[erlang-bugs] mnesia:read returns undefined when getting write lock when sticky_write lock is held

[Dan Gudmundsson]

Thanks for the bug report and thanks for the excellent tests.

/Dan

On Thu, Dec 1, 2011 at 6:24 PM, Magnus Henoch
<magnus.henoch@REDACTED> wrote:
> Hi all,
>
> I noticed that mnesia:read would return the atom undefined instead of
> a list of records in some cases, and managed to reduce it to the test
> case below, which triggers the error in both R14B01 and R15A.
>
> This seems to happen when one node holds a sticky lock on a table, and
> another node calls mnesia:read(Table, Key, write), i.e. reading while
> getting a write lock.  (It wasn't intentional, I was in the process of
> removing sticky locks and had forgotten one place.)
>
> This test case:
>
> -module(foo).
>
> -include_lib("eunit/include/eunit.hrl").
>
> -export([foo/0]).
>
> foo() ->
>    net_kernel:start([foo@REDACTED, shortnames]),
>    {ok, Slave} = slave:start_link(localhost, slave),
>    mnesia:start(),
>    try
>        ?assertEqual(ok, rpc:call(Slave, mnesia, start, [])),
>        ?assertMatch({ok, _}, mnesia:change_config(extra_db_nodes, [Slave])),
>        ?debugVal(mnesia:create_table(foo, [{ram_copies, [node(), Slave]}])),
>        ?assertEqual(ok, rpc:call(Slave, mnesia, wait_for_tables, [[foo], 1000])),
>        ?assertEqual({atomic, ok},
>                     ?debugVal(
>                        mnesia:transaction(
>                          fun() ->
>                                  mnesia:write(foo, {foo, 42, a}, sticky_write)
>                          end))),
>        ?assertEqual({atomic, [{foo, 42, a}]},
>                     ?debugVal(
>                        rpc:call(
>                          Slave, mnesia, transaction,
>                          [fun mnesia:read/3, [foo, 42, write]])))
>    after
>        mnesia:stop(),
>        mnesia:delete_schema([node(), Slave]),
>        slave:stop(Slave)
>    end.
>
> produces this output:
>
> Erlang R15A (erts-5.9.pre) [source] [64-bit] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]
>
> Eshell V5.9.pre  (abort with ^G)
> 1> foo:foo().
> foo.erl:14:<0.32.0>: mnesia : create_table ( foo , [ { ram_copies , [ node ( ) , Slave ] } ] ) = {atomic,
>                                                                             ok}
> foo.erl:17:<0.32.0>: mnesia : transaction ( fun ( ) -> mnesia : write ( foo , { foo , 42 , a } , sticky_write ) end ) = {atomic,
>                                                                                                    ok}
> foo.erl:23:<0.32.0>: rpc : call ( Slave , mnesia , transaction , [ fun mnesia : read / 3 , [ foo , 42 , write ] ] ) = {atomic,
>                                                                                                  undefined}
>
> =INFO REPORT==== 1-Dec-2011::17:22:22 ===
>    application: mnesia
>    exited: stopped
>    type: temporary
> ** exception error: {assertEqual_failed,[{module,foo},
>                                         {line,26},
>                                         {expression,"? debugVal ( rpc : call ( Slave , mnesia , transaction , [ fun mnesia : read / 3 , [ foo , 42 , write ] ] ) )"},
>                                         {expected,{atomic,[{foo,42,a}]}},
>                                         {value,{atomic,undefined}}]}
>     in function  foo:'-foo/0-fun-8-'/2 (foo.erl, line 26)
>     in call from foo:foo/0 (foo.erl, line 22)
>
>
> --
> Magnus Henoch
> Erlang Solutions Ltd
> http://www.erlang-solutions.com/
>
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