suggest when user may want annex.stalldetection

When annex.stalldetection is not enabled, and a likely stall is detected,
display a suggestion to enable it.

Note that the progress meter display is not taken down when displaying
the message, so it will display like this:

	0%    8 B                 0 B/s
	  Transfer seems to have stalled. To handle stalling transfers, configure annex.stalldetection
	0%    10 B                0 B/s

Although of course if it's really stalled, it will never update
again after the message. Taking down the progress meter and starting
a new one doesn't seem too necessary given how unusual this is,
also this does help show the state it was at when it stalled.

Use of uninterruptibleCancel here is ok, the thread it's canceling
only does STM transactions and sleeps. The annex thread that gets
forked off is separate to avoid it being canceled, so that it
can be joined back at the end.

A module cycle required moving from dupState the precaching of the
remote list. Doing it at startConcurrency should cover all the cases
where the remote list is used in concurrent actions.

This commit was sponsored by Kevin Mueller on Patreon.

Logs/Transfer.hs

@@ -1,6 +1,6 @@
 {- git-annex transfer information files and lock files
  -
- - Copyright 2012-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -27,7 +27,7 @@ import Annex.Perms
 
 import Data.Time.Clock
 import Data.Time.Clock.POSIX
-import Control.Concurrent
+import Control.Concurrent.STM
 import qualified Data.ByteString.Char8 as B8
 import qualified System.FilePath.ByteString as P
 
@@ -58,23 +58,25 @@ percentComplete t info =
  - the transfer info file. Also returns the file it'll be updating, 
  - an action that sets up the file with appropriate permissions,
  - which should be run after locking the transfer lock file, but
- - before using the callback, and a MVar that can be used to read
- - the number of bytesComplete. -}
-mkProgressUpdater :: Transfer -> TransferInfo -> Annex (MeterUpdate, RawFilePath, Annex (), MVar Integer)
+ - before using the callback, and a TVar that can be used to read
+ - the number of bytes processed so far. -}
+mkProgressUpdater :: Transfer -> TransferInfo -> Annex (MeterUpdate, RawFilePath, Annex (), TVar (Maybe BytesProcessed))
 mkProgressUpdater t info = do
 	tfile <- fromRepo $ transferFile t
 	let createtfile = void $ tryNonAsync $ writeTransferInfoFile info tfile
-	mvar <- liftIO $ newMVar 0
-	return (liftIO . updater (fromRawFilePath tfile) mvar, tfile, createtfile, mvar)
+	tvar <- liftIO $ newTVarIO Nothing
+	loggedtvar <- liftIO $ newTVarIO 0
+	return (liftIO . updater (fromRawFilePath tfile) tvar loggedtvar, tfile, createtfile, tvar)
   where
-	updater tfile mvar b = modifyMVar_ mvar $ \oldbytes -> do
-		let newbytes = fromBytesProcessed b
-		if newbytes - oldbytes >= mindelta
-			then do
-				let info' = info { bytesComplete = Just newbytes }
-				_ <- tryIO $ updateTransferInfoFile info' tfile
-				return newbytes
-			else return oldbytes
+	updater tfile tvar loggedtvar new = do
+		old <- atomically $ swapTVar tvar (Just new)
+		let oldbytes = maybe 0 fromBytesProcessed old
+		let newbytes = fromBytesProcessed new
+		when (newbytes - oldbytes >= mindelta) $ do
+			let info' = info { bytesComplete = Just newbytes }
+			_ <- tryIO $ updateTransferInfoFile info' tfile
+			atomically $ writeTVar loggedtvar newbytes
+
 	{- The minimum change in bytesComplete that is worth
 	 - updating a transfer info file for is 1% of the total
 	 - keySize, rounded down. -}