summaryrefslogtreecommitdiff
path: root/Git.hs
blob: b226bce0f28220e14410fd26afda994c448bd678 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
{- git repository handling 
 -
 - This is written to be completely independant of git-annex and should be
 - suitable for other uses.
 -
 - Copyright 2010,2011 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

module Git (
	Repo,
	repoFromCwd,
	repoFromAbsPath,
	repoFromUnknown,
	repoFromUrl,
	localToUrl,
	repoIsUrl,
	repoIsSsh,
	repoIsHttp,
	repoIsLocalBare,
	repoDescribe,
	repoLocation,
	workTree,
	workTreeFile,
	gitDir,
	urlPath,
	urlHost,
	urlPort,
	urlHostUser,
	urlAuthority,
	urlScheme,
	configGet,
	configMap,
	configRead,
	hConfigRead,
	configStore,
	configTrue,
	gitCommandLine,
	run,
	runBool,
	pipeRead,
	pipeWrite,
	pipeWriteRead,
	pipeNullSplit,
	attributes,
	remotes,
	remotesAdd,
	repoRemoteName,
	repoRemoteNameSet,
	checkAttr,
	decodeGitFile,
	encodeGitFile,
	repoAbsPath,
	reap,
	useIndex,
	hashObject,
	getSha,
	shaSize,
	commit,

	prop_idempotent_deencode
) where

import Control.Monad (unless, when)
import System.Directory
import System.FilePath
import System.Posix.Directory
import System.Posix.User
import System.Posix.Process
import System.Path
import System.Cmd.Utils
import IO (bracket_, try)
import Data.String.Utils
import System.IO
import qualified Data.Map as M hiding (map, split)
import Network.URI
import Data.Maybe
import Data.Char
import Data.Word (Word8)
import Codec.Binary.UTF8.String (encode)
import Text.Printf
import Data.List (isInfixOf, isPrefixOf, isSuffixOf)
import System.Exit
import System.Posix.Env (setEnv, unsetEnv, getEnv)

import Utility

{- There are two types of repositories; those on local disk and those
 - accessed via an URL. -}
data RepoLocation = Dir FilePath | Url URI | Unknown
	deriving (Show, Eq)

data Repo = Repo {
	location :: RepoLocation,
	config :: M.Map String String,
	remotes :: [Repo],
	-- remoteName holds the name used for this repo in remotes
	remoteName :: Maybe String 
} deriving (Show, Eq)

newFrom :: RepoLocation -> Repo
newFrom l = 
	Repo {
		location = l,
		config = M.empty,
		remotes = [],
		remoteName = Nothing
	}

{- Local Repo constructor, requires an absolute path to the repo be
 - specified. -}
repoFromAbsPath :: FilePath -> IO Repo
repoFromAbsPath dir
	| "/" `isPrefixOf` dir = do
 		-- Git always looks for "dir.git" in preference to
		-- to "dir", even if dir ends in a "/".
		let canondir = dropTrailingPathSeparator dir
		let dir' = canondir ++ ".git"
		e <- doesDirectoryExist dir'
		if e
			then ret dir'
			else if "/.git" `isSuffixOf` canondir
				then do
					-- When dir == "foo/.git", git looks
					-- for "foo/.git/.git", and failing
					-- that, uses "foo" as the repository.
					e' <- doesDirectoryExist $ dir </> ".git"
					if e'
						then ret dir
						else ret $ takeDirectory canondir
				else ret dir
	| otherwise = error $ "internal error, " ++ dir ++ " is not absolute"
	where
		ret = return . newFrom . Dir

{- Remote Repo constructor. Throws exception on invalid url. -}
repoFromUrl :: String -> IO Repo
repoFromUrl url
	| startswith "file://" url = repoFromAbsPath $ uriPath u
	| otherwise = return $ newFrom $ Url u
		where
			u = fromMaybe bad $ parseURI url
			bad = error $ "bad url " ++ url

{- Creates a repo that has an unknown location. -}
repoFromUnknown :: Repo
repoFromUnknown = newFrom Unknown

