reorganize and refactor lock code

Added a convenience Utility.LockFile that is not a windows/posix
portability shim, but still manages to cut down on the boilerplate around
locking.

This commit was sponsored by Johan Herland.

1 files changed, 75 insertions, 0 deletions

diff --git a/Utility/LockFile/Windows.hs b/Utility/LockFile/Windows.hs
new file mode 100644
index 000000000..73c248b03
--- /dev/null
+++ b/Utility/LockFile/Windows.hs
@@ -0,0 +1,75 @@
+{- Windows lock files
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - License: BSD-2-clause
+ -}
+
+module Utility.LockFile.Windows (
+	lockShared,
+	lockExclusive,
+	dropLock,
+	waitToLock,
+	LockHandle
+) where
+
+import System.Win32.Types
+import System.Win32.File
+import Control.Concurrent
+
+type LockFile = FilePath
+
+type LockHandle = HANDLE
+
+{- Tries to lock a file with a shared lock, which allows other processes to
+ - also lock it shared. Fails is the file is exclusively locked. -}
+lockShared :: LockFile -> IO (Maybe LockHandle)
+lockShared = openLock fILE_SHARE_READ
+
+{- Tries to take an exclusive lock on a file. Fails if another process has
+ - a shared or exclusive lock.
+ -
+ - Note that exclusive locking also prevents the file from being opened for
+ - read or write by any other progess. So for advisory locking of a file's
+ - content, a different LockFile should be used. -}
+lockExclusive :: LockFile -> IO (Maybe LockHandle)
+lockExclusive = openLock fILE_SHARE_NONE
+
+{- Windows considers just opening a file enough to lock it. This will
+ - create the LockFile if it does not already exist.
+ -
+ - Will fail if the file is already open with an incompatable ShareMode.
+ - Note that this may happen if an unrelated process, such as a virus
+ - scanner, even looks at the file. See http://support.microsoft.com/kb/316609
+ -
+ - Note that createFile busy-waits to try to avoid failing when some other
+ - process briefly has a file open. But that would make checking locks
+ - much more expensive, so is not done here. Thus, the use of c_CreateFile.
+ -
+ - Also, passing Nothing for SECURITY_ATTRIBUTES ensures that the lock file
+ - is not inheerited by any child process.
+ -}
+openLock :: ShareMode -> LockFile -> IO (Maybe LockHandle)
+openLock sharemode f = do
+	h <- withTString f $ \c_f ->
+		c_CreateFile c_f gENERIC_READ sharemode security_attributes
+			oPEN_ALWAYS fILE_ATTRIBUTE_NORMAL (maybePtr Nothing)
+	return $ if h == iNVALID_HANDLE_VALUE
+		then Nothing
+		else Just h
+  where
+	security_attributes = maybePtr Nothing
+
+dropLock :: LockHandle -> IO ()
+dropLock = closeHandle
+
+{- If the initial lock fails, this is a BUSY wait, and does not
+ - guarentee FIFO order of waiters. In other news, Windows is a POS. -}
+waitToLock :: IO (Maybe LockHandle) -> IO LockHandle
+waitToLock locker = takelock
+  where
+	takelock = go =<< locker
+	go (Just lck) = return lck
+	go Nothing = do
+		threadDelay (500000) -- half a second
+		takelock