard_id = ShardId, db = DB, schema = Schema0, shard = Shard0, cf_refs = CFRefs0} = S0,
 
@@ -583,7 +593,7 @@ handle_add_generation(S0, Since) ->
     end.
 
 -spec handle_update_config(server_state(), emqx_ds:time(), emqx_ds:create_db_opts()) ->
-    server_state().
+    server_state() | {error, overlaps_existing_generations}.
 handle_update_config(S0 = #s{schema = Schema}, Since, Options) ->
     Prototype = maps:get(storage, Options),
     S = S0#s{schema = Schema#{prototype := Prototype}},
@@ -719,7 +729,7 @@ db_dir({DB, ShardId}) ->
     filename:join([emqx_ds:base_dir(), atom_to_list(DB), binary_to_list(ShardId)]).
 
 -spec update_last_until(Schema, emqx_ds:time()) ->
-    Schema | {error, exists | nonmonotonic}
+    Schema | {error, exists | overlaps_existing_generations}
 when
     Schema :: shard_schema() | shard().
 update_last_until(Schema = #{current_generation := GenId}, Until) ->
@@ -729,7 +739,7 @@ update_last_until(Schema = #{current_generation := GenId}, Until) ->
         #{since := Until} ->
             {error, exists};
         #{since := CurrentSince} when CurrentSince > Until ->
-            {error, nonmonotonic}
+            {error, overlaps_existing_generations}
     end.
 
 run_post_creation_actions(