[couchdb] 01/01: Fix error on race condition in mem3 startup

[wo...@apache.org]

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wohali pushed a commit to branch 3402-mem3-race
in repository https://gitbox.apache.org/repos/asf/couchdb.git

commit 1aa48ef245a9f5d8d336d43070767363ef092ed1
Author: Joan Touzet <jo...@atypical.net>
AuthorDate: Sun Apr 30 02:49:20 2017 -0400

    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 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
---
 src/mem3/src/mem3_util.erl | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/mem3/src/mem3_util.erl b/src/mem3/src/mem3_util.erl
index 2cd444d..71ef5b6 100644
--- 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.
 
 

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