diff --git a/libvips/foreign/heifload.c b/libvips/foreign/heifload.c
index 08cdc68c..802fdc66 100644
--- a/libvips/foreign/heifload.c
+++ b/libvips/foreign/heifload.c
@@ -872,6 +872,9 @@ vips_foreign_load_heif_load( VipsForeignLoad *load )
 		vips_image_write( t[1], load->real ) )
 		return( -1 );
 
+	if( vips_source_decode( heif->source ) )
+		return( -1 );
+
 	return( 0 );
 }
 
diff --git a/libvips/iofuncs/source.c b/libvips/iofuncs/source.c
index b6cbc36b..cd0eb73c 100644
--- a/libvips/iofuncs/source.c
+++ b/libvips/iofuncs/source.c
@@ -42,8 +42,8 @@
  */
 
 /*
-#define TEST_SANITY
 #define VIPS_DEBUG
+#define TEST_SANITY
  */
 
 #ifdef HAVE_CONFIG_H
@@ -639,13 +639,15 @@ vips_source_decode( VipsSource *source )
 
 	SANITY( source );
 
-	/* We have finished reading the header. We can discard the header bytes
-	 * we saved.
-	 */
 	if( !source->decode ) {
 		source->decode = TRUE;
-		VIPS_FREEF( g_byte_array_unref, source->header_bytes ); 
 		VIPS_FREEF( g_byte_array_unref, source->sniff ); 
+
+		/* If this is still a pipe source, we can discard the header 
+		 * bytes we saved.
+		 */
+		if( source->is_pipe ) 
+			VIPS_FREEF( g_byte_array_unref, source->header_bytes ); 
 	}
 
 	vips_source_minimise( source );
@@ -655,6 +657,25 @@ vips_source_decode( VipsSource *source )
 	return( 0 );
 }
 
+#ifdef VIPS_DEBUG
+static void
+vips_source_print( VipsSource *source )
+{
+	printf( "vips_source_print: %p\n", source );
+	printf( "  source->read_position = %zd\n", source->read_position );
+	printf( "  source->is_pipe = %d\n", source->is_pipe );
+	printf( "  source->length = %zd\n", source->length );
+	printf( "  source->data = %p\n", source->data );
+	printf( "  source->header_bytes = %p\n", source->header_bytes );
+	if( source->header_bytes ) 
+		printf( "  source->header_bytes->len = %d\n", 
+			source->header_bytes->len );
+	printf( "  source->sniff = %p\n", source->sniff );
+	if( source->sniff )
+		printf( "  source->sniff->len = %d\n", source->sniff->len );
+}
+#endif /*VIPS_DEBUG*/
+
 /**
  * vips_source_read:
  * @source: source to operate on
@@ -761,31 +782,13 @@ vips_source_read( VipsSource *source, void *buffer, size_t length )
 	return( total_read );
 }
 
-#ifdef DEBUG
-static void
-vips_source_print( VipsSource *source )
-{
-	printf( "vips_source_print: %p\n", source );
-	printf( "  source->read_position = %zd\n", source->read_position );
-	printf( "  source->is_pipe = %d\n", source->is_pipe );
-	printf( "  source->length = %zd\n", source->length );
-	printf( "  source->data = %p\n", source->data );
-	printf( "  source->header_bytes = %p\n", source->header_bytes );
-	if( source->header_bytes ) 
-		printf( "  source->header_bytes->len = %d\n", 
-			source->header_bytes->len );
-	printf( "  source->sniff = %p\n", source->sniff );
-	if( source->sniff )
-		printf( "  source->sniff->len = %d\n", source->sniff->len );
-}
-#endif /*DEBUG*/
-
 /* Read to a position. 
  *
  * target == -1 means read to end of source -- useful for forcing a pipe into
- * memory, for example. 
+ * memory, for example. This will always set length to the pipe length.
  *
- * If we hit EOF, set length on the pipe.
+ * If we hit EOF and we're buffering, set length on the pipe and turn it into
+ * a memory source.
  *
  * read_position is left somewhere indeterminate.
  */
@@ -800,6 +803,7 @@ vips_source_pipe_read_to_position( VipsSource *source, gint64 target )
 	 * length).
 	 */
 	g_assert( source->length == -1 );
+	g_assert( source->is_pipe );
 
 	while( target == -1 ||
 		source->read_position < target ) {
@@ -813,6 +817,19 @@ vips_source_pipe_read_to_position( VipsSource *source, gint64 target )
 			/* No more bytes available, we must be at EOF.
 			 */
 			source->length = source->read_position;
+
+			/* Have we been buffering the whole thing? We can
+			 * become a memory source.
+			 */
+			if( source->header_bytes ) {
+				source->data = source->header_bytes->data;
+				source->is_pipe = FALSE;
+
+				/* TODO ... we could close more fds here.
+				 */
+				vips_source_minimise( source );
+			}
+
 			break;
 		}
 
