compositor: set scheduler to SCHED_RR before dropping CAP_SYS_NICE

src/utils/nice.rs

@@ -0,0 +1,40 @@
+use {
+    crate::{compositor::config_dir, config::have_config_so},
+    c::sched_setscheduler,
+    std::{
+        env, mem,
+        sync::atomic::{AtomicBool, Ordering::Relaxed},
+    },
+    uapi::c::{self, SCHED_RESET_ON_FORK, SCHED_RR, sched_param},
+};
+
+static DID_ELEVATE_SCHEDULER: AtomicBool = AtomicBool::new(false);
+
+pub const JAY_NO_REALTIME: &str = "JAY_NO_REALTIME";
+
+pub fn elevate_scheduler() {
+    if env::var(JAY_NO_REALTIME).as_deref().unwrap_or_default() == "1" {
+        return;
+    }
+    if have_config_so(config_dir().as_deref()) && dont_allow_realtime_config_so() {
+        return;
+    }
+    let mut param = unsafe { mem::zeroed::<sched_param>() };
+    param.sched_priority = 1;
+    let res = unsafe { sched_setscheduler(0, SCHED_RR | SCHED_RESET_ON_FORK, &param) };
+    if res == 0 {
+        DID_ELEVATE_SCHEDULER.store(true, Relaxed);
+    }
+}
+
+pub fn did_elevate_scheduler() -> bool {
+    DID_ELEVATE_SCHEDULER.load(Relaxed)
+}
+
+fn dont_allow_realtime_config_so() -> bool {
+    option_env!(jay_allow_realtime_config_so!()).unwrap_or_default() != "1"
+}
+
+pub fn dont_allow_config_so() -> bool {
+    did_elevate_scheduler() && dont_allow_realtime_config_so()
+}