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

common/pango.c

@@ -109,10 +109,9 @@ void get_text_size(cairo_t *cairo, const char *font, int *width, int *height,
 	free(buf);
 }
 
-void get_text_metrics(const char *font, int *height, int *baseline) {
+void get_text_metrics(const PangoFontDescription *description, int *height, int *baseline) {
 	cairo_t *cairo = cairo_create(NULL);
 	PangoContext *pango = pango_cairo_create_context(cairo);
-	PangoFontDescription *description = pango_font_description_from_string(font);
 	// When passing NULL as a language, pango uses the current locale.
 	PangoFontMetrics *metrics = pango_context_get_metrics(pango, description, NULL);
 
@@ -120,7 +119,6 @@ void get_text_metrics(const char *font, int *height, int *baseline) {
 	*height = *baseline + pango_font_metrics_get_descent(metrics) / PANGO_SCALE;
 
 	pango_font_metrics_unref(metrics);
-	pango_font_description_free(description);
 	g_object_unref(pango);
 	cairo_destroy(cairo);
 }

include/pango.h

@@ -17,7 +17,7 @@ PangoLayout *get_pango_layout(cairo_t *cairo, const char *font,
 		const char *text, double scale, bool markup);
 void get_text_size(cairo_t *cairo, const char *font, int *width, int *height,
 		int *baseline, double scale, bool markup, const char *fmt, ...);
-void get_text_metrics(const char *font, int *height, int *baseline);
+void get_text_metrics(const PangoFontDescription *font, int *height, int *baseline);
 void render_text(cairo_t *cairo, const char *font,
 		double scale, bool markup, const char *fmt, ...);
 

include/sway/config.h

@@ -17,6 +17,7 @@
 #include "sway/input/tablet.h"
 #include "sway/tree/root.h"
 #include "wlr-layer-shell-unstable-v1-protocol.h"
+#include <pango/pangocairo.h>
 
 // TODO: Refactor this shit
 
@@ -504,7 +505,8 @@ struct sway_config {
 	char *floating_scroll_right_cmd;
 	enum sway_container_layout default_orientation;
 	enum sway_container_layout default_layout;
-	char *font;
+	char *font; // Use mostly for IPC.
+	PangoFontDescription *font_description; // Used internally for rendering and validating.
 	int font_height;
 	int font_baseline;
 	bool pango_markup;

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);
 }

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();