Try not to thumbnail FIFOs

Unless there is a writer, this may block forever.

And if there is one, we're somewhat likely to break something.

fiv-io.c

@@ -1775,7 +1775,7 @@ open_libraw(const char *data, gsize len, GError **error)
 
 	int width = image->width, height = image->height;
 	cairo_surface_t *surface =
-		cairo_image_surface_create(CAIRO_FORMAT_ARGB32, width, height);
+		cairo_image_surface_create(CAIRO_FORMAT_RGB24, width, height);
 	cairo_status_t surface_status = cairo_surface_status(surface);
 	if (surface_status != CAIRO_STATUS_SUCCESS) {
 		set_error(error, cairo_status_to_string(surface_status));

fiv-thumbnail.c

@@ -281,6 +281,12 @@ fiv_thumbnail_extract(GFile *target, FivThumbnailSize max_size, GError **error)
 	// TODO(p): Implement our own thumbnail extractors.
 	set_error(error, "unsupported file");
 #else  // HAVE_LIBRAW
+	// In this case, g_mapped_file_get_contents() returns NULL, causing issues.
+	if (!g_mapped_file_get_length(mf)) {
+		set_error(error, "empty file");
+		goto fail;
+	}
+
 	libraw_data_t *iprc = libraw_init(
 		LIBRAW_OPIONS_NO_MEMERR_CALLBACK | LIBRAW_OPIONS_NO_DATAERR_CALLBACK);
 	if (!iprc) {
@@ -290,12 +296,42 @@ fiv_thumbnail_extract(GFile *target, FivThumbnailSize max_size, GError **error)
 
 	int err = 0;
 	if ((err = libraw_open_buffer(iprc, (void *) g_mapped_file_get_contents(mf),
-			 g_mapped_file_get_length(mf))) ||
-		(err = libraw_unpack_thumb(iprc))) {
+			 g_mapped_file_get_length(mf)))) {
 		set_error(error, libraw_strerror(err));
 		goto fail_libraw;
 	}
 
+#if LIBRAW_VERSION >= LIBRAW_MAKE_VERSION(0, 21, 0)
+	if (!iprc->thumbs_list.thumbcount) {
+		set_error(error, "no thumbnails found");
+		goto fail_libraw;
+	}
+
+	// The old libraw_unpack_thumb() goes for the largest thumbnail,
+	// but we currently want the smallest thumbnail.
+	// TODO(p): To handle the ugly IFD0 thumbnail of NEF,
+	// try to go for the second smallest size. Remember to reflect tflip.
+	int best_index = 0;
+	float best_pixels = INFINITY;
+	for (int i = 0; i < iprc->thumbs_list.thumbcount; i++) {
+		float pixels = (float) iprc->thumbs_list.thumblist[i].twidth *
+			(float) iprc->thumbs_list.thumblist[i].theight;
+		if (pixels && pixels < best_pixels) {
+			best_index = i;
+			best_pixels = pixels;
+		}
+	}
+	if ((err = libraw_unpack_thumb_ex(iprc, best_index))) {
+		set_error(error, libraw_strerror(err));
+		goto fail_libraw;
+	}
+#else
+	if ((err = libraw_unpack_thumb(iprc))) {
+		set_error(error, libraw_strerror(err));
+		goto fail_libraw;
+	}
+#endif
+
 	libraw_processed_image_t *image = libraw_dcraw_make_mem_thumb(iprc, &err);
 	if (!image) {
 		set_error(error, libraw_strerror(err));
@@ -447,6 +483,7 @@ fiv_thumbnail_produce_for_search(
 static cairo_surface_t *
 produce_fallback(GFile *target, FivThumbnailSize size, GError **error)
 {
+	// Note that this comes with a TOCTTOU problem.
 	goffset filesize = 0;
 	GFileInfo *info = g_file_query_info(target,
 		G_FILE_ATTRIBUTE_STANDARD_NAME "," G_FILE_ATTRIBUTE_STANDARD_SIZE,
@@ -497,6 +534,13 @@ fiv_thumbnail_produce(GFile *target, FivThumbnailSize max_size, GError **error)
 		return NULL;
 	}
 
+	// TODO(p): Use open(O_RDONLY | O_NONBLOCK | _O_BINARY), fstat(),
+	// g_mapped_file_new_from_fd(), and reset the non-blocking flag on the file.
+	if (!S_ISREG(st.st_mode)) {
+		set_error(error, "not a regular file");
+		return NULL;
+	}
+
 	GError *e = NULL;
 	GMappedFile *mf = g_mapped_file_new(path, FALSE, &e);
 	if (!mf) {
@@ -505,7 +549,13 @@ fiv_thumbnail_produce(GFile *target, FivThumbnailSize max_size, GError **error)
 		return produce_fallback(target, max_size, error);
 	}
 
+	// In this case, g_mapped_file_get_bytes() has NULL data, causing issues.
 	gsize filesize = g_mapped_file_get_length(mf);
+	if (!filesize) {
+		set_error(error, "empty file");
+		return NULL;
+	}
+
 	gboolean color_managed = FALSE;
 	cairo_surface_t *surface =
 		render(target, g_mapped_file_get_bytes(mf), &color_managed, error);