From a4145c2947d6b6c7e29757e2e477dc81dc53702d Mon Sep 17 00:00:00 2001
From: "Paul J. Davis" <paul.joseph.davis@gmail.com>
Date: Tue, 7 May 2019 15:06:06 -0500
Subject: Fix epoch mismatch errors (#2027)

Originally we posited that duplicate UUIDs could never be created.
However due to operations interventions its possible to unintentionally
copy UUIDs in ways that violate this assumption. Instead of crashing the
process we just reset the seq to zero and log a warning as was done in
the other instances when we have mismatches in the epoch history.
---
 src/couch/src/couch_db.erl | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/src/couch/src/couch_db.erl b/src/couch/src/couch_db.erl
index ab38eb895..52c3fbb76 100644
--- a/src/couch/src/couch_db.erl
+++ b/src/couch/src/couch_db.erl
@@ -1560,7 +1560,14 @@ calculate_start_seq(Db, _Node, {Seq, Uuid, EpochNode}) ->
 calculate_start_seq(Db, _Node, {replace, OriginalNode, Uuid, Seq}) ->
     case is_prefix(Uuid, couch_db:get_uuid(Db)) of
         true ->
-            start_seq(get_epochs(Db), OriginalNode, Seq);
+            try
+                start_seq(get_epochs(Db), OriginalNode, Seq)
+            catch throw:epoch_mismatch ->
+                couch_log:warning("~p start_seq duplicate uuid on node: ~p "
+                    "db: ~p, seq: ~p, uuid: ~p, epoch_node: ~p",
+                    [?MODULE, node(), Db#db.name, Seq, Uuid, OriginalNode]),
+                0
+            end;
         false ->
             {replace, OriginalNode, Uuid, Seq}
     end.
@@ -1607,8 +1614,8 @@ start_seq([{_, NewSeq}, {OrigNode, _} | _], OrigNode, Seq) when Seq > NewSeq ->
     NewSeq;
 start_seq([_ | Rest], OrigNode, Seq) ->
     start_seq(Rest, OrigNode, Seq);
-start_seq([], OrigNode, Seq) ->
-    erlang:error({epoch_mismatch, OrigNode, Seq}).
+start_seq([], _OrigNode, _Seq) ->
+    throw(epoch_mismatch).
 
 
 fold_docs(Db, UserFun, UserAcc) ->
@@ -1986,7 +1993,8 @@ calculate_start_seq_test_() ->
             t_calculate_start_seq_uuid_mismatch(),
             t_calculate_start_seq_is_owner(),
             t_calculate_start_seq_not_owner(),
-            t_calculate_start_seq_raw()
+            t_calculate_start_seq_raw(),
+            t_calculate_start_seq_epoch_mismatch()
         ]
     }.
 
@@ -2031,6 +2039,14 @@ t_calculate_start_seq_raw() ->
         ?assertEqual(13, Seq)
     end).
 
+t_calculate_start_seq_epoch_mismatch() ->
+    ?_test(begin
+        Db = test_util:fake_db([]),
+        SeqIn = {replace, not_this_node, get_uuid(Db), 42},
+        Seq = calculate_start_seq(Db, node1, SeqIn),
+        ?assertEqual(0, Seq)
+    end).
+
 is_owner_test() ->
     ?assertNot(is_owner(foo, 1, [])),
     ?assertNot(is_owner(foo, 1, [{foo, 1}])),
-- 
cgit v1.2.1