CoW probing

Improved probing when CoW copies can be made between files on the same
drive. Now supports CoW between BTRFS subvolumes. And, falls back to rsync
instead of using cp when CoW won't work, eg copies between repos on the
same EXT4 filesystem.

Rather than trying cp --reflink=always for each file copied to a remote,
it's tried once and if it fails it falls back to using rsync thereafter
for the lifetime of the Remote object. That avoids overhead of calling cp
which while small, will add up over a large number of files.

This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.

Utility/CopyFile.hs

@@ -1,12 +1,13 @@
 {- file copying
  -
- - Copyright 2010-2014 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2019 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
 
 module Utility.CopyFile (
 	copyFileExternal,
+	copyCoW,
 	createLinkOrCopy,
 	CopyMetaData(..)
 ) where
@@ -22,6 +23,17 @@ data CopyMetaData
 	| CopyAllMetaData
 	deriving (Eq)
 
+copyMetaDataParams :: CopyMetaData -> [CommandParam]
+copyMetaDataParams meta = map snd $ filter fst
+	[ (allmeta && BuildInfo.cp_a, Param "-a")
+	, (allmeta && BuildInfo.cp_p && not BuildInfo.cp_a
+		, Param "-p")
+	, (not allmeta && BuildInfo.cp_preserve_timestamps
+		, Param "--preserve=timestamps")
+	]
+  where
+	allmeta = meta == CopyAllMetaData
+
 {- The cp command is used, because I hate reinventing the wheel,
  - and because this allows easy access to features like cp --reflink. -}
 copyFileExternal :: CopyMetaData -> FilePath -> FilePath -> IO Bool
@@ -30,15 +42,33 @@ copyFileExternal meta src dest = do
 		removeFile dest
 	boolSystem "cp" $ params ++ [File src, File dest]
   where
-	params = map snd $ filter fst
-		[ (BuildInfo.cp_reflink_auto, Param "--reflink=auto")
-		, (allmeta && BuildInfo.cp_a, Param "-a")
-		, (allmeta && BuildInfo.cp_p && not BuildInfo.cp_a
-			, Param "-p")
-		, (not allmeta && BuildInfo.cp_preserve_timestamps
-			, Param "--preserve=timestamps")
-		]
-	allmeta = meta == CopyAllMetaData
+	params
+		| BuildInfo.cp_reflink_supported =
+			Param "--reflink=auto" : copyMetaDataParams meta
+		| otherwise = copyMetaDataParams meta
+
+{- When a filesystem supports CoW (and cp does), uses it to make
+ - an efficient copy of a file. Otherwise, returns False. -}
+copyCoW :: CopyMetaData -> FilePath -> FilePath -> IO Bool
+copyCoW meta src dest
+	| BuildInfo.cp_reflink_supported = do
+		whenM (doesFileExist dest) $
+			removeFile dest
+		-- When CoW is not supported, cp will complain to stderr,
+		-- so have to discard its stderr.
+		ok <- catchBoolIO $ do
+			withQuietOutput createProcessSuccess $
+				proc "cp" $ toCommand $
+					params ++ [File src, File dest]
+			return True
+		-- When CoW is not supported, cp creates the destination
+		-- file but leaves it empty.
+		unless ok $
+			void $ tryIO $ removeFile dest
+		return ok
+	| otherwise = return False
+  where
+	params = Param "--reflink=always" : copyMetaDataParams meta
 
 {- Create a hard link if the filesystem allows it, and fall back to copying
  - the file. -}