From 026c049512cfdca600fbf75a69706a5c72ad9a61 Mon Sep 17 00:00:00 2001
From: Chris Frey <cdfrey@foursquare.net>
Date: Thu, 9 Feb 2012 15:46:11 -0500
Subject: [PATCH] desktop: made the thread locks more forgiving when
 pre-loading

When loading a large database from a slow device, the old single
lock system blocked unnecessarily.  This commit allows for locking
of the map and locking of the load, so accessing pre-loaded data
is still possible during background device access.
---
 desktop/src/Mode_Browse.cc | 38 +++++++++++++++++++++++++++++++-------
 desktop/src/Mode_Browse.h  |  1 +
 2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/desktop/src/Mode_Browse.cc b/desktop/src/Mode_Browse.cc
index cd604bf4..8405ad8f 100644
--- a/desktop/src/Mode_Browse.cc
+++ b/desktop/src/Mode_Browse.cc
@@ -393,21 +393,45 @@ DBMap::DBMap(ThreadableDesktop &tdesktop)
 	if( pthread_mutex_init(&m_map_mutex, NULL) ) {
 		throw Barry::Error("Failed to create map mutex");
 	}
+
+	if( pthread_mutex_init(&m_load_mutex, NULL) ) {
+		throw Barry::Error("Failed to create load mutex");
+	}
 }
 
 DBMap::DBCachePtr DBMap::LoadDBCache(const std::string &dbname)
 {
-	scoped_lock lock(m_map_mutex);
+	// first, check for pre-loaded data, before the load lock,
+	// to make sure we return pre-loaded data with utmost haste
+	{
+		scoped_lock map_lock(m_map_mutex);
 
-	MapType::iterator i = m_map.find(dbname);
-	if( i != m_map.end() )
-		return i->second;
+		MapType::iterator i = m_map.find(dbname);
+		if( i != m_map.end() )
+			return i->second;
+	}
+
+	// if not found, lock and load, but be careful, since we
+	// don't want to open a window here for loading a db twice
+	scoped_lock load_lock(m_load_mutex);
 
-	// do not unlock here, since we only want to load this
-	// data once, not provide a window for loading it twice
+	// check again for pre-loaded data, since between
+	// map.unlock and load.lock there could have been
+	// another successful load
+	{
+		scoped_lock map_lock(m_map_mutex);
 
-	// not found, time to load it up
+		MapType::iterator i = m_map.find(dbname);
+		if( i != m_map.end() )
+			return i->second;
+	}
+
+	// do the load, without map.lock, since this can take a
+	// significant amount of time
 	DBCachePtr p( new DBCache(m_tdesktop, dbname) );
+
+	// lock once more to update the map, and then done
+	scoped_lock map_lock(m_map_mutex);
 	m_map[dbname] = p;
 	return p;
 }
diff --git a/desktop/src/Mode_Browse.h b/desktop/src/Mode_Browse.h
index ced49b88..4569a3db 100644
--- a/desktop/src/Mode_Browse.h
+++ b/desktop/src/Mode_Browse.h
@@ -311,6 +311,7 @@ private:
 	ThreadableDesktop &m_tdesktop;
 	MapType m_map;
 	pthread_mutex_t m_map_mutex;
+	pthread_mutex_t m_load_mutex;
 
 public:
 	DBMap(ThreadableDesktop &tdesktop);
-- 
2.11.4.GIT