Fix unix changing from real-time to normal policy (#39)

* Adds a failing unit test for unix scheduling policy

No effect when changing from real-time to normal policy.

* Fixes behavior when changing from real-time to normal policy

src/unix.rs

@@ -596,19 +596,29 @@ pub fn set_thread_priority_and_policy(
                     e => Err(Error::OS(e)),
                 }
             } else {
-                // If this is a normal-scheduled thread, the priority is
-                // set via niceness.
-                set_errno(0);
+                // Normal priority threads must be set with static priority 0.
+                let params = ScheduleParams { sched_priority: 0 }.into_posix();
 
-                let ret = unsafe { libc::setpriority(libc::PRIO_PROCESS, 0, fixed_priority) };
-                if ret == 0 {
-                    return Ok(());
+                let ret = unsafe {
+                    libc::pthread_setschedparam(
+                        native,
+                        policy.to_posix(),
+                        &params as *const libc::sched_param,
+                    )
+                };
+
+                if ret != 0 {
+                    return Err(Error::OS(ret));
                 }
 
-                match errno() {
-                    0 => Ok(()),
-                    e => Err(Error::OS(e)),
+                // Normal priority threads adjust relative priority through niceness.
+                set_errno(0);
+                let ret = unsafe { libc::setpriority(libc::PRIO_PROCESS, 0, fixed_priority) };
+                if ret != 0 {
+                    return Err(Error::OS(errno()));
                 }
+
+                Ok(())
             }
         }
     }
@@ -820,6 +830,44 @@ mod tests {
         assert!(thread_schedule_policy_param(thread_id).is_ok());
     }
 
+    // Running this test requires CAP_SYS_NICE.
+    #[test]
+    fn change_between_realtime_and_normal_policies_requires_capabilities() {
+        use crate::ThreadPriorityOsValue;
+
+        const TEST_PRIORITY: u8 = 15;
+
+        let realtime_policy = ThreadSchedulePolicy::Realtime(RealtimeThreadSchedulePolicy::Fifo);
+        let normal_policy = ThreadSchedulePolicy::Normal(NormalThreadSchedulePolicy::Other);
+
+        // While we may desire an OS-specific priority, the reported value is always crossplatform.
+        let desired_priority = ThreadPriority::Os(ThreadPriorityOsValue(TEST_PRIORITY as _));
+        let expected_priority = ThreadPriority::Crossplatform(ThreadPriorityValue(TEST_PRIORITY));
+
+        let thread = std::thread::current();
+        thread
+            .set_priority_and_policy(realtime_policy, desired_priority)
+            .expect("to set realtime fifo policy");
+
+        assert_eq!(thread.get_schedule_policy(), Ok(realtime_policy));
+        assert_eq!(thread.get_priority(), Ok(expected_priority));
+
+        thread
+            .set_priority_and_policy(normal_policy, desired_priority)
+            .expect("to set normal other policy");
+
+        assert_eq!(thread.get_schedule_policy(), Ok(normal_policy));
+
+        // On linux, normal priority threads always have static priority 0. Instead the "nice" value is used.
+        #[cfg(not(target_os = "linux"))]
+        assert_eq!(thread.get_priority(), Ok(expected_priority));
+        #[cfg(target_os = "linux")]
+        {
+            let nice = unsafe { libc::getpriority(0, 0) };
+            assert_eq!(nice, TEST_PRIORITY as i32);
+        }
+    }
+
     #[test]
     #[cfg(target_os = "linux")]
     fn set_deadline_policy() {