@@ -853,8 +870,6 @@ vips_source_read_to_memory( VipsSource *source )
 	byte_array = g_byte_array_new();
 	g_byte_array_set_size( byte_array, source->length );
 
-	/* Read in a series of chunks to reduce stress upstream.
-	 */
 	read_position = 0;
 	q = byte_array->data;
 	while( read_position < source->length ) {
@@ -887,34 +902,6 @@ vips_source_read_to_memory( VipsSource *source )
 	return( 0 );
 }
 
-/* Read the entire pipe into memory and turn this into a memory source.
- */
-static int
-vips_source_pipe_to_memory( VipsSource *source )
-{
-	VIPS_DEBUG_MSG( "vips_source_pipe_to_memory:\n" );
-
-	g_assert( source->is_pipe );
-	g_assert( !source->blob );
-	g_assert( !source->decode );
-	g_assert( source->header_bytes );
-
-	if( vips_source_pipe_read_to_position( source, -1 ) )
-		return( -1 );
-
-	/* Steal the header_bytes pointer and turn into a memory source.
-	 */
-	source->length = source->header_bytes->len;
-	source->data = source->header_bytes->data;
-	source->is_pipe = FALSE;
-
-	/* TODO ... we could close more fds here.
-	 */
-	vips_source_minimise( source );
-
-	return( 0 );
-}
-
 static int
 vips_source_descriptor_to_memory( VipsSource *source )
 {
@@ -1002,7 +989,7 @@ vips_source_map( VipsSource *source, size_t *length_out )
 				return( NULL );
 		}
 		else {
-			if( vips_source_pipe_to_memory( source ) )
+			if( vips_source_pipe_read_to_position( source, -1 ) )
 				return( NULL );
 		}
 	}
@@ -1113,8 +1100,7 @@ vips_source_seek( VipsSource *source, gint64 offset, int whence )
 			/* We have to read the whole source into memory to get
 			 * the length.
 			 */
-			if( source->length == -1 &&
-				vips_source_pipe_to_memory( source ) )
+			if( vips_source_pipe_read_to_position( source, -1 ) )
 				return( -1 );
 
 			new_pos = source->length + offset;
@@ -1130,6 +1116,13 @@ vips_source_seek( VipsSource *source, gint64 offset, int whence )
 			return( -1 );
 	}
 
+	/* For pipes, we have to fake seek by reading to that point. This
+	 * might hit EOF and turn the pipe into a memory source.
+	 */
+	if( source->is_pipe &&
+		vips_source_pipe_read_to_position( source, new_pos ) )
+		return( -1 );
+
 	/* Don't allow out of range seeks.
 	 */
 	if( new_pos < 0 ||
@@ -1140,28 +1133,6 @@ vips_source_seek( VipsSource *source, gint64 offset, int whence )
                 return( -1 );
 	}
 
-	/* For pipes, we have to fake seek by reading to that point.
-	 */
-	if( source->is_pipe ) {
-		if( vips_source_pipe_read_to_position( source, new_pos ) )
-			return( -1 );
-
-		/* We've hit EOF in the pipe, and we've got the whole thing 
-		 * buffered. Steal the header_bytes pointer and turn into a 
-		 * memory source.
-		 */
-		if( source->length != -1 &&
-			source->header_bytes ) {
-			source->length = source->header_bytes->len;
-			source->data = source->header_bytes->data;
-			source->is_pipe = FALSE;
-
-			/* TODO ... we could close more fds here.
-			 */
-			vips_source_minimise( source );
-		}
-	}
-
 	source->read_position = new_pos;
 
 	VIPS_DEBUG_MSG( "    new_pos = %" G_GINT64_FORMAT "\n", new_pos );