io: always cleanup AsyncFd registration list on deregister

tokio/src/runtime/io/driver.rs

@@ -296,7 +296,8 @@ impl Handle {
         source: &mut impl Source,
     ) -> io::Result<()> {
         // Deregister the source with the OS poller **first**
-        self.registry.deregister(source)?;
+        // Cleanup ALWAYS happens
+        let os_result = self.registry.deregister(source);
 
         if self
             .registrations
@@ -307,7 +308,7 @@ impl Handle {
 
         self.metrics.dec_fd_count();
 
-        Ok(())
+        os_result // Return error after cleanup
     }
 
     fn release_pending_registrations(&self) {

tokio/tests/io_async_fd_memory_leak.rs

@@ -0,0 +1,188 @@
+//! Regression test for issue #7563 - Memory leak when fd closed before AsyncFd drop
+//!
+//! This test uses a custom global allocator to track actual memory usage,
+//! avoiding false positives from RSS measurements which include freed-but-retained memory.
+
+#![cfg(all(unix, target_os = "linux"))]
+
+use std::alloc::{GlobalAlloc, Layout, System};
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+/// A tracking allocator that counts bytes currently allocated
+struct TrackingAllocator;
+
+static ALLOCATED: AtomicUsize = AtomicUsize::new(0);
+
+unsafe impl GlobalAlloc for TrackingAllocator {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        let ptr = unsafe { System.alloc(layout) };
+        if !ptr.is_null() {
+            ALLOCATED.fetch_add(layout.size(), Ordering::Relaxed);
+        }
+        ptr
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        unsafe { System.dealloc(ptr, layout) };
+        ALLOCATED.fetch_sub(layout.size(), Ordering::Relaxed);
+    }
+
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+        let new_ptr = unsafe { System.realloc(ptr, layout, new_size) };
+        if !new_ptr.is_null() {
+            // Subtract old size, add new size
+            if new_size > layout.size() {
+                ALLOCATED.fetch_add(new_size - layout.size(), Ordering::Relaxed);
+            } else {
+                ALLOCATED.fetch_sub(layout.size() - new_size, Ordering::Relaxed);
+            }
+        }
+        new_ptr
+    }
+}
+
+#[global_allocator]
+static GLOBAL: TrackingAllocator = TrackingAllocator;
+
+fn allocated_bytes() -> usize {
+    ALLOCATED.load(Ordering::Relaxed)
+}
+
+#[tokio::test]
+async fn memory_leak_when_fd_closed_before_drop() {
+    use nix::sys::socket::{self, AddressFamily, SockFlag, SockType};
+    use std::os::unix::io::{AsRawFd, RawFd};
+    use std::sync::Arc;
+    use tokio::io::unix::AsyncFd;
+
+    struct RawFdWrapper {
+        fd: RawFd,
+    }
+
+    impl AsRawFd for RawFdWrapper {
+        fn as_raw_fd(&self) -> RawFd {
+            self.fd
+        }
+    }
+
+    struct ArcFd(Arc<RawFdWrapper>);
+
+    impl AsRawFd for ArcFd {
+        fn as_raw_fd(&self) -> RawFd {
+            self.0.as_raw_fd()
+        }
+    }
+
+    fn set_nonblocking(fd: RawFd) {
+        unsafe {
+            let flags = libc::fcntl(fd, libc::F_GETFL);
+            libc::fcntl(fd, libc::F_SETFL, flags | libc::O_NONBLOCK);
+        }
+    }
+
+    // Warm up - let runtime and allocator stabilize
+    for _ in 0..100 {
+        tokio::task::yield_now().await;
+    }
+
+    const ITERATIONS: usize = 1000;
+
+    // Phase 1: Warm up allocations
+    for _ in 0..ITERATIONS {
+        let (fd_a, _fd_b) = socket::socketpair(
+            AddressFamily::Unix,
+            SockType::Stream,
+            None,
+            SockFlag::empty(),
+        )
+        .unwrap();
+
+        let raw_fd = fd_a.as_raw_fd();
+        set_nonblocking(raw_fd);
+        std::mem::forget(fd_a);
+
+        let wrapper = Arc::new(RawFdWrapper { fd: raw_fd });
+        let async_fd = AsyncFd::new(ArcFd(wrapper)).unwrap();
+
+        // Close fd before dropping AsyncFd