arrayjoin signalling minimise also frees memory

By making the sequential line cache non-persistent, and only minimising
when the read point is well past the image.

On large arrayjoin operations, this saves many GB of memory.

See https://github.com/kleisauke/net-vips/issues/135

ChangeLog

@@ -4,7 +4,8 @@
 - fix thumbnail with small image plus crop plus no upsize [Andrewsville]
 - rename speed / reduction-effort / etc. params as "effort"
 - add gifsave [lovell]
-- arrayjoin minimises inputs during processing
+- arrayjoin minimises inputs during sequential processing, saving a lot of 
+  memory and file descriptors
 
 16/8/21 started 8.11.4
 - fix off-by-one error in new rank fast path

libvips/conversion/arrayjoin.c

@@ -68,6 +68,7 @@ typedef struct _VipsArrayjoin {
 
 	int down;
 	VipsRect *rects;
+	gboolean *minimised;
 
 } VipsArrayjoin;
 
@@ -119,23 +120,20 @@ vips_arrayjoin_gen( VipsRegion *or, void *seq,
 	}
 
 	if( vips_image_is_sequential( conversion->out ) ) {
-		/* In sequential mode, we can minimise an input once we've
-		 * fetched the final line of pixels from it.
+		/* In sequential mode, we can minimise an input once our
+		 * generate point is well past the end of it. This can save a
+		 * lot of memory and file descriptors on large image arrays.
 		 *
-		 * Find all inputs whose final line is inside this rect and
-		 * shut them down.
+		 * minimise_all is quite expensive, so only trigger once for
+		 * each input.
 		 */
-		for( i = 0; i < n; i++ ) {
-			VipsRect final_line = { 
-				join->rects[i].left,
-				VIPS_RECT_BOTTOM( &join->rects[i] ) - 1,
-				join->rects[i].width,
-				1
-			};
-
-			if( vips_rect_includesrect( &final_line, r ) ) 
+		for( i = 0; i < n; i++ ) 
+			if( !join->minimised[i] &&
+				r->top > VIPS_RECT_BOTTOM( &join->rects[i] ) +
+				       256 ) {
 				vips_image_minimise_all( in[i] );
-		}
+				join->minimised[i] = TRUE;
+			}
 	}
 
 	return( 0 );
@@ -247,6 +245,12 @@ vips_arrayjoin_build( VipsObject *object )
 				output_width - join->rects[i].left;
 	}
 
+	/* A thing to track which inputs we've signalled minimise on.
+	 */
+	join->minimised = VIPS_ARRAY( join, n, gboolean ); 
+	for( i = 0; i < n; i++ ) 
+		join->minimised[i] = FALSE;
+
 	/* Each image must be cropped and aligned within an @hspacing by
 	 * @vspacing box.
 	 */

libvips/conversion/sequential.c

@@ -23,6 +23,8 @@
  * 	- re-enable skipahead now we have the single-thread-first-tile idea
  * 6/3/17
  * 	- deprecate @trace, @access now seq is much simpler
+ * 6/9/21
+ * 	- don't set "persistent", it can cause huge memory use
  */
 
 /*
@@ -192,14 +194,17 @@ vips_sequential_build( VipsObject *object )
 	if( VIPS_OBJECT_CLASS( vips_sequential_parent_class )->build( object ) )
 		return( -1 );
 
+	/* We've gone forwards and backwards on sequential caches being
+	 * persistent. Persistent caches can be useful if you want to eg.
+	 * make several crop() operations on a seq image source, but they use
+	 * a lot of memory with eg. arrayjoin.
+	 *
+	 * On balance, if you want to make many crops from one source, use a
+	 * RANDOM image.
+	 */
 	if( vips_linecache( sequential->in, &t, 
 		"tile_height", sequential->tile_height,
 		"access", VIPS_ACCESS_SEQUENTIAL,
-		/* We need seq caches to persist across minimise in case
-		 * someone is trying to read an image with a series of crop
-		 * operations.
-		 */
-		"persistent", TRUE,
 		NULL ) )
 		return( -1 );