Pass custom settings to the DB's SESSION (#1063)

- allows queries to refer to current_setting('app.settings.foo') to retrieve variables
- useful for 12-factor apps (app data can be in environment)
- provides workaround for AWS Relational Database Service (RDS) not
  allowing `ALTER DATABASE SET 'app.[KEY]' TO '[VALUE]'` on database.

src/PostgREST/Config.hs

@@ -76,6 +76,7 @@ data AppConfig = AppConfig {
   , configMaxRows           :: Maybe Integer
   , configReqCheck          :: Maybe Text
   , configQuiet             :: Bool
+  , configSettings          :: [(Text, Text)]
   }
 
 defaultCorsPolicy :: CorsResourcePolicy
@@ -136,6 +137,7 @@ readOptions = do
           <*> (join . fmap coerceInt <$> C.key "max-rows")
           <*> (mfilter (/= "") <$> C.key "pre-request")
           <*> pure False
+          <*> (fmap parsedPairToTextPair <$> C.subassocs "app.settings")
 
   case mAppConf of
     Nothing -> do
@@ -145,6 +147,13 @@ readOptions = do
       return appConf
 
   where
+    parsedPairToTextPair :: (Name, Value) -> (Text, Text)
+    parsedPairToTextPair (k, v) = (k, newValue)
+      where
+        newValue = case v of
+          String textVal -> textVal
+          _ -> show v
+
     parseJwtAudience :: Name -> C.ConfigParserM (Maybe StringOrURI)
     parseJwtAudience k =
       C.key k >>= \case
@@ -159,9 +168,9 @@ readOptions = do
     coerceInt (String x) = readMaybe $ toS x
     coerceInt _          = Nothing
 
-    coerceBool ::  Value -> Maybe Bool
+    coerceBool :: Value -> Maybe Bool
     coerceBool (Bool b)   = Just b
-    coerceBool (String x) = readMaybe $ toS x
+    coerceBool (String b) = readMaybe $ toS b
     coerceBool _          = Nothing
 
     opts = info (helper <*> pathParser) $
@@ -218,7 +227,7 @@ pathParser =
 
 -- | Tells the minimum PostgreSQL version required by this version of PostgREST
 minimumPgVersion :: PgVersion
-minimumPgVersion = PgVersion 90300 "9.3"
+minimumPgVersion = PgVersion 90400 "9.4"
 
 pgVersion96 :: PgVersion
 pgVersion96 = PgVersion 90600 "9.6"

src/PostgREST/DbStructure.hs

@@ -9,6 +9,7 @@ module PostgREST.DbStructure (
 , accessibleProcs
 , schemaDescription
 , getPgVersion
+, fillSessionWithSettings
 ) where
 
 import qualified Hasql.Decoders                as HD
@@ -18,9 +19,11 @@ import qualified Hasql.Query                   as H
 import           Control.Applicative
 import qualified Data.HashMap.Strict           as M
 import           Data.List                     (elemIndex)
+import qualified Data.List                     as List
 import           Data.Maybe                    (fromJust)
 import           Data.Text                     (split, strip,
-                                                breakOn, dropAround, splitOn)
+                                                breakOn, dropAround,
+                                                splitOn)
 import qualified Data.Text                     as T
 import qualified Hasql.Session                 as H
 import           PostgREST.Types
@@ -30,6 +33,9 @@ import           GHC.Exts                      (groupWith)
 import           Protolude
 import           Unsafe (unsafeHead)
 
+import           Data.Functor.Contravariant    (contramap)
+import           Contravariant.Extras          (contrazip2)
+
 getDbStructure :: Schema -> PgVersion -> H.Session DbStructure
 getDbStructure schema pgVer = do
   tabs <- H.query () allTables
@@ -747,3 +753,17 @@ getPgVersion = H.query () $ H.statement sql HE.unit versionRow False
   where
     sql = "SELECT current_setting('server_version_num')::integer, current_setting('server_version')"
     versionRow = HD.singleRow $ PgVersion <$> HD.value HD.int4 <*> HD.value HD.text
+
+fillSessionWithSettings :: [(Text, Text)] -> H.Session ()
+fillSessionWithSettings settings =
+    -- Send all of the config settings to the set_config function, using pgsql's `unnest` to transform arrays of values
+    H.query settings $ H.statement "SELECT set_config(k, v, false) FROM unnest($1, $2) AS f1(k, v)" encoder HD.unit False
+  
+  where
+    -- Take a list of (key, value) pairs and encode each as an array to later bind to the query
+    -- see Insert Many section at https://hackage.haskell.org/package/hasql-1.1.1/docs/Hasql-Encoders.html
+    encoder = contramap List.unzip $ contrazip2 (vector HE.text) (vector HE.text)
+      where
+        vector value =
+          HE.value $ HE.array $ HE.arrayDimension foldl' $ HE.arrayValue value
+