{- Converts a Local Repo into a remote repo, using the reference repo
 - which is assumed to be on the same host. -}
localToUrl :: Repo -> Repo -> Repo
localToUrl reference r
	| not $ repoIsUrl reference = error "internal error; reference repo not url"
	| repoIsUrl r = r
	| otherwise = r { location = Url $ fromJust $ parseURI absurl }
	where
		absurl =
			urlScheme reference ++ "//" ++
			urlAuthority reference ++
			workTree r

{- User-visible description of a git repo. -}
repoDescribe :: Repo -> String
repoDescribe Repo { remoteName = Just name } = name
repoDescribe Repo { location = Url url } = show url
repoDescribe Repo { location = Dir dir } = dir
repoDescribe Repo { location = Unknown } = "UNKNOWN"

{- Location of the repo, either as a path or url. -}
repoLocation :: Repo -> String
repoLocation Repo { location = Url url } = show url
repoLocation Repo { location = Dir dir } = dir
repoLocation Repo { location = Unknown } = undefined

{- Constructs and returns an updated version of a repo with
 - different remotes list. -}
remotesAdd :: Repo -> [Repo] -> Repo
remotesAdd repo rs = repo { remotes = rs }

{- Returns the name of the remote that corresponds to the repo, if
 - it is a remote. -}
repoRemoteName :: Repo -> Maybe String
repoRemoteName Repo { remoteName = Just name } = Just name
repoRemoteName _ = Nothing

{- Sets the name of a remote based on the git config key, such as
   "remote.foo.url". -}
repoRemoteNameSet :: Repo -> String -> Repo
repoRemoteNameSet r k = r { remoteName = Just basename }
	where
		basename = join "." $ reverse $ drop 1 $
				reverse $ drop 1 $ split "." k

{- Some code needs to vary between URL and normal repos,
 - or bare and non-bare, these functions help with that. -}
repoIsUrl :: Repo -> Bool
repoIsUrl Repo { location = Url _ } = True
repoIsUrl _ = False

repoIsSsh :: Repo -> Bool
repoIsSsh Repo { location = Url url } 
	| uriScheme url == "ssh:" = True
	-- git treats these the same as ssh
	| uriScheme url == "git+ssh:" = True
	| uriScheme url == "ssh+git:" = True
	| otherwise = False
repoIsSsh _ = False

repoIsHttp :: Repo -> Bool
repoIsHttp Repo { location = Url url } 
	| uriScheme url == "http:" = True
	| uriScheme url == "https:" = True
	| otherwise = False
repoIsHttp _ = False

configAvail ::Repo -> Bool
configAvail Repo { config = c } = c /= M.empty

repoIsLocalBare :: Repo -> Bool
repoIsLocalBare r@(Repo { location = Dir _ }) = configAvail r && configBare r
repoIsLocalBare _ = False

assertLocal :: Repo -> a -> a
assertLocal repo action = 
	if not $ repoIsUrl repo
		then action
		else error $ "acting on URL git repo " ++  repoDescribe repo ++ 
				" not supported"
assertUrl :: Repo -> a -> a
assertUrl repo action = 
	if repoIsUrl repo
		then action
		else error $ "acting on local git repo " ++  repoDescribe repo ++ 
				" not supported"

configBare :: Repo -> Bool
configBare repo = maybe unknown configTrue $ M.lookup "core.bare" $ config repo
	where
		unknown = error $ "it is not known if git repo " ++
			repoDescribe repo ++
			" is a bare repository; config not read"

{- Path to a repository's gitattributes file. -}
attributes :: Repo -> String
attributes repo
	| configBare repo = workTree repo ++ "/info/.gitattributes"
	| otherwise = workTree repo ++ "/.gitattributes"

{- Path to a repository's .git directory, relative to its workTree. -}
gitDir :: Repo -> String
gitDir repo
	| configBare repo = ""
	| otherwise = ".git"

{- Path to a repository's --work-tree, that is, its top.
 -
 - Note that for URL repositories, this is the path on the remote host. -}
workTree :: Repo -> FilePath
workTree r@(Repo { location = Url _ }) = urlPath r
workTree (Repo { location = Dir d }) = d
workTree Repo { location = Unknown } = undefined

