avoid displaying the scanning annexed files message when repo is not large

Avoids users thinking this scan is a big deal, when it's not in the
majority of repos.

showSideActionAfter has some ugly caveats, since it has to display in
the background of another action. I could not see a better way to do it
and it works fine in this particular case. It also doesn't really belong
in Annex.Concurrent, but cannot go in Messages due to an import loop.

Sponsored-by: Dartmouth College's Datalad project

Annex/Concurrent.hs

@@ -1,6 +1,6 @@
 {- git-annex concurrent state
  -
- - Copyright 2015-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -19,8 +19,10 @@ import Types.Concurrency
 import Types.CatFileHandles
 import Annex.CheckAttr
 import Annex.CheckIgnore
+import Utility.ThreadScheduler
 
 import qualified Data.Map as M
+import Control.Concurrent.Async
 
 setConcurrency :: ConcurrencySetting -> Annex ()
 setConcurrency (ConcurrencyCmdLine s) = setConcurrency' s ConcurrencyCmdLine
@@ -98,3 +100,23 @@ mergeState st = do
 		uncurry addCleanupAction
 	Annex.Queue.mergeFrom st'
 	changeState $ \s -> s { errcounter = errcounter s + errcounter st' }
+
+{- Display a message, only when the action runs for a long enough
+ - amount of time.
+ - 
+ - The action should not display any other messages, progress, etc;
+ - if it did there could be some scrambling of the display since the
+ - message display could happen at the same time as other output,
+ - or after it.
+ -}
+showSideActionAfter :: Microseconds -> String -> Annex a -> Annex a
+showSideActionAfter t m a = do
+	waiter <- liftIO $ async $ unboundDelay t
+	let display = liftIO (waitCatch waiter) >>= \case
+		Left _ -> return ()
+		Right _ -> showSideAction m
+	displayer <- liftIO . async =<< forkState display
+	let cleanup = do
+		liftIO $ cancel waiter
+		join (liftIO (wait displayer))
+	a `finally` cleanup

Annex/Init.hs

@@ -37,6 +37,7 @@ import Annex.UUID
 import Annex.WorkTree
 import Annex.Fixup
 import Annex.Path
+import Annex.Concurrent
 import Config
 import Config.Files
 import Config.Smudge
@@ -133,8 +134,8 @@ initialize' mversion = checkInitializeAllowed $ do
 		then configureSmudgeFilter
 		else deconfigureSmudgeFilter
 	unlessM isBareRepo $ do
-		showSideAction "scanning for annexed files"
-		scanAnnexedFiles
+		showSideActionAfter oneSecond "scanning for annexed files" $
+			scanAnnexedFiles
 		hookWrite postCheckoutHook
 		hookWrite postMergeHook
 	AdjustedBranch.checkAdjustedClone >>= \case

Utility/ThreadScheduler.hs

@@ -15,6 +15,7 @@ module Utility.ThreadScheduler (
 	threadDelaySeconds,
 	waitForTermination,
 	oneSecond,
+	unboundDelay,
 ) where
 
 import Control.Monad

doc/todo/init__58__do_not_bother_scanning_if_in_git-annex_branch/comment_2_2cbfb22e8f1d89d4f78ade565554cb1c._comment

@@ -0,0 +1,9 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 2"""
+ date="2021-06-04T17:14:42Z"
+ content="""
+Made the scanning message not be displayed unless it takes at least 1
+second. Of course, if some test suite is still looking at that message,
+it will break..
+"""]]