Serialize use of C magic library, which is not thread safe.

This fixes failures uploading to S3 when using -J.

This commit was sponsored by Denis Dzyubenko on Patreon.

Annex/Magic.hs

@@ -1,6 +1,6 @@
 {- Interface to libmagic
  -
- - Copyright 2019 Joey Hess <id@joeyh.name>
+ - Copyright 2019-2020 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -21,6 +21,8 @@ import Control.Monad.IO.Class
 #ifdef WITH_MAGICMIME
 import Magic
 import Utility.Env
+import Control.Concurrent
+import System.IO.Unsafe (unsafePerformIO)
 import Common
 #else
 type Magic = ()
@@ -41,7 +43,7 @@ initMagicMime = return Nothing
 
 getMagicMime :: Magic -> FilePath -> IO (Maybe (MimeType, MimeEncoding))
 #ifdef WITH_MAGICMIME
-getMagicMime m f = Just . parse <$> magicFile m f
+getMagicMime m f = Just . parse <$> magicConcurrentSafe (magicFile m f)
   where
 	parse s = 
 		let (mimetype, rest) = separate (== ';') s
@@ -58,3 +60,15 @@ getMagicMimeType m f = liftIO $ fmap fst <$> getMagicMime m f
 
 getMagicMimeEncoding :: MonadIO m => Magic -> FilePath -> m(Maybe MimeEncoding)
 getMagicMimeEncoding m f = liftIO $ fmap snd <$> getMagicMime m f
+
+#ifdef WITH_MAGICMIME
+{-# NOINLINE mutex #-}
+mutex :: MVar ()
+mutex = unsafePerformIO $ newMVar ()
+
+-- Work around a bug, the library is not concurrency safe and will
+-- sometimes access the wrong memory if multiple ones are called at the
+-- same time.
+magicConcurrentSafe :: IO a -> IO a
+magicConcurrentSafe = bracket_ (takeMVar mutex) (putMVar mutex ())
+#endif

CHANGELOG

@@ -9,6 +9,8 @@ git-annex (8.20200909) UNRELEASED; urgency=medium
     to determine which batch request each response corresponds to.
   * aws-0.22 improved its support for setting etags, which improves
     support for versioned S3 buckets.
+  * Serialize use of C magic library, which is not thread safe.
+    This fixes failures uploading to S3 when using -J.
 
  -- Joey Hess <id@joeyh.name>  Mon, 14 Sep 2020 18:34:37 -0400
 

doc/bugs/concurrent_git-annex-copy_to_s3_special_remote_fails/comment_7_d2943f675bbfe86b78b30e4fb17b7fb4._comment

@@ -0,0 +1,20 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 7"""
+ date="2020-09-17T21:03:27Z"
+ content="""
+I instrumented git-annex to output the content-type it's sending,
+and it IS corrupted before hitting the http stack. It seems that libmagic
+is returning garbage.
+
+Perhaps it's not thread safe? If so, it might be *writing* to the wrong
+location too in some cases, so could explain all this weird behavior.
+And S3 remote is almost the only part of git-annex that uses libmagic, and the
+only part that uses it concurrently...
+
+So, I added a mutex around uses of it. Problem went away. Bug sent to
+library maintainer, and for now I hope this fixes the problem.
+
+If you were seeing hangs, crashes, etc, please try with a git-annex
+autobuild made after this comment..
+"""]]