{- Given a relative or absolute filename inside a git repository's
 - workTree, calculates the name to use to refer to that file to git.
 -
 - This is complicated because the best choice can vary depending on
 - whether the cwd is in a subdirectory of the git repository, or not.
 -
 - For example, when adding a file "/tmp/repo/foo", it's best to refer
 - to it as "foo" if the cwd is outside the repository entirely
 - (this avoids a gotcha with using the full path name when /tmp/repo
 - is itself a symlink). But, if the cwd is "/tmp/repo/subdir",
 - it's best to refer to "../foo".
 -}
workTreeFile :: Repo -> FilePath -> IO FilePath
workTreeFile repo@(Repo { location = Dir d }) file = do
	cwd <- getCurrentDirectory
	let file' = absfile cwd
	unless (inrepo file') $
		error $ file ++ " is not located inside git repository " ++ absrepo
	if inrepo $ addTrailingPathSeparator cwd
		then return $ relPathDirToFile cwd file'
		else return $ drop (length absrepo) file'
	where
		-- normalize both repo and file, so that repo
		-- will be substring of file
		absrepo = maybe bad addTrailingPathSeparator $ absNormPath "/" d
		absfile c = fromMaybe file $ secureAbsNormPath c file
		inrepo f = absrepo `isPrefixOf` f
		bad = error $ "bad repo" ++ repoDescribe repo
workTreeFile repo _ = assertLocal repo $ error "internal"

{- Path of an URL repo. -}
urlPath :: Repo -> String
urlPath Repo { location = Url u } = uriPath u
urlPath repo = assertUrl repo $ error "internal"

{- Scheme of an URL repo. -}
urlScheme :: Repo -> String
urlScheme Repo { location = Url u } = uriScheme u
urlScheme repo = assertUrl repo $ error "internal"

{- Work around a bug in the real uriRegName
 - <http://trac.haskell.org/network/ticket/40> -}
uriRegName' :: URIAuth -> String
uriRegName' a = fixup $ uriRegName a
	where
		fixup x@('[':rest)
			| rest !! len == ']' = take len rest
			| otherwise = x
			where
				len  = length rest - 1
		fixup x = x

{- Hostname of an URL repo. -}
urlHost :: Repo -> String
urlHost = urlAuthPart uriRegName'

{- Port of an URL repo, if it has a nonstandard one. -}
urlPort :: Repo -> Maybe Integer
urlPort r = 
	case urlAuthPart uriPort r of
		":" -> Nothing
		(':':p) -> Just (read p)
		_ -> Nothing

{- Hostname of an URL repo, including any username (ie, "user@host") -}
urlHostUser :: Repo -> String
urlHostUser r = urlAuthPart uriUserInfo r ++ urlAuthPart uriRegName' r

{- The full authority portion an URL repo. (ie, "user@host:port") -}
urlAuthority :: Repo -> String
urlAuthority Repo { location = Url u } = uriUserInfo a ++ uriRegName' a ++ uriPort a
	where
		a = fromMaybe (error $ "bad url " ++ show u) (uriAuthority u)
urlAuthority repo = assertUrl repo $ error "internal"

{- Applies a function to extract part of the uriAuthority of an URL repo. -}
urlAuthPart :: (URIAuth -> a) -> Repo -> a
urlAuthPart a Repo { location = Url u } = a auth
	where
		auth = fromMaybe (error $ "bad url " ++ show u) (uriAuthority u)
urlAuthPart _ repo = assertUrl repo $ error "internal"

{- Constructs a git command line operating on the specified repo. -}
gitCommandLine :: Repo -> [CommandParam] -> [CommandParam]
gitCommandLine repo@(Repo { location = Dir d} ) params =
	-- force use of specified repo via --git-dir and --work-tree
	[ Param ("--git-dir=" ++ d ++ "/" ++ gitDir repo)
	, Param ("--work-tree=" ++ d)
	] ++ params
gitCommandLine repo _ = assertLocal repo $ error "internal"

{- Runs git in the specified repo. -}
runBool :: Repo -> String -> [CommandParam] -> IO Bool
runBool repo subcommand params = assertLocal repo $
	boolSystem "git" $ gitCommandLine repo $ Param subcommand : params

{- Runs git in the specified repo, throwing an error if it fails. -}
run :: Repo -> String -> [CommandParam] -> IO ()
run repo subcommand params = assertLocal repo $
	runBool repo subcommand params
		>>! error $ "git " ++ show params ++ " failed"

