init: Display an additional message when it detects a filesystem that allows writing to files whose write bit is not set.

Annex/Init.hs

@@ -146,32 +146,40 @@ probeCrippledFileSystem :: Annex Bool
 probeCrippledFileSystem = do
 	tmp <- fromRepo gitAnnexTmpMiscDir
 	createAnnexDirectory tmp
-	liftIO $ probeCrippledFileSystem' tmp
+	probeCrippledFileSystem' tmp
 
-probeCrippledFileSystem' :: FilePath -> IO Bool
+probeCrippledFileSystem' :: FilePath -> Annex Bool
 #ifdef mingw32_HOST_OS
 probeCrippledFileSystem' _ = return True
 #else
 probeCrippledFileSystem' tmp = do
 	let f = tmp </> "gaprobe"
-	writeFile f ""
+	liftIO $ writeFile f ""
 	uncrippled <- probe f
-	void $ tryIO $ allowWrite f
-	removeFile f
+	void $ liftIO $ tryIO $ allowWrite f
+	liftIO $ removeFile f
 	return $ not uncrippled
   where
 	probe f = catchBoolIO $ do
 		let f2 = f ++ "2"
-		nukeFile f2
-		createSymbolicLink f f2
-		nukeFile f2
-		preventWrite f
+		liftIO $ nukeFile f2
+		liftIO $ createSymbolicLink f f2
+		liftIO $ nukeFile f2
+		liftIO $ preventWrite f
 		-- Should be unable to write to the file, unless
 		-- running as root, but some crippled
 		-- filesystems ignore write bit removals.
-		ifM ((== 0) <$> getRealUserID)
+		ifM ((== 0) <$> liftIO getRealUserID)
 			( return True
-			, not <$> catchBoolIO (writeFile f "2" >> return True)
+			, do
+				r <- liftIO $ catchBoolIO $
+					writeFile f "2" >> return True
+				if r
+					then do
+						warning "Filesystem allows writing to files whose write bit is not set."
+						return False
+					else return True
+
 			)
 #endif
 

CHANGELOG

@@ -1,6 +1,8 @@
 git-annex (6.20170819) UNRELEASED; urgency=medium
 
   * Support building with feed-1.0, while still supporting older versions.
+  * init: Display an additional message when it detects a filesystem that
+    allows writing to files whose write bit is not set.
 
  -- Joey Hess <id@joeyh.name>  Mon, 28 Aug 2017 12:20:59 -0400
 

Test.hs

@@ -147,7 +147,7 @@ runner = Just go
 		exitWith exitcode
 	runsubprocesstests opts (Just _) = isolateGitConfig $ do
 		ensuretmpdir
-		crippledfilesystem <- Annex.Init.probeCrippledFileSystem' tmpdir
+		crippledfilesystem <- annexeval $ Annex.Init.probeCrippledFileSystem' tmpdir
 		case tryIngredients ingredients (tastyOptionSet opts) (tests crippledfilesystem opts) of
 			Nothing -> error "No tests found!?"
 			Just act -> ifM act

doc/bugs/Git-annex_not_properly_detecting_symlink_capability_in_CryFS.mdwn

@@ -60,3 +60,4 @@ git-annex-6.20170519-1.fc26.x86_64
 
 I love git-annex.
 
+> [[done]] --[[Joey]]

doc/bugs/Git-annex_not_properly_detecting_symlink_capability_in_CryFS/comment_1_78f6f047578974df0dd73740ab05ee8c._comment

@@ -0,0 +1,17 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2017-08-28T17:06:35Z"
+ content="""
+git-annex has detected problems with the filesystem, since symlinks
+work I think it detected that the filesystem allows writing to a
+file even when the file's write bit is not set.
+
+In that situation, a git-annex repository using symlinks will apprear to
+work, but if you have an annexed file "foo" in there, and run "echo hi >
+foo", it will follow the symlink, and overwrite the annexed file, ignoring
+the lack of write bit. This can result in data loss in an unexpected way,
+which is why git-annex avoids using the symlinks in this situation.
+
+I've added an additional message to make more clear what is happening.
+"""]]