[GitHub] eiri commented on a change in pull request #496: Couchdb 3287 pluggable storage engines

[gi...@git.apache.org]

eiri commented on a change in pull request #496: Couchdb 3287 pluggable storage engines
URL: https://github.com/apache/couchdb/pull/496#discussion_r119694870
 
 

 ##########
 File path: src/couch/src/couch_server.erl
 ##########
 @@ -559,6 +596,115 @@ db_closed(Server, Options) ->
         true -> Server
     end.
 
+
+get_configured_engines() ->
+    ConfigEntries = config:get("couchdb_engines"),
+    Engines = lists:flatmap(fun({Extension, ModuleStr}) ->
+        try
+            [{Extension, list_to_atom(ModuleStr)}]
+        catch _T:_R ->
+            []
+        end
+    end, ConfigEntries),
+    case Engines of
+        [] ->
+            [{"couch", couch_bt_engine}];
+        Else ->
+            Else
+    end.
+
+
+get_engine(Server, DbName, Options) ->
+    #server{
+        root_dir = RootDir,
+        engines = Engines
+    } = Server,
+    case couch_util:get_value(engine, Options) of
+        Ext when is_binary(Ext) ->
+            ExtStr = binary_to_list(Ext),
+            case lists:keyfind(ExtStr, 1, Engines) of
+                {ExtStr, Engine} ->
+                    Path = make_filepath(RootDir, DbName, ExtStr),
+                    {ok, {Engine, Path}};
+                false ->
+                    {error, {invalid_engine_extension, Ext}}
+            end;
+        _ ->
+            get_engine(Server, DbName)
+    end.
+
+
+get_engine(Server, DbName) ->
+    #server{
+        root_dir = RootDir,
+        engines = Engines
+    } = Server,
+    Possible = get_possible_engines(DbName, RootDir, Engines),
+    case Possible of
+        [] ->
+            get_default_engine(Server, DbName);
+        [Engine] ->
+            {ok, Engine};
+        _ ->
+            erlang:error(engine_conflict)
+    end.
+
+
+get_possible_engines(DbName, RootDir, Engines) ->
+    lists:foldl(fun({Extension, Engine}, Acc) ->
+        Path = make_filepath(RootDir, DbName, Extension),
+        case couch_db_engine:exists(Engine, Path) of
+            true ->
+                [{Engine, Path} | Acc];
+            false ->
+                Acc
+        end
+    end, [], Engines).
+
+
+get_default_engine(Server, DbName) ->
+    #server{
+        root_dir = RootDir,
+        engines = Engines
+    } = Server,
+    Default = {couch_bt_engine, make_filepath(RootDir, DbName, "couch")},
+    case config:get("couchdb", "default_engine") of
+        Extension when is_list(Extension) ->
+            case lists:keyfind(Extension, 1, Engines) of
+                {Extension, Module} ->
+                    {ok, {Module, make_filepath(RootDir, DbName, Extension)}};
+                false ->
+                    Fmt = "Invalid storage engine extension ~s,"
+                            " configured engine extensions are: ~s",
+                    Exts = [E || {E, _} <- Engines],
+                    Args = [Extension, string:join(Exts, ", ")],
+                    couch_log:error(Fmt, Args),
+                    {ok, Default}
+            end;
+        _ ->
+            {ok, Default}
+    end.
+
+
+make_filepath(RootDir, DbName, Extension) when is_binary(RootDir) ->
+    make_filepath(binary_to_list(RootDir), DbName, Extension);
+make_filepath(RootDir, DbName, Extension) when is_binary(DbName) ->
+    make_filepath(RootDir, binary_to_list(DbName), Extension);
+make_filepath(RootDir, DbName, Extension) when is_binary(Extension) ->
+    make_filepath(RootDir, DbName, binary_to_list(Extension));
+make_filepath(RootDir, DbName, Extension) ->
+    filename:join([RootDir, "./" ++ DbName ++ "." ++ Extension]).
 
 Review comment:
   I think this should be just `filename:join([RootDir, DbName ++ "." ++ Extension])`.
   
   If the current directory bit here just for a case when `RootDir` is an empty string, to make it into a relative path, then it's not going to work, because `filename:join/1` will overwrite that:
   ```erlang
   1> filename:join(["", "./" ++ "koi" ++ "." ++ "couch"]).
   "/koi.couch"
   ```
 
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