experiment with removal of seq stall

TODO

@@ -39,6 +39,20 @@
 
   sinkmemory will need to have something added
 
+  so ... just swap vips_sequentiial for vips_linecache, look at adding
+  interlocks to sinkmemory, test, possibly increase margins
+
+  what will do skipahead now? we have to build this into linecache ... or maybe
+  leave seq there and just strip out the stalling and sync code
+
+  make tiff2vips track and verify y_pos 
+
+  foreign operations affected
+
+	  tiff2vips.c
+	  jpeg2vips.c
+	  radiance.c
+	  vipspng.c
 
 - vips linecache has access there twice!
 

libvips/foreign/jpeg2vips.c

@@ -552,7 +552,7 @@ read_jpeg_generate( VipsRegion *or,
 	g_assert( r->height == VIPS_MIN( 8, or->im->Ysize - r->top ) ); 
 
 	/* And check that y_pos is correct. It should be, since we are inside
-	 * a vips_sequential().
+	 * a vips_linecache().
 	 */
 	if( r->top != jpeg->y_pos ) {
 		VIPS_GATE_STOP( "read_jpeg_generate: work" );
@@ -674,7 +674,7 @@ read_jpeg_image( ReadJpeg *jpeg, VipsImage *out )
 	if( vips_image_generate( t[0], 
 		NULL, read_jpeg_generate, NULL, 
 		jpeg, NULL ) ||
-		vips_sequential( t[0], &t[1], 
+		vips_linecache( t[0], &t[1], 
 			"tile_height", 8,
 			"access", jpeg->readbehind ? 
 				VIPS_ACCESS_SEQUENTIAL : 

libvips/foreign/radiance.c

@@ -1155,7 +1155,7 @@ vips__rad_load( const char *filename, VipsImage *out, gboolean readbehind )
 	if( vips_image_generate( t[0], 
 		NULL, rad2vips_generate, NULL, 
 		read, NULL ) ||
-		vips_sequential( t[0], &t[1], 
+		vips_linecache( t[0], &t[1], 
 			"tile_height", 8,
 			"access", readbehind ? 
 				VIPS_ACCESS_SEQUENTIAL : 

libvips/foreign/tiff2vips.c

@@ -1924,7 +1924,7 @@ rtiff_read_stripwise( Rtiff *rtiff, VipsImage *out )
 		vips_image_generate( t[0], 
 			NULL, rtiff_stripwise_generate, NULL, 
 			rtiff, NULL ) ||
-		vips_sequential( t[0], &t[1], 
+		vips_linecache( t[0], &t[1], 
 			"tile_height", rtiff->header.rows_per_strip,
 			"access", rtiff->readbehind ? 
 				VIPS_ACCESS_SEQUENTIAL : 

libvips/foreign/vipspng.c

@@ -497,7 +497,7 @@ png2vips_generate( VipsRegion *or,
 	g_assert( r->height == VIPS_MIN( 8, or->im->Ysize - r->top ) ); 
 
 	/* And check that y_pos is correct. It should be, since we are inside
-	 * a vips_sequential().
+	 * a vips_linecache().
 	 */
 	if( r->top != read->y_pos ) {
 		vips_error( "vipspng", 
@@ -590,7 +590,7 @@ png2vips_image( Read *read, VipsImage *out )
 			vips_image_generate( t[0], 
 				NULL, png2vips_generate, NULL, 
 				read, NULL ) ||
-			vips_sequential( t[0], &t[1], 
+			vips_linecache( t[0], &t[1], 
 				"tile_height", 8,
 				"access", read->readbehind ? 
 					VIPS_ACCESS_SEQUENTIAL :