From 74740da17c8aa3acfd521fbd9d377000d4d94626 Mon Sep 17 00:00:00 2001
From: Christian Hergert <chergert@redhat.com>
Date: Wed, 6 Mar 2024 13:52:01 -0800
Subject: [PATCH] glib/gmain: use ppoll() when possible

If our GPollFunc is set to g_poll() then we can optionally use a poll()
alternative with higher precision. ppoll() provides poll() equivalence
but with timeouts in nanoseconds.

With more precise polling timouts, frame clocks and other timing sensitive
APIs are not restricted to a minimum of 1 millisecond timeout.
---
 glib/gmain.c | 23 +++++++++++++++++++++++
 meson.build  | 13 +++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/glib/gmain.c b/glib/gmain.c
index fe735a656..af343c9ca 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -69,6 +69,10 @@
 #include <errno.h>
 #include <string.h>
 
+#ifdef HAVE_POLL_H
+#include <poll.h>
+#endif
+
 #ifdef HAVE_PIDFD
 #include <sys/syscall.h>
 #include <sys/wait.h>
@@ -4548,6 +4552,25 @@ g_main_context_poll_unlocked (GMainContext *context,
 #endif
       poll_func = context->poll_func;
 
+#if defined(HAVE_PPOLL) && defined(HAVE_POLL)
+      if (poll_func == g_poll)
+        {
+          struct timespec spec;
+          struct timespec *spec_p = NULL;
+
+          if (timeout_usec > -1)
+            {
+              spec.tv_sec = timeout_usec / G_USEC_PER_SEC;
+              spec.tv_nsec = (timeout_usec % G_USEC_PER_SEC) * 1000L;
+              spec_p = &spec;
+            }
+
+          UNLOCK_CONTEXT (context);
+          ret = ppoll ((struct pollfd *)fds, n_fds, spec_p, NULL);
+          LOCK_CONTEXT (context);
+        }
+      else
+#endif
         {
           int timeout_msec = round_timeout_to_msec (timeout_usec);
 
diff --git a/meson.build b/meson.build
index 753454209..0ba7ca131 100644
--- a/meson.build
+++ b/meson.build
@@ -1008,6 +1008,19 @@ if cc.links('''#include <sys/eventfd.h>
   glib_conf.set('HAVE_EVENTFD', 1)
 endif
 
+# Check for ppoll(2)
+if cc.links('''#define _GNU_SOURCE
+               #include <poll.h>
+               #include <stddef.h>
+               int main (int argc, char ** argv) {
+                 struct pollfd fds[1] = {{0}};
+                 struct timespec ts = {0};
+                 ppoll (fds, 1, NULL, NULL);
+                 return 0;
+               }''', name : 'ppoll(2) system call')
+  glib_conf.set('HAVE_PPOLL', 1)
+endif
+
 # Check for pidfd_open(2)
 if cc.links('''#include <sys/syscall.h>
                #include <sys/wait.h>