virtio-devices: Test activator failure releases barrier

Feed BadActivate into VirtioPciDeviceActivator and assert that the
error propagates, device_activated stays false, DEVICE_NEEDS_RESET is
set in status, a single Config interrupt is delivered, and a thread
waiting on the activation barrier unblocks. The barrier release is
the deadlock fixed by the NEEDS_RESET on activation failure change.

Assisted-by: Claude:Opus-4.7
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

virtio-devices/src/transport/pci_device.rs

@@ -1365,6 +1365,7 @@ mod unit_tests {
     use vm_device::interrupt::InterruptSourceConfig;
 
     use super::*;
+    use crate::{ActivateError, DEVICE_NEEDS_RESET};
 
     struct TestInterruptSourceGroup {
         event_fd: EventFd,
@@ -1594,4 +1595,30 @@ mod unit_tests {
         };
         (activator, status, device_activated, interrupt, barrier)
     }
+
+    #[test]
+    fn activate_failure_marks_needs_reset_and_releases_barrier() {
+        let (activator, status, device_activated, interrupt, barrier) =
+            make_activator(Err(ActivateError::BadActivate));
+
+        // Simulate the vCPU thread blocked on the activation
+        // barrier after writing DRIVER_OK.
+        let waiter = std::thread::spawn(move || barrier.wait());
+
+        let result = activator.activate();
+
+        assert!(matches!(result, Err(ActivateError::BadActivate)));
+        assert!(!device_activated.load(Ordering::SeqCst));
+        assert_ne!(
+            status.load(Ordering::SeqCst) & (DEVICE_NEEDS_RESET as u8),
+            0
+        );
+        let triggers = interrupt.triggers.lock().unwrap();
+        assert_eq!(triggers.len(), 1);
+        assert!(matches!(triggers[0], VirtioInterruptType::Config));
+
+        // The barrier waiter must complete, showing the activator
+        // did not deadlock the vCPU thread.
+        waiter.join().expect("barrier waiter deadlocked");
+    }
 }