Reject font values that are invalid for pango

Use pango to parse font configuration early, and reject the command as
invalid if the value is invalid for pango. Since we're already parsing
the font into a `PangoFontDescription`, keep that instance around and
avoid re-parsing the font each time we render text.

Fixes: https://github.com/swaywm/sway/issues/6805

2 files changed, 25 insertions, 1 deletions

diff --git a/sway/commands/font.c b/sway/commands/font.c
index 3eda0a9c..74bb6b9f 100644
--- a/sway/commands/font.c
+++ b/sway/commands/font.c
@@ -4,6 +4,7 @@
 #include "sway/config.h"
 #include "log.h"
 #include "stringop.h"
+#include <pango/pangocairo.h>
 
 struct cmd_results *cmd_font(int argc, char **argv) {
 	struct cmd_results *error = NULL;
@@ -22,6 +23,28 @@ struct cmd_results *cmd_font(int argc, char **argv) {
 		config->font = font;
 	}
 
+	// Parse the font early so we can reject it if it's not valid for pango.
+	// Also avoids re-parsing each time we render text.
+	PangoFontDescription *font_description = pango_font_description_from_string(config->font);
+
+	const char *family = pango_font_description_get_family(font_description);
+	if (family == NULL) {
+		pango_font_description_free(font_description);
+		return cmd_results_new(CMD_FAILURE, "Invalid font family.");
+	}
+
+	const gint size = pango_font_description_get_size(font_description);
+	if (size == 0) {
+		pango_font_description_free(font_description);
+		return cmd_results_new(CMD_FAILURE, "Invalid font size.");
+	}
+
+	if (config->font_description != NULL) {
+		pango_font_description_free(config->font_description);
+	}
+
+	config->font_description = font_description;
 	config_update_font_height();
+
 	return cmd_results_new(CMD_SUCCESS, NULL);
 }
diff --git a/sway/config.c b/sway/config.c
index 8220ece0..b41dd871 100644
--- a/sway/config.c
+++ b/sway/config.c
@@ -243,6 +243,7 @@ static void config_defaults(struct sway_config *config) {
 	config->default_layout = L_NONE;
 	config->default_orientation = L_NONE;
 	if (!(config->font = strdup("monospace 10"))) goto cleanup;
+	config->font_description = pango_font_description_from_string(config->font);
 	config->urgent_timeout = 500;
 	config->focus_on_window_activation = FOWA_URGENT;
 	config->popup_during_fullscreen = POPUP_SMART;
@@ -1006,7 +1007,7 @@ int workspace_output_cmp_workspace(const void *a, const void *b) {
 void config_update_font_height(void) {
 	int prev_max_height = config->font_height;
 
-	get_text_metrics(config->font, &config->font_height, &config->font_baseline);
+	get_text_metrics(config->font_description, &config->font_height, &config->font_baseline);
 
 	if (config->font_height != prev_max_height) {
 		arrange_root();