make VipsArea threadsafe

google maps output needs threaded create / destroy of VipsArea (used to
hold the background colour). We need a lock on VipsArea::count.

Fixes https://github.com/jcupitt/libvips/issues/46

Thanks fuho

ChangeLog

@@ -32,6 +32,7 @@
 - added vips_bandbool(), with vips_bandand(), _bandor(), _bandeor() as
   convenience functions
 - added scRGB colourspace, linear light float space with sRGB primaries
+- VipsArea is threadsafe
 
 14/11/12 started 7.30.6
 - capture tiff warnings earlier

libvips/foreign/dzsave.c

@@ -1046,6 +1046,17 @@ vips_foreign_save_dz_build( VipsObject *object )
 			VIPS_SETSTR( dz->suffix, ".jpg" );
 	}
 
+	/* Default to white background. 
+	 */
+	if( dz->layout == VIPS_FOREIGN_DZ_LAYOUT_GOOGLE &&
+		!vips_object_argument_isset( object, "background" ) ) {
+		VipsArrayDouble *background; 
+
+		background = vips_array_double_newv( 1, 255.0 );
+		g_object_set( object, "background", background, NULL );
+		vips_area_unref( background ); 
+	}
+
 	if( dz->overlap >= dz->tile_size || 
 		dz->overlap >= dz->tile_size ) {
 		vips_error( "dzsave", 
@@ -1231,9 +1242,6 @@ vips_foreign_save_dz_init( VipsForeignSaveDz *dz )
 	dz->overlap = 1;
 	dz->tile_size = 256;
 	dz->tile_count = 0;
-	dz->background = 
-		vips_area_new_array( G_TYPE_DOUBLE, sizeof( double ), 1 ); 
-	((double *) (dz->background->data))[0] = 255;
 	dz->depth = VIPS_FOREIGN_DZ_DEPTH_1PIXEL; 
 }
 

libvips/include/vips/type.h

@@ -66,9 +66,13 @@ typedef struct _VipsArea {
 
 	/*< private >*/
 
-	/* Reference count.
+	/* Reference count and lock. 
+	 *
+	 * We could use an atomic int, but this is not a high-traffic data
+	 * structure, so a simple GMutex is OK.
 	 */
 	int count;
+	GMutex *lock;		
 
 	/* Things like ICC profiles need their own free functions.
 	 */

libvips/iofuncs/type.c

@@ -147,6 +147,8 @@ static int vips_area_number = 0;
 VipsArea *
 vips_area_copy( VipsArea *area )
 {
+	g_mutex_lock( area->lock );
+
 	g_assert( area->count >= 0 );
 
 	area->count += 1;
@@ -155,12 +157,16 @@ vips_area_copy( VipsArea *area )
 	printf( "vips_area_copy: %p count = %d\n", area, area->count );
 #endif /*DEBUG*/
 
+	g_mutex_unlock( area->lock );
+
 	return( area );
 }
 
 void
 vips_area_unref( VipsArea *area )
 {
+	g_mutex_lock( area->lock );
+
 	g_assert( area->count > 0 );
 
 	area->count -= 1;
@@ -176,6 +182,10 @@ vips_area_unref( VipsArea *area )
 			area->free_fn = NULL;
 		}
 
+		g_mutex_unlock( area->lock );
+
+		VIPS_FREEF( vips_g_mutex_free, area->lock );
+
 		g_free( area );
 
 #ifdef DEBUG
@@ -184,6 +194,8 @@ vips_area_unref( VipsArea *area )
 			vips_area_number );
 #endif /*DEBUG*/
 	}
+	else
+		g_mutex_unlock( area->lock );
 }
 
 /**
@@ -205,6 +217,7 @@ vips_area_new( VipsCallbackFn free_fn, void *data )
 
 	area = g_new( VipsArea, 1 );
 	area->count = 1;
+	area->lock = vips_g_mutex_new();
 	area->length = 0;
 	area->data = data;
 	area->free_fn = free_fn;