Add timespec helpers

Borrow timespec helpers from Weston [1]. We'll re-use these helpers for
adaptive sync in a future commit.

Rename refresh_nsec to next_refresh_nsec, because this value only
applies to the next output presentation.

[1]: 467e6b9883/shared/timespec-util.h

common/util.c

@@ -7,6 +7,7 @@
 #include <string.h>
 #include <strings.h>
 #include <wayland-server-protocol.h>
+#include <time.h>
 #include "log.h"
 #include "util.h"
 
@@ -102,3 +103,40 @@ bool sway_set_cloexec(int fd, bool cloexec) {
 	}
 	return true;
 }
+
+static const int64_t NSEC_PER_SEC = 1000000000;
+
+void timespec_sub(struct timespec *r, const struct timespec *a,
+		const struct timespec *b) {
+	r->tv_sec = a->tv_sec - b->tv_sec;
+	r->tv_nsec = a->tv_nsec - b->tv_nsec;
+	if (r->tv_nsec < 0) {
+		r->tv_sec--;
+		r->tv_nsec += NSEC_PER_SEC;
+	}
+}
+
+int64_t timespec_sub_to_nsec(const struct timespec *a,
+		const struct timespec *b) {
+	struct timespec r;
+	timespec_sub(&r, a, b);
+	return timespec_to_nsec(&r);
+}
+
+void timespec_add_nsec(struct timespec *r, const struct timespec *a,
+		int64_t nsec) {
+	r->tv_sec = a->tv_sec + (nsec / NSEC_PER_SEC);
+	r->tv_nsec = a->tv_nsec + (nsec % NSEC_PER_SEC);
+
+	if (r->tv_nsec >= NSEC_PER_SEC) {
+		r->tv_sec++;
+		r->tv_nsec -= NSEC_PER_SEC;
+	} else if (r->tv_nsec < 0) {
+		r->tv_sec--;
+		r->tv_nsec += NSEC_PER_SEC;
+	}
+}
+
+int64_t timespec_to_nsec(const struct timespec *t) {
+	return (int64_t)t->tv_sec * NSEC_PER_SEC + t->tv_nsec;
+}

include/sway/output.h

@@ -54,7 +54,7 @@ struct sway_output {
 	} events;
 
 	struct timespec last_presentation;
-	uint32_t refresh_nsec;
+	uint32_t next_refresh_nsec;
 	int max_render_time; // In milliseconds
 	struct wl_event_source *repaint_timer;
 	bool surface_needs_frame;

include/util.h

@@ -37,4 +37,13 @@ const char *sway_wl_output_subpixel_to_string(enum wl_output_subpixel subpixel);
 
 bool sway_set_cloexec(int fd, bool cloexec);
 
+struct timespec;
+
+void timespec_sub(struct timespec *r, const struct timespec *a,
+	const struct timespec *b);
+int64_t timespec_sub_to_nsec(const struct timespec *a, const struct timespec *b);
+void timespec_add_nsec(struct timespec *r, const struct timespec *a,
+	int64_t nsec);
+int64_t timespec_to_nsec(const struct timespec *t);
+
 #endif

sway/desktop/output.c

@@ -29,6 +29,7 @@
 #include "sway/tree/root.h"
 #include "sway/tree/view.h"
 #include "sway/tree/workspace.h"
+#include "util.h"
 
 struct sway_output *output_by_name_or_id(const char *name_or_id) {
 	for (int i = 0; i < root->outputs->length; ++i) {
@@ -590,28 +591,15 @@ static void damage_handle_frame(struct wl_listener *listener, void *user_data) {
 			= wlr_backend_get_presentation_clock(server.backend);
 		clock_gettime(presentation_clock, &now);
 
-		const long NSEC_IN_SECONDS = 1000000000;
-		struct timespec predicted_refresh = output->last_presentation;
-		predicted_refresh.tv_nsec += output->refresh_nsec % NSEC_IN_SECONDS;
-		predicted_refresh.tv_sec += output->refresh_nsec / NSEC_IN_SECONDS;
-		if (predicted_refresh.tv_nsec >= NSEC_IN_SECONDS) {
-			predicted_refresh.tv_sec += 1;
-			predicted_refresh.tv_nsec -= NSEC_IN_SECONDS;
-		}
+		struct timespec predicted_refresh;
+		timespec_add_nsec(&predicted_refresh, &output->last_presentation,
+			output->next_refresh_nsec);
 
 		// If the predicted refresh time is before the current time then
 		// there's no point in delaying.
-		//
-		// We only check tv_sec because if the predicted refresh time is less
-		// than a second before the current time, then msec_until_refresh will
-		// end up slightly below zero, which will effectively disable the delay
-		// without potential disastrous negative overflows that could occur if
-		// tv_sec was not checked.
-		if (predicted_refresh.tv_sec >= now.tv_sec) {
-			long nsec_until_refresh
-				= (predicted_refresh.tv_sec - now.tv_sec) * NSEC_IN_SECONDS
-					+ (predicted_refresh.tv_nsec - now.tv_nsec);
-
+		int64_t nsec_until_refresh =
+			timespec_sub_to_nsec(&predicted_refresh, &now);
+		if (nsec_until_refresh >= 0) {
 			// We want msec_until_refresh to be conservative, that is, floored.
 			// If we have 7.9 msec until refresh, we better compute the delay
 			// as if we had only 7 msec, so that we don't accidentally delay
@@ -862,7 +850,7 @@ static void handle_present(struct wl_listener *listener, void *data) {
 	}
 
 	output->last_presentation = *output_event->when;
-	output->refresh_nsec = output_event->refresh;
+	output->next_refresh_nsec = output_event->refresh;
 }
 
 void handle_new_output(struct wl_listener *listener, void *data) {