improve fail-on handling for GIF load

fail-on truncated and warning now works for GIF load

ChangeLog

@@ -18,6 +18,7 @@ master
 - threadpools size dynamically with load
 - operations can hint threadpool size
 - support for N-colour ICC profiles
+- GIF load supports truncated frames [tlsa]
 
 11/10/22 started 8.13.3
 - improve rules for 16-bit heifsave [johntrunc]

libvips/foreign/nsgifload.c

@@ -359,19 +359,44 @@ vips_foreign_load_nsgif_header( VipsForeignLoad *load )
 		return( -1 );
 	vips_source_minimise( gif->source );
 
+	/* Treat errors from _scan() as warnings. If libnsgif really can't do
+	 * something it'll fail gracefully later when we try to read out 
+	 * frame data.
+	 */
 	result = nsgif_data_scan( gif->anim, size, (void *) data );
 	VIPS_DEBUG_MSG( "nsgif_data_scan() = %s\n", nsgif_strerror( result ) );
+	switch( result ) {
+	case NSGIF_ERR_END_OF_DATA:
+		if( load->fail_on >= VIPS_FAIL_ON_TRUNCATED ) {
+			vips_foreign_load_nsgif_error( gif, result ); 
+			return( -1 );
+		}
+		else
+			g_warning( "%s", nsgif_strerror( result ) );
+		break;
+
+	case NSGIF_OK:
+		break;
+
+	default:
+		if( load->fail_on >= VIPS_FAIL_ON_WARNING ) {
+			vips_foreign_load_nsgif_error( gif, result ); 
+			return( -1 );
+		}
+		else
+			g_warning( "%s", nsgif_strerror( result ) );
+		break;
+	}
+
+	/* Tell libnsgif that that's all the data we have. This will let us
+	 * read out any truncated final frames.
+	 */
 	nsgif_data_complete( gif->anim );
+
 	gif->info = nsgif_get_info( gif->anim );
 #ifdef VERBOSE
 	print_animation( gif->anim, gif->info );
 #endif /*VERBOSE*/
-	if( result != NSGIF_OK &&
-		load->fail_on >= VIPS_FAIL_ON_WARNING ) {
-		vips_foreign_load_nsgif_error( gif, result ); 
-		return( -1 );
-	}
-
 	if( !gif->info->frame_count ) {
 		vips_error( class->nickname, "%s", _( "no frames in GIF" ) );
 		return( -1 );

test/test-suite/test_foreign.py

@@ -7,7 +7,7 @@ import pytest
 
 import pyvips
 from helpers import \
-    JPEG_FILE, SRGB_FILE, MATLAB_FILE, PNG_FILE, TIF_FILE, OME_FILE, \
+    IMAGES, JPEG_FILE, SRGB_FILE, MATLAB_FILE, PNG_FILE, TIF_FILE, OME_FILE, \
     ANALYZE_FILE, GIF_FILE, WEBP_FILE, EXR_FILE, FITS_FILE, OPENSLIDE_FILE, \
     PDF_FILE, SVG_FILE, SVGZ_FILE, SVG_GZ_FILE, GIF_ANIM_FILE, DICOM_FILE, \
     BMP_FILE, NIFTI_FILE, ICO_FILE, TGA_FILE, SGI_FILE, AVIF_FILE, \
@@ -1012,6 +1012,32 @@ class TestForeign:
         x2 = pyvips.Image.new_from_file(GIF_ANIM_DISPOSE_PREVIOUS_EXPECTED_PNG_FILE)
         assert (x1 - x2).abs().max() == 0
 
+    @skip_if_no("gifload")
+    def test_gifload_truncated(self):
+        # should load with just a warning
+        truncated_gif = os.path.join(IMAGES, "truncated.gif")
+        im = pyvips.Image.new_from_file(truncated_gif)
+        assert im.width == 575
+
+        # should fail on truncation and warning
+        with pytest.raises(Exception):
+            im = pyvips.Image.new_from_file(truncated_gif, fail_on="warning")
+        with pytest.raises(Exception):
+            im = pyvips.Image.new_from_file(truncated_gif, fail_on="truncated")
+
+    @skip_if_no("gifload")
+    def test_gifload_frame_error(self):
+        # should load with just a warning
+        truncated_gif = os.path.join(IMAGES, "garden.gif")
+        im = pyvips.Image.new_from_file(truncated_gif)
+        assert im.width == 800
+
+        # should fail on warning only
+        im = pyvips.Image.new_from_file(truncated_gif, fail_on="truncated")
+        assert im.width == 800
+        with pytest.raises(Exception):
+            im = pyvips.Image.new_from_file(truncated_gif, fail_on="warning")
+
     @skip_if_no("svgload")
     def test_svgload(self):
         def svg_valid(im):