From b8c051c89faac248135d2a635e48943b655a161b Mon Sep 17 00:00:00 2001
From: Scott Lystig Fritchie <slfritchie@snookles.com>
Date: Sat, 20 Sep 2014 18:01:25 +0900
Subject: [PATCH] Fix broken sequencer semantics.

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.
---
 .../tango-prototype/src/corfurl_sequencer.erl  | 18 +++++++++++++++++-
 .../test/corfurl_sequencer_test.erl            | 17 +++++++++++------
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/prototype/tango-prototype/src/corfurl_sequencer.erl b/prototype/tango-prototype/src/corfurl_sequencer.erl
index 6e869e1..b73ae06 100644
--- a/prototype/tango-prototype/src/corfurl_sequencer.erl
+++ b/prototype/tango-prototype/src/corfurl_sequencer.erl
@@ -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() ->
diff --git a/prototype/tango-prototype/test/corfurl_sequencer_test.erl b/prototype/tango-prototype/test/corfurl_sequencer_test.erl
index f30cba6..789a6d5 100644
--- a/prototype/tango-prototype/test/corfurl_sequencer_test.erl
+++ b/prototype/tango-prototype/test/corfurl_sequencer_test.erl
@@ -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