{- Runs a git subcommand and returns its output, lazily. 
 -
 - Note that this leaves the git process running, and so zombies will
 - result unless reap is called.
 -}
pipeRead :: Repo -> [CommandParam] -> IO String
pipeRead repo params = assertLocal repo $ do
	(_, s) <- pipeFrom "git" $ toCommand $ gitCommandLine repo params
	return s

{- Runs a git subcommand, feeding it input.
 - You should call either getProcessStatus or forceSuccess on the PipeHandle. -}
pipeWrite :: Repo -> [CommandParam] -> String -> IO PipeHandle
pipeWrite repo params s = assertLocal repo $
	pipeTo "git" (toCommand $ gitCommandLine repo params) s

{- Runs a git subcommand, feeding it input, and returning its output.
 - You should call either getProcessStatus or forceSuccess on the PipeHandle. -}
pipeWriteRead :: Repo -> [CommandParam] -> String -> IO (PipeHandle, String)
pipeWriteRead repo params s = assertLocal repo $
	pipeBoth "git" (toCommand $ gitCommandLine repo params) s

{- Reaps any zombie git processes. -}
reap :: IO ()
reap = do
	-- throws an exception when there are no child processes
	r <- catch (getAnyProcessStatus False True) (\_ -> return Nothing)
	maybe (return ()) (const reap) r

{- Forces git to use the specified index file.
 - Returns an action that will reset back to the default
 - index file. -}
useIndex :: FilePath -> IO (IO ())
useIndex index = do
	res <- try $ getEnv var
	setEnv var index True
	return $ reset res
	where
		var = "GIT_INDEX_FILE"
		reset (Right (Just v)) = setEnv var v True
		reset _ = unsetEnv var

{- Injects some content into git, returning its hash. -}
hashObject :: Repo -> String -> IO String
hashObject repo content = getSha subcmd $ do
	(h, s) <- pipeWriteRead repo (map Param params) content
	length s `seq` do
		forceSuccess h
		reap -- XXX unsure why this is needed
		return s
	where
		subcmd = "hash-object"
		params = [subcmd, "-w", "--stdin"]

{- Runs an action that causes a git subcommand to emit a sha, and strips
   any trailing newline, returning the sha. -}
getSha :: String -> IO String -> IO String
getSha subcommand a = do
	t <- a
	let t' = if last t == '\n'
		then take (length t - 1) t
		else t
	when (length t' /= shaSize) $
		error $ "failed to read sha from git " ++ subcommand ++ " (" ++ t' ++ ")"
	return t'

{- Size of a git sha. -}
shaSize :: Int
shaSize = 40

{- Commits the index into the specified branch, 
 - with the specified parent refs. -}
commit :: Repo -> String -> String -> [String] -> IO ()
commit g message newref parentrefs = do
	tree <- getSha "write-tree" $
		pipeRead g [Param "write-tree"]
	sha <- getSha "commit-tree" $ ignorehandle $
		pipeWriteRead g (map Param $ ["commit-tree", tree] ++ ps) message
	run g "update-ref" [Param newref, Param sha]
	where
		ignorehandle a = return . snd =<< a
		ps = concatMap (\r -> ["-p", r]) parentrefs

{- Reads null terminated output of a git command (as enabled by the -z 
 - parameter), and splits it into a list of files/lines/whatever. -}
pipeNullSplit :: Repo -> [CommandParam] -> IO [FilePath]
pipeNullSplit repo params = do
	fs0 <- pipeRead repo params
	return $ split0 fs0
	where
		split0 s = filter (not . null) $ split "\0" s

{- Runs git config and populates a repo with its config. -}
configRead :: Repo -> IO Repo
configRead repo@(Repo { location = Dir d }) = do
	{- Cannot use pipeRead because it relies on the config having
	   been already read. Instead, chdir to the repo. -}
	cwd <- getCurrentDirectory
	bracket_ (changeWorkingDirectory d)
		(\_ -> changeWorkingDirectory cwd) $
			pOpen ReadFromPipe "git" ["config", "--list"] $
				hConfigRead repo
configRead r = assertLocal r $ error "internal"

{- Reads git config from a handle and populates a repo with it. -}
hConfigRead :: Repo -> Handle -> IO Repo
hConfigRead repo h = do
	val <- hGetContentsStrict h
	configStore repo val

