Fix error on race condition in mem3 startup

wohali · wohali · commit 81ee7c5ac71e · 2017-05-02T14:14:58.000-04:00
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 makes `mem3_util:ensure_enxists/1` more robust in the face of a race to create `_dbs`. Fixes COUCHDB-3402. Approved by @davisp and @iilyak
diff --git a/src/mem3/src/mem3_util.erl b/src/mem3/src/mem3_util.erl
@@ -214,11 +214,12 @@ shard_info(DbName) ->
 ensure_exists(DbName) when is_list(DbName) ->
     ensure_exists(list_to_binary(DbName));
 ensure_exists(DbName) ->
-    case couch_db:open(DbName, [nologifmissing, sys_db | [?ADMIN_CTX]]) of
+    Options = [nologifmissing, sys_db, {create_if_missing, true}, ?ADMIN_CTX],
+    case couch_db:open(DbName, Options) of
     {ok, Db} ->
         {ok, Db};
-    _ ->
-        couch_server:create(DbName, [?ADMIN_CTX])
+    file_exists ->
+        couch_db:open(DbName, [sys_db, ?ADMIN_CTX])
     end.
 
 
