DRY out CouchDB FDB prefix fetching

It was suggested in another PR's discussion:

https://github.com/apache/couchdb/pull/2107#pullrequestreview-274431487

3 files changed, 12 insertions, 18 deletions

diff --git a/src/couch_jobs/src/couch_jobs_fdb.erl b/src/couch_jobs/src/couch_jobs_fdb.erl
index 6903801a2..00a8ddf72 100644
--- a/src/couch_jobs/src/couch_jobs_fdb.erl
+++ b/src/couch_jobs/src/couch_jobs_fdb.erl
@@ -615,10 +615,7 @@ init_jtx(undefined) ->
     fabric2_fdb:transactional(fun(Tx) -> init_jtx(Tx) end);
 
 init_jtx({erlfdb_transaction, _} = Tx) ->
-    Root = erlfdb_directory:root(),
-    Dir = fabric2_server:fdb_directory(),
-    CouchDB = erlfdb_directory:create_or_open(Tx, Root, Dir),
-    LayerPrefix = erlfdb_directory:get_name(CouchDB),
+    LayerPrefix = fabric2_fdb:create_or_open_couchdb_dir(Tx),
     Jobs = erlfdb_tuple:pack({?JOBS}, LayerPrefix),
     Version = erlfdb:wait(erlfdb:get(Tx, ?METADATA_VERSION_KEY)),
     % layer_prefix, md_version and tx here match db map fields in fabric2_fdb
diff --git a/src/fabric/src/fabric2_fdb.erl b/src/fabric/src/fabric2_fdb.erl
index 5c58da482..5471f99f2 100644
--- a/src/fabric/src/fabric2_fdb.erl
+++ b/src/fabric/src/fabric2_fdb.erl
@@ -24,6 +24,8 @@
     delete/1,
     exists/1,
 
+    create_or_open_couchdb_dir/1,
+
     list_dbs/4,
 
     get_info/1,
@@ -274,11 +276,15 @@ exists(#{name := DbName} = Db) when is_binary(DbName) ->
     end.
 
 
-list_dbs(Tx, Callback, AccIn, Options) ->
+create_or_open_couchdb_dir(Tx) ->
     Root = erlfdb_directory:root(),
     Dir = fabric2_server:fdb_directory(),
     CouchDB = erlfdb_directory:create_or_open(Tx, Root, Dir),
-    LayerPrefix = erlfdb_directory:get_name(CouchDB),
+    erlfdb_directory:get_name(CouchDB).
+
+
+list_dbs(Tx, Callback, AccIn, Options) ->
+    LayerPrefix = create_or_open_couchdb_dir(Tx),
     Prefix = erlfdb_tuple:pack({?ALL_DBS}, LayerPrefix),
     fold_range({tx, Tx}, Prefix, fun({K, _V}, Acc) ->
         {DbName} = erlfdb_tuple:unpack(K, Prefix),
@@ -781,10 +787,7 @@ debug_cluster(Start, End) ->
 
 
 init_db(Tx, DbName, Options) ->
-    Root = erlfdb_directory:root(),
-    Dir = fabric2_server:fdb_directory(),
-    CouchDB = erlfdb_directory:create_or_open(Tx, Root, Dir),
-    Prefix = erlfdb_directory:get_name(CouchDB),
+    Prefix = create_or_open_couchdb_dir(Tx),
     Version = erlfdb:wait(erlfdb:get(Tx, ?METADATA_VERSION_KEY)),
     #{
         name => DbName,
diff --git a/src/fabric/src/fabric2_txids.erl b/src/fabric/src/fabric2_txids.erl
index 06704f021..f1a75243c 100644
--- a/src/fabric/src/fabric2_txids.erl
+++ b/src/fabric/src/fabric2_txids.erl
@@ -44,10 +44,7 @@ start_link() ->
 
 
 create(Tx, undefined) ->
-    Root = erlfdb_directory:root(),
-    Dir = fabric2_server:fdb_directory(),
-    CouchDB = erlfdb_directory:create_or_open(Tx, Root, Dir),
-    Prefix = erlfdb_directory:get_name(CouchDB),
+    Prefix = fabric2_fdb:create_or_open_couchdb_dir(Tx),
     create(Tx, Prefix);
 
 create(_Tx, LayerPrefix) ->
@@ -136,10 +133,7 @@ clean(St, NeedsSweep) ->
 
 
 sweep(Tx, {Mega, Secs, Micro}) ->
-    Root = erlfdb_directory:root(),
-    Dir = fabric2_server:fdb_directory(),
-    CouchDB = erlfdb_directory:create_or_open(Tx, Root, Dir),
-    Prefix = erlfdb_directory:get_name(CouchDB),
+    Prefix = fabric2_fdb:create_or_open_couchdb_dir(Tx),
     StartKey = erlfdb_tuple:pack({?TX_IDS}, Prefix),
     EndKey = erlfdb_tuple:pack({?TX_IDS, Mega, Secs, Micro}, Prefix),
     erlfdb:set_option(Tx, next_write_no_write_conflict_range),