Add ability to map GUC to http response headers

src/PostgREST/Types.hs

@@ -4,6 +4,7 @@ import           Protolude
 import qualified GHC.Show
 import           Data.Aeson
 import qualified Data.ByteString.Lazy as BL
+import qualified Data.CaseInsensitive as CI
 import qualified Data.HashMap.Strict  as M
 import           Data.Tree
 import qualified Data.Vector          as V
@@ -216,6 +217,22 @@ type NodeName = Text
 -- Rpc query param, only used for GET rpcs
 type RpcQParam = (Text, Text)
 
+{-|
+  Custom guc header, it's obtained by parsing the json in a:
+  `SET LOCAL "response.headers" = '[{"Set-Cookie": ".."}]'
+-}
+newtype GucHeader = GucHeader (Text, Text)
+
+instance FromJSON GucHeader where
+  parseJSON (Object o) = case headMay (M.toList o) of
+    Just (k, String s) | M.size o == 1 -> pure $ GucHeader (k, s)
+                       | otherwise     -> mzero
+    _ -> mzero
+  parseJSON _          = mzero
+
+toHeaders :: [GucHeader] -> [Header]
+toHeaders = map $ \(GucHeader (k, v)) -> (CI.mk $ toS k, toS v)
+
 {-|
   This type will hold information about which particular 'Relation' between two tables to choose when there are multiple ones.
   Specifically, it will contain the name of the foreign key or the join table in many to many relations.