{- Stores a git config into a repo, returning the new version of the repo.
 - The git config may be multiple lines, or a single line. Config settings
 - can be updated inrementally. -}
configStore :: Repo -> String -> IO Repo
configStore repo s = do
	let repo' = repo { config = configParse s `M.union` config repo }
	rs <- configRemotes repo'
	return $ repo' { remotes = rs }

{- Parses git config --list output into a config map. -}
configParse :: String -> M.Map String String
configParse s = M.fromList $ map pair $ lines s
	where
		pair l = (key l, val l)
		key l = head $ keyval l
		val l = join sep $ drop 1 $ keyval l
		keyval l = split sep l :: [String]
		sep = "="

{- Calculates a list of a repo's configured remotes, by parsing its config. -}
configRemotes :: Repo -> IO [Repo]
configRemotes repo = mapM construct remotepairs
	where
		remotepairs = M.toList $ filterremotes $ config repo
		filterremotes = M.filterWithKey (\k _ -> isremote k)
		isremote k = startswith "remote." k && endswith ".url" k
		construct (k,v) = do
			r <- gen v
			return $ repoRemoteNameSet r k
		gen v	| scpstyle v = repoFromUrl $ scptourl v
			| isURI v = repoFromUrl v
			| otherwise = repoFromRemotePath v repo
		-- git remotes can be written scp style -- [user@]host:dir
		scpstyle v = ":" `isInfixOf` v && not ("//" `isInfixOf` v)
		scptourl v = "ssh://" ++ host ++ slash dir
			where
				bits = split ":" v
				host = head bits
				dir = join ":" $ drop 1 bits
				slash d	| d == "" = "/~/" ++ dir
					| head d == '/' = dir
					| head d == '~' = '/':dir
					| otherwise = "/~/" ++ dir

{- Checks if a string from git config is a true value. -}
configTrue :: String -> Bool
configTrue s = map toLower s == "true"

{- Returns a single git config setting, or a default value if not set. -}
configGet :: Repo -> String -> String -> String
configGet repo key defaultValue = 
	M.findWithDefault defaultValue key (config repo)

{- Access to raw config Map -}
configMap :: Repo -> M.Map String String
configMap = config

{- Efficiently looks up a gitattributes value for each file in a list. -}
checkAttr :: Repo -> String -> [FilePath] -> IO [(FilePath, String)]
checkAttr repo attr files = do
	-- git check-attr wants files that are absolute (or relative to the
	-- top of the repo). But we're passed files relative to the current
	-- directory. Convert to absolute, and then convert the filenames
	-- in its output back to relative.
	cwd <- getCurrentDirectory
	let top = workTree repo
	let absfiles = map (absPathFrom cwd) files
	(_, fromh, toh) <- hPipeBoth "git" (toCommand params)
        _ <- forkProcess $ do
		hClose fromh
                hPutStr toh $ join "\0" absfiles
                hClose toh
                exitSuccess
        hClose toh
	s <- hGetContents fromh
	return $ map (topair cwd top) $ lines s
	where
		params = gitCommandLine repo [Param "check-attr", Param attr, Params "-z --stdin"]
		topair cwd top l = (relfile, value)
			where 
				relfile
					| startswith cwd' file = drop (length cwd') file
					| otherwise = relPathDirToFile top' file
				file = decodeGitFile $ join sep $ take end bits
				value = bits !! end
				end = length bits - 1
				bits = split sep l
				sep = ": " ++ attr ++ ": "
				cwd' = cwd ++ "/"
				top' = top ++ "/"

{- Some git commands output encoded filenames. Decode that (annoyingly
 - complex) encoding. -}
decodeGitFile :: String -> FilePath
decodeGitFile [] = []
decodeGitFile f@(c:s)
	-- encoded strings will be inside double quotes
	| c == '"' = unescape ("", middle)
	| otherwise = f
	where
		e = '\\'
		middle = take (length s - 1) s
		unescape (b, []) = b
		-- look for escapes starting with '\'
		unescape (b, v) = b ++ beginning ++ unescape (decode rest)
			where
				pair = span (/= e) v
				beginning = fst pair
				rest = snd pair
		isescape x = x == e
		-- \NNN is an octal encoded character
		decode (x:n1:n2:n3:rest)
			| isescape x && alloctal = (fromoctal, rest)
				where
					alloctal = isOctDigit n1 &&
						isOctDigit n2 &&
						isOctDigit n3
					fromoctal = [chr $ readoctal [n1, n2, n3]]
					readoctal o = read $ "0o" ++ o :: Int
		-- \C is used for a few special characters
		decode (x:nc:rest)
			| isescape x = ([echar nc], rest)
			where
				echar 'a' = '\a'
				echar 'b' = '\b'
				echar 'f' = '\f'
				echar 'n' = '\n'
				echar 'r' = '\r'
				echar 't' = '\t'
				echar 'v' = '\v'
				echar a = a
		decode n = ("", n)

{- Should not need to use this, except for testing decodeGitFile. -}
encodeGitFile :: FilePath -> String
encodeGitFile s = foldl (++) "\"" (map echar s) ++ "\""
	where
		e c = '\\' : [c]
		echar '\a' = e 'a'
		echar '\b' = e 'b'
		echar '\f' = e 'f'
		echar '\n' = e 'n'
		echar '\r' = e 'r'
		echar '\t' = e 't'
		echar '\v' = e 'v'
		echar '\\' = e '\\'
		echar '"'  = e '"'
		echar x
			| ord x < 0x20 = e_num x -- low ascii
			| ord x >= 256 = e_utf x
			| ord x > 0x7E = e_num x -- high ascii
			| otherwise = [x]        -- printable ascii
			where 
				showoctal i = '\\' : printf "%03o" i
				e_num c = showoctal $ ord c
				-- unicode character is decomposed to
				-- Word8s and each is shown in octal
				e_utf c = showoctal =<< (encode [c] :: [Word8])

{- for quickcheck -}
prop_idempotent_deencode :: String -> Bool
prop_idempotent_deencode s = s == decodeGitFile (encodeGitFile s)

{- Constructs a Repo from the path specified in the git remotes of
 - another Repo. -}
repoFromRemotePath :: FilePath -> Repo -> IO Repo
repoFromRemotePath dir repo = do
	dir' <- expandTilde dir
	repoFromAbsPath $ workTree repo </> dir'

{- Git remotes can have a directory that is specified relative
 - to the user's home directory, or that contains tilde expansions.
 - This converts such a directory to an absolute path.
 - Note that it has to run on the system where the remote is.
 -}
repoAbsPath :: FilePath -> IO FilePath
repoAbsPath d = do
	d' <- expandTilde d
	h <- myHomeDir
	return $ h </> d'

expandTilde :: FilePath -> IO FilePath
expandTilde = expandt True
	where
		expandt _ [] = return ""
		expandt _ ('/':cs) = do
			v <- expandt True cs
			return ('/':v)
		expandt True ('~':'/':cs) = do
			h <- myHomeDir
			return $ h </> cs
		expandt True ('~':cs) = do
			let (name, rest) = findname "" cs
			u <- getUserEntryForName name
			return $ homeDirectory u </> rest
		expandt _ (c:cs) = do
			v <- expandt False cs
			return (c:v)
		findname n [] = (n, "")
		findname n (c:cs)
			| c == '/' = (n, cs)
			| otherwise = findname (n++[c]) cs

{- Finds the current git repository, which may be in a parent directory. -}
repoFromCwd :: IO Repo
repoFromCwd = getCurrentDirectory >>= seekUp isRepoTop >>= maybe norepo makerepo
	where
		makerepo = return . newFrom . Dir
		norepo = error "Not in a git repository."

seekUp :: (FilePath -> IO Bool) -> FilePath -> IO (Maybe FilePath)
seekUp want dir = do
	ok <- want dir
	if ok
		then return $ Just dir
		else case parentDir dir of
			"" -> return Nothing
			d -> seekUp want d

isRepoTop :: FilePath -> IO Bool
isRepoTop dir = do
	r <- isRepo
	b <- isBareRepo
	return (r || b)
	where
		isRepo = gitSignature ".git" ".git/config"
		isBareRepo = gitSignature "objects" "config"
		gitSignature subdir file = do
			s <- (doesDirectoryExist (dir ++ "/" ++ subdir))
			f <- (doesFileExist (dir ++ "/" ++ file))
			return (s && f)