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Fix broken sequencer semantics.

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It occurred to me today that I implemented the sequencer incorrectly and
hadn't yet noticed because I don't have any tests that are
complex/interleaved/perhaps-non-deterministic to find the problem.
The problem is that the sequencer's current implementation only keeps
track of the last LPN for any Tango stream.

The fix is to do what the paper actually says: the sequencer keeps a
*list* of the last $K$ LPNs for each stream.  Derp.  Yes, that's really
necessary to avoid a pretty simple race condition with 2 actors
simultaneously updating a single Tango stream.

1st commit: fix the implementation and the smoke test.  The
broken-everything-else will be repaired in later commits.
This commit is contained in:
Scott Lystig Fritchie 2014-09-20 18:01:25 +09:00
parent 940012cef1
commit b8c051c89f
2 changed files with 28 additions and 7 deletions
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18
prototype/tango-prototype/src/corfurl_sequencer.erl
View file

@ -109,7 +109,7 @@ init({FLUs, TypeOrSeed}) ->
end.
handle_call({get, NumPages, StreamList, LC}, _From, {Tab, MLP}) ->
[ets:insert(Tab, {Stream, MLP}) || Stream <- StreamList],
update_stream_tails(Tab, StreamList, MLP),
NewLC = lclock_update(LC),
{reply, {{ok, MLP}, NewLC}, {Tab, MLP + NumPages}};
handle_call({get, NumPages, StreamList, LC}, _From,
@ -164,6 +164,22 @@ get_max_logical_page(FLUs) ->
FLU <- FLUs,
{ok, Ps} <- [corfurl_flu:status(FLU)]]).
update_stream_tails(Tab, StreamList, LPN) ->
[begin
OldBackPs = try ets:lookup_element(Tab, Stream, 2)
catch error:badarg -> []
end,
NewBackPs = add_back_pointer(OldBackPs, LPN),
ets:insert(Tab, {Stream, NewBackPs})
end || Stream <- StreamList].
add_back_pointer([D,C,B,_A|_], New) ->
[New,D,C,B];
add_back_pointer([], New) ->
[New];
add_back_pointer(BackPs, New) ->
[New|BackPs].
-ifdef(PULSE).
lclock_init() ->

17
prototype/tango-prototype/test/corfurl_sequencer_test.erl
View file

@ -65,15 +65,20 @@ smoke_test() ->
MLP4 = MLP0 + 4,
{ok, Sequencer} = ?M:start_link(FLUs),
try
[{Stream9, Tail9}] = StreamTails = [{9, 99999}],
[{Stream9, Tails9}] = StreamTails = [{9, [1125, 1124, 1123]}],
ok = ?M:set_tails(Sequencer, StreamTails),
{ok, [Tail9]} = ?M:get_tails(Sequencer, [Stream9]),
{ok, [Tails9]} = ?M:get_tails(Sequencer, [Stream9]),
{ok, MLP1} = ?M:get(Sequencer, 2),
{ok, MLP3} = ?M:get(Sequencer, 1, [2]),
{ok, MLP4} = ?M:get(Sequencer, 1, [1]),
{ok, LPN1} = ?M:get(Sequencer, 2),
{ok, LPN3} = ?M:get(Sequencer, 1, [2]),
{ok, LPN4} = ?M:get(Sequencer, 1, [1]),
{ok, LPN5} = ?M:get(Sequencer, 1, [2]),
{ok, LPN6} = ?M:get(Sequencer, 1, [2]),
{ok, LPN7} = ?M:get(Sequencer, 1, [2]),
{ok, LPN8} = ?M:get(Sequencer, 1, [2]),
{ok, [MLP4, MLP3]} = ?M:get_tails(Sequencer, [1,2])
{ok, [[LPN4], [LPN8, LPN7, LPN6, LPN5]]} = ?M:get_tails(Sequencer,
[1,2])
after
?M:stop(Sequencer)
end