Add basic access to the PostgREST version through SQL

src/PostgREST/AppState.hs

@@ -41,10 +41,12 @@ import Data.IORef         (IORef, atomicWriteIORef, newIORef,
 import Data.Time          (ZonedTime, defaultTimeLocale, formatTime,
                            getZonedTime)
 import Data.Time.Clock    (UTCTime, getCurrentTime)
+import Network.URI        (parseURI, uriQuery)
 
 import PostgREST.Config           (AppConfig (..))
 import PostgREST.Config.PgVersion (PgVersion (..), minimumPgVersion)
 import PostgREST.SchemaCache      (SchemaCache)
+import PostgREST.Version          (prettyVersion)
 
 import Protolude
 
@@ -111,10 +113,17 @@ destroy = destroyPool
 
 initPool :: AppConfig -> IO SQL.Pool
 initPool AppConfig{..} =
-  SQL.acquire configDbPoolSize timeoutMicroseconds $ toUtf8 configDbUri
+  SQL.acquire configDbPoolSize timeoutMicroseconds configDbUriAndAppName
   where
     timeoutMicroseconds = (* oneSecond) <$> configDbPoolAcquisitionTimeout
     oneSecond = 1000000
+    configDbUriAndAppName = toUtf8 configDbUri <>
+      case uriQuery <$> parseURI (toS configDbUri) of
+        Nothing  -> " " <> appNameParam
+        Just ""  -> "?" <> appNameParam
+        Just "?" -> appNameParam
+        _        -> "&" <> appNameParam
+    appNameParam = "fallback_application_name='PostgREST " <> prettyVersion <> "'"
 
 -- | Run an action with a database connection.
 usePool :: AppState -> SQL.Session a -> IO (Either SQL.UsageError a)