[GitHub] davisp commented on issue #501: Fix error on race condition in mem3 startup

[gi...@git.apache.org]

davisp commented on issue #501: Fix error on race condition in mem3 startup
URL: https://github.com/apache/couchdb/pull/501#issuecomment-298252919
 
 
   I think the bigger issue here is that `mem3_util:ensure_exists/1` is racey in that if multiple concurrent clients attempt to ensure a missing database exists simultaneously only one of them will succeed. A better approach would be to allow for an expected file_exists return and for the client that receives it to try re-opening a second time. Something along these lines:
   
   ```diff
   diff --git a/src/mem3/src/mem3_util.erl b/src/mem3/src/mem3_util.erl
   index 4e1b5fd7d..deb2c6640 100644
   --- a/src/mem3/src/mem3_util.erl
   +++ b/src/mem3/src/mem3_util.erl
   @@ -224,11 +224,21 @@ shard_info(DbName) ->
    ensure_exists(DbName) when is_list(DbName) ->
        ensure_exists(list_to_binary(DbName));
    ensure_exists(DbName) ->
   -    case couch_db:open(DbName, [nologifmissing, sys_db | [?ADMIN_CTX]]) of
   -    {ok, Db} ->
   -        {ok, Db};
   -    _ ->
   -        couch_server:create(DbName, [?ADMIN_CTX])
   +    Options = [nologifmissing, sys_db, ?ADMIN_CTX],
   +    case couch_db:open(DbName, Options) of
   +        {ok, Db} ->
   +            {ok, Db};
   +        _ ->
   +            case couch_server:create(DbName, Options) of
   +                {ok, Db} ->
   +                    {ok, Db};
   +                file_exists ->
   +                    % Try opening again in case there was a race to
   +                    % create the database.
   +                    couch_db:open(DbName, Options);
   +                Error ->
   +                    Error
   +            end
        end.
   ```
 
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