Fix error on race condition in mem3 startup

During mem3 startup, 2 paths attempt to call `couch_server:create/2` on
`_dbs`:

```
gen_server:init_it/6
  -> mem3_shards:init/1
    -> mem3_shards:get_update_seq/0
      -> couch_server:create/2
```

and

```
mem3_sync:initial_sync/1
  -> mem3_shards:fold/2
    -> couch_server:create/2
```

Normally, the first path completes before the second. If the second path
finishes first, the first path fails because it does not expect a
`file_exists` response.

This patch simply retries mem3_util:ensure_exists/1 once if it gets back
a `file_exists` response. Any failures past this point are not handled.

Fixes COUCHDB-3402.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/mem3/src/mem3_shards.erl b/src/mem3/src/mem3_shards.erl
index 3c2001b1b..7182f09da 100644
--- a/src/mem3/src/mem3_shards.erl
+++ b/src/mem3/src/mem3_shards.erl
@@ -315,7 +315,12 @@ fold_fun(#doc_info{}=DI, _, {Db, UFun, UAcc}) ->
 
 get_update_seq() ->
     DbName = config:get("mem3", "shards_db", "_dbs"),
-    {ok, Db} = mem3_util:ensure_exists(DbName),
+    case mem3_util:ensure_exists(DbName) of
+    {ok, Db} ->
+        {ok, Db};
+    file_exists ->
+        {ok, Db} = mem3_util:ensure_exists(DbName)
+    end,
     couch_db:close(Db),
     Db#db.update_seq.