automatic stall detection

annex.stalldetection can now be set to "true" to make git-annex do
automatic stall detection when it detects a remote is updating its transfer
progress consistently enough.

This commit was sponsored by Luke Shumaker on Patreon.

Types/StallDetection.hs

@@ -1,6 +1,6 @@
 {- types for stall detection
  -
- - Copyright 2020 Joey Hess <id@joeyh.name>
+ - Copyright 2020-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -10,20 +10,32 @@ module Types.StallDetection where
 import Utility.DataUnits
 import Utility.HumanTime
 import Utility.Misc
+import Git.Config
 
--- Unless the given number of bytes have been sent over the given
--- amount of time, there's a stall.
-data StallDetection = StallDetection ByteSize Duration
+data StallDetection
+	= StallDetection ByteSize Duration
+	-- ^ Unless the given number of bytes have been sent over the given
+	-- amount of time, there's a stall.
+	| ProbeStallDetection
+	-- ^ Used when unsure how frequently transfer progress is updated,
+	-- or how fast data can be sent.
 	deriving (Show)
 
 -- Parse eg, "0KiB/60s"
-parseStallDetection :: String -> Either String StallDetection
-parseStallDetection s = 
-	let (bs, ds) = separate (== '/') s
-	in do
+--
+-- Also, it can be set to "true" (or other git config equivilants)
+-- to enable ProbeStallDetection. 
+-- And "false" (and other git config equivilants) explicitly
+-- disable stall detection.
+parseStallDetection :: String -> Either String (Maybe StallDetection)
+parseStallDetection s = case isTrueFalse s of
+	Nothing -> do
+		let (bs, ds) = separate (== '/') s
 		b <- maybe 
 			(Left $ "Unable to parse stall detection amount " ++ bs)
 			Right
 			(readSize dataUnits bs)
 		d <- parseDuration ds
-		return (StallDetection b d)
+		return (Just (StallDetection b d))
+	Just True -> Right (Just ProbeStallDetection)
+	Just False -> Right Nothing