ptp-helper: Set thread priority to time-critical on Windows

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3889>

subprojects/gstreamer/libs/gst/helpers/ptp/ffi.rs

@@ -546,6 +546,8 @@ pub mod windows {
     pub const FILE_TYPE_CHAR: u32 = 0x0002;
     pub const FILE_TYPE_PIPE: u32 = 0x0003;
 
+    pub const THREAD_PRIORITY_TIME_CRITICAL: i32 = 15;
+
     #[link(name = "kernel32")]
     extern "system" {
         pub fn GetStdHandle(nstdhandle: i32) -> HANDLE;
@@ -597,6 +599,9 @@ pub mod windows {
             lpmem: *mut c_void,
             dwbytes: usize,
         ) -> *mut c_void;
+
+        pub fn SetThreadPriority(pthread: HANDLE, npriority: i32) -> i32;
+        pub fn GetCurrentThread() -> HANDLE;
     }
 
     pub const BCRYPT_USE_SYSTEM_PREFERRED_RNG: u32 = 0x00000002;

subprojects/gstreamer/libs/gst/helpers/ptp/main.rs

@@ -29,6 +29,7 @@ mod io;
 mod net;
 mod privileges;
 mod rand;
+mod thread;
 
 use error::{Context, Error};
 use rand::rand;
@@ -127,6 +128,8 @@ fn main() -> Result<(), Error> {
     let general_socket =
         create_socket(PTP_GENERAL_PORT).context("Failed creating general socket")?;
 
+    thread::set_priority().context("Failed to set thread priority")?;
+
     privileges::drop().context("Failed dropping privileges")?;
 
     let clock_id = join_multicast(&args, &event_socket, &general_socket)

subprojects/gstreamer/libs/gst/helpers/ptp/thread.rs

@@ -0,0 +1,35 @@
+// GStreamer
+//
+// Copyright (C) 2015-2023 Sebastian Dröge <sebastian@centricular.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla Public License, v2.0.
+// If a copy of the MPL was not distributed with this file, You can obtain one at
+// <https://mozilla.org/MPL/2.0/>.
+//
+// SPDX-License-Identifier: MPL-2.0
+
+use crate::error::Error;
+
+pub fn set_priority() -> Result<(), Error> {
+    #[cfg(windows)]
+    {
+        use std::io;
+
+        use crate::{bail, ffi::windows::*};
+
+        // SAFETY: Getting a handle to the current thread is safe at any time
+        let thread = unsafe { GetCurrentThread() };
+
+        // SAFETY: SetThreadPriority() requires a valid thread handle, which was given above,
+        // and will return 0 on errors.
+        unsafe {
+            if SetThreadPriority(thread, THREAD_PRIORITY_TIME_CRITICAL) == 0 {
+                bail!(
+                    source: io::Error::last_os_error(),
+                    "Failed to set thread priority"
+                );
+            }
+        }
+    }
+    Ok(())
+}