Fix deadlock during content examination.

Before this fix, the following situation could happen in threads
A and B:

A: Some DONE signal happens; this triggers setup_pieces which
   takes a lock on the player mutex.

B: FFmpegContent::examine takes a lock on the content mutex.
B: FFmpegContent::examine adds a stream
B: That causes STREAMS PENDING to be emitted.
B: This tries to take a lock on the player mutex so it can update _suspended

A: setup_pieces tries to access some content information, hence
   tries to take a lock on the content mutex.

Now B is holding the CL and awaiting the PL and A is holding
the PL and awaiting the CL.

It feels like the root cause of this is that while setup_pieces
is happening another change (which would itself cause setup_pieces)
is announced, and this isn't dealt with properly.

There are two steps here; _suspended is protected with an atomic
rather than using _mutex, and also it can cope with being updated
recursively.

Backported from df48c75c38dd788835a93540aea243a2dac4bb10 in v2.15.x

2 files changed, 6 insertions, 8 deletions

diff --git a/src/lib/player.cc b/src/lib/player.cc
index 30b49d186..1d83b9b5a 100644
--- a/src/lib/player.cc
+++ b/src/lib/player.cc
@@ -85,7 +85,7 @@ int const PlayerProperty::DCP_DECODE_REDUCTION = 704;
 Player::Player (shared_ptr<const Film> film, shared_ptr<const Playlist> playlist)
 	: _film (film)
 	, _playlist (playlist)
-	, _suspended (false)
+	, _suspended (0)
 	, _ignore_video (false)
 	, _ignore_audio (false)
 	, _ignore_text (false)
@@ -237,19 +237,17 @@ void
 Player::playlist_content_change (ChangeType type, int property, bool frequent)
 {
 	if (type == CHANGE_TYPE_PENDING) {
-		boost::mutex::scoped_lock lm (_mutex);
 		/* The player content is probably about to change, so we can't carry on
 		   until that has happened and we've rebuilt our pieces.  Stop pass()
 		   and seek() from working until then.
 		*/
-		_suspended = true;
+		++_suspended;
 	} else if (type == CHANGE_TYPE_DONE) {
 		/* A change in our content has gone through.  Re-build our pieces. */
 		setup_pieces ();
-		_suspended = false;
+		--_suspended;
 	} else if (type == CHANGE_TYPE_CANCELLED) {
-		boost::mutex::scoped_lock lm (_mutex);
-		_suspended = false;
+		--_suspended;
 	}
 
 	Change (type, property, frequent);
diff --git a/src/lib/player.h b/src/lib/player.h
index 8a81146c9..65ab1279f 100644
--- a/src/lib/player.h
+++ b/src/lib/player.h
@@ -145,8 +145,8 @@ private:
 	boost::shared_ptr<const Film> _film;
 	boost::shared_ptr<const Playlist> _playlist;
 
-	/** true if we are suspended (i.e. pass() and seek() do nothing) */
-	bool _suspended;
+	/** > 0 if we are suspended (i.e. pass() and seek() do nothing) */
+	boost::atomic<int> _suspended;
 	std::list<boost::shared_ptr<Piece> > _pieces;
 
 	/** Size of the image in the DCP (e.g. 1990x1080 for flat) */