Enable HASURA_GRAPHQL_PG_CONN_LIFETIME by default with a default of 6…

…00 sec

See: hasura/pg-client-hs#26

We're still experiencing segfaults in certain configurations from the
libpq shrinking approach. This should be very safe, but is also
inadequate in several ways:

- negotiating a new connection is quite heavyweight compared to
  allocating a 1mb buffer
- thundering herd issues (TODO maybe add some jitter here)
- memory is not reclaimed promptly; there is no good default here.

CHANGELOG.md

@@ -121,6 +121,7 @@ This release contains the [PDV refactor (#4111)](https://github.com/hasura/graph
 - server: accept only non-negative integers for batch size and refetch interval (close #5653) (#5759)
 - server: fix bug which arised when renaming a table which had a manual relationship defined (close #4158)
 - server: limit the length of event trigger names (close #5786)
+- server: enable HASURA_GRAPHQL_PG_CONN_LIFETIME by default to reclaim memory
 - server: Configurable websocket keep-alive interval. Add `--websocket-keepalive` command-line flag
           and handle `HASURA_GRAPHQL_WEBSOCKET_KEEPALIVE` env variable (fix #3539) 
 **NOTE:** If you have event triggers with names greater than 42 chars, then you should update their names to avoid running into Postgres identifier limit bug (#5786)

server/cabal.project

@@ -43,7 +43,7 @@ package graphql-engine
 source-repository-package
   type: git
   location: https://github.com/hasura/pg-client-hs.git
-  tag: 633a06ba9c1e6a561b0ad0d25951fcdf6dd054ca
+  tag: 08b02fbb4e7c7e1ac44325ef1bd107725f59255f
 
 source-repository-package
   type: git

server/src-lib/Hasura/Server/Init.hs

@@ -201,7 +201,10 @@ mkServeOptions rso = do
       -- hard throughput cap at 1000RPS when db queries take 50ms on average:
       conns <- fromMaybe 50 <$> withEnv c (fst pgConnsEnv)
       iTime <- fromMaybe 180 <$> withEnv i (fst pgTimeoutEnv)
-      connLifetime <- withEnv cl (fst pgConnLifetimeEnv)
+      connLifetime <- withEnv cl (fst pgConnLifetimeEnv) <&> \case
+        Nothing -> Just 600 -- Not set by user; use the default timeout
+        Just 0  -> Nothing  -- user wants to disable PG_CONN_LIFETIME
+        Just n  -> Just n   -- user specified n seconds lifetime
       allowPrepare <- fromMaybe True <$> withEnv p (fst pgUsePrepareEnv)
       return $ Q.ConnParams stripes conns iTime allowPrepare connLifetime
 
@@ -393,7 +396,8 @@ pgConnLifetimeEnv :: (String, String)
 pgConnLifetimeEnv =
   ( "HASURA_GRAPHQL_PG_CONN_LIFETIME"
   , "Time from connection creation after which the connection should be destroyed and a new one "
-    <> "created. (default: none)"
+    <> "created. A value of 0 indicates we should never destroy an active connection. If 0 is "
+    <> "passed, memory from large query results may not be reclaimed. (default: 600 sec)"
   )
 
 pgUsePrepareEnv :: (String, String)