pollBatch: wait 100ms for the write to complete

Summary:
We're incurring a minimum 1s delay for every write currently, because
the first poll will always fail and the caller (often SendQueue.hs)
waits 1s before polling again.

This makes tests run slowly, in particular.

Workaround: block for max 100ms in pollBatch to allow writes that
complete quickly to avoid the poll/retry roundtrip.

Reviewed By: donsbot

Differential Revision: D51206446

fbshipit-source-id: 8faa5d609cd586c0fd3f5c9b789289561c482307

glean/db/Glean/Database/Writes.hs

@@ -55,6 +55,7 @@ import qualified Data.Text.Encoding as Text
 import qualified Data.UUID as UUID
 import qualified Data.UUID.V4 as UUID
 import System.Clock
+import System.Timeout
 
 import ServiceData.GlobalStats
 import ServiceData.Types
@@ -319,7 +320,11 @@ pollBatch env@Env{..} handle = do
   r <- HashMap.lookup handle <$> readTVarIO envWrites
   case r of
     Just write -> do
-      s <- tryReadMVar (writeWait write)
+      -- for tiny writes that will complete in a few ms, we would like
+      -- to wait synchronously. Otherwise we'll return a Retry to the
+      -- caller which will wait 1s before polling again. In particular
+      -- all those one-second delays make tests run slowly.
+      s <- timeout 100000 $ readMVar (writeWait write)
       case s of
         Just x -> do
           atomically $ void $ updateTVar envWrites $ HashMap.delete handle