gitignore support for the assistant and watcher

Requires git 1.8.4 or newer. When it's installed, a background
git check-ignore process is run, and used to efficiently check ignores
whenever a new file is added.

Thanks to Adam Spiers, for getting the necessary support into git for this.

A complication is what to do about files that are gitignored but have
been checked into git anyway. git commands assume the ignore has been
overridden in this case, and not need any more overriding to commit a
changed version.

However, for the assistant to do the same, it would have to run git ls-files
to check if the ignored file is in git. This is somewhat expensive. Or it
could use the running git-cat-file process to query the file that way,
but that requires transferring the whole file content over a pipe, so it
can be quite expensive too, for files that are not git-annex
symlinks.

Now imagine if the user knows that a file or directory tree will be getting
frequent changes, and doesn't want the assistant to sync it, so gitignores
it. The assistant could overload the system with repeated ls-files checks!

So, I've decided that the assistant will not automatically commit changes
to files that are gitignored. This is a tradeoff. Hopefully it won't be a
problem to adjust .gitignore settings to not ignore files you want the
assistant to autocommit, or to manually git annex add files that are listed
in .gitignore.

(This could be revisited if git-annex gets access to an interface to check
the content of the index w/o forking a git command. This could be libgit2,
or perhaps a separate git cat-file --batch-check process, so it wouldn't
need to ship over the whole file content.)

This commit was sponsored by Francois Marier. Thanks!

1 files changed, 71 insertions, 0 deletions

diff --git a/Git/CheckIgnore.hs b/Git/CheckIgnore.hs
new file mode 100644
index 000000000..2ab7cb3dc
--- /dev/null
+++ b/Git/CheckIgnore.hs
@@ -0,0 +1,71 @@
+{- git check-ignore interface
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Git.CheckIgnore (
+	CheckIgnoreHandle,
+	checkIgnoreStart,
+	checkIgnoreStop,
+	checkIgnored
+) where
+
+import Common
+import Git
+import Git.Command
+import qualified Git.Version
+import qualified Utility.CoProcess as CoProcess
+
+import System.IO.Error
+
+type CheckIgnoreHandle = CoProcess.CoProcessHandle
+
+{- Starts git check-ignore running, and returns a handle.
+ -
+ - This relies on git check-ignore --non-matching -v outputting
+ - lines for both matching an non-matching files. Also relies on
+ - GIT_FLUSH behavior flushing the output buffer when git check-ignore
+ - is piping to us.
+ -
+ - The first version of git to support what we need is 1.8.4.
+ - Nothing is returned if an older git is installed.
+ -}
+checkIgnoreStart :: Repo -> IO (Maybe CheckIgnoreHandle)
+checkIgnoreStart repo = ifM supportedGitVersion
+	( Just <$> (CoProcess.rawMode =<< gitCoProcessStart True params repo)
+	, return Nothing
+	)
+  where
+	params =
+		[ Param "check-ignore" 
+		, Params "-z --stdin --verbose --non-matching"
+		]
+
+supportedGitVersion :: IO Bool
+supportedGitVersion = do
+	v <- Git.Version.installed
+	return $ v >= Git.Version.normalize "1.8.4"
+
+checkIgnoreStop :: CheckIgnoreHandle -> IO ()
+checkIgnoreStop = CoProcess.stop
+
+{- Returns True if a file is ignored. -}
+checkIgnored :: CheckIgnoreHandle -> FilePath -> IO Bool
+checkIgnored h file = CoProcess.query h send (receive "")
+  where
+	send to = do
+		hPutStr to $ file ++ "\0"
+		hFlush to
+	receive c from = do
+		s <- hGetSomeString from 1024
+		if null s
+			then eofError
+			else do
+				let v = c ++ s
+				maybe (receive v from) return (parse v)
+	parse s = case segment (== '\0') s of
+		(_source:_line:pattern:_pathname:_eol:[]) -> Just $ not $ null pattern
+		_ -> Nothing
+	eofError = ioError $ mkIOError userErrorType "git cat-file EOF" Nothing Nothing