Improve pid locking code to work on filesystems that don't support hard links.

Probing for hard link support in the pid locking code is redundant since
git-annex init already probes that. But, it didn't seem worth threading
that data through; the pid locking code runs at most once per git-annex
process, and only on unusual filesystems. Optimising a single hard link
and unlink isn't worth it.

This commit was sponsored by Francois Marier on Patreon.

CHANGELOG

@@ -49,6 +49,8 @@ git-annex (6.20170102) UNRELEASED; urgency=medium
   * import: Added --reinject-duplicates.
   * Added git template directory to Linux standalone tarball and OSX
     app bundle.
+  * Improve pid locking code to work on filesystems that don't support hard
+    links.
 
  -- Joey Hess <id@joeyh.name>  Fri, 06 Jan 2017 15:22:06 -0400
 

Utility/LockFile/PidLock.hs

@@ -150,14 +150,30 @@ tryLock lockfile = trySideLock lockfile $ \sidelock -> do
 -- open(2) suggests that link can sometimes appear to fail
 -- on NFS but have actually succeeded, and the way to find out is to stat
 -- the file and check its link count etc.
+--
+-- However, not all filesystems support hard links. So, first probe
+-- to see if they are supported. If not, use open with O_EXCL.
 linkToLock :: SideLockHandle -> FilePath -> FilePath -> IO Bool
 linkToLock Nothing _ _ = return False
 linkToLock (Just _) src dest = do
-	_ <- tryIO $ createLink src dest
-	ifM (catchBoolIO checklinked)
-		( catchBoolIO $ not <$> checkInsaneLustre dest
-		, return False
-		)
+	let probe = src ++ ".lnk"
+	v <- tryIO $ createLink src probe
+	nukeFile probe
+	case v of
+		Right _ -> do
+			_ <- tryIO $ createLink src dest
+			ifM (catchBoolIO checklinked)
+				( catchBoolIO $ not <$> checkInsaneLustre dest
+				, return False
+				)
+		Left _ -> catchBoolIO $ do
+			fd <- openFd dest WriteOnly
+				(Just $ combineModes readModes)
+				(defaultFileFlags {exclusive = True})
+			h <- fdToHandle fd
+			readFile src >>= hPutStr h
+			hClose h
+			return True
   where
 	checklinked = do
 		x <- getSymbolicLinkStatus src

doc/forum/git_annex_init_timeout/comment_2_bf15b686b07c5e6e95796affa497a541._comment

@@ -0,0 +1,13 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 2"""
+ date="2017-02-10T18:52:59Z"
+ content="""
+I was able to reproduce the problem with a FAT filesystem
+mounted on Linux, if I manually enabled annex.pidlock
+before git-annex init. Hard linking does not work on FAT, so that
+matches my earlier guess.
+
+I've committed a fix for this, so try again with a recent autobuild of
+git-annex and it will probably fix your problem.
+"""]]