extract is sequential again

a hack in seq.c will skip ahead if the very first read is not at line 0

ChangeLog

@@ -13,8 +13,6 @@
 - VipsSequential has an integrated cache and stalls out of order threads
 - add a line cache ... sizes up dynamically with request size
 - tilecache / linecache use a hash table not a linear list
-- due to out of order stalling, extract_area is no longer a sequential 
-  operation, sadly
 
 20/7/12 started 7.30.0
 - support "rs" mode in vips7

TODO

@@ -1,6 +1,3 @@
-- sometimes still errors in .jpg
-
-
 blocking bugs
 =============
 

libvips/conversion/extract.c

@@ -191,6 +191,8 @@ vips_extract_area_class_init( VipsExtractAreaClass *class )
 	vobject_class->description = _( "extract an area from an image" );
 	vobject_class->build = vips_extract_area_build;
 
+	operation_class->flags = VIPS_OPERATION_SEQUENTIAL;
+
 	VIPS_ARG_IMAGE( class, "input", 0, 
 		_( "Input" ), 
 		_( "Input image" ),

libvips/conversion/sequential.c

@@ -115,8 +115,10 @@ retry:
 
 	VIPS_DEBUG_MSG( "thread %p has lock ...\n", g_thread_self() ); 
 
-	if( r->top > sequential->y_pos ) {
-		/* This is for stuff in the future, stall.
+	if( r->top > sequential->y_pos && 
+		sequential->y_pos > 0 ) {
+		/* We have started reading (y_pos > 0) and this request is for 
+		 * stuff beyond that, stall.
 		 */
 		VIPS_DEBUG_MSG( "thread %p stalling ...\n", g_thread_self() ); 
 		g_cond_wait( sequential->ready, sequential->lock );
@@ -126,6 +128,28 @@ retry:
 		goto retry;
 	}
 
+	/* This is a request for something some way down the image, and we've
+	 * not read anything yet. Probably the operation is something like
+	 * extract_area and we should skip the initial part of the image. In
+	 * fact we read to cache.
+	 */
+	if( r->top > sequential->y_pos ) {
+		VipsRect area;
+
+		VIPS_DEBUG_MSG( "thread %p skipping to line %d ...\n", 
+			g_thread_self(),
+			r->top );
+
+		area.left = 0;
+		area.top = sequential->y_pos;
+		area.width = 1;
+		area.height = r->top - sequential->y_pos;
+		if( vips_region_prepare( ir, &area ) )
+			return( -1 );
+
+		sequential->y_pos = VIPS_RECT_BOTTOM( &area );
+	}
+
 	/* This is a request for old or present pixels -- serve from cache.
 	 * This may trigger further, sequential reads.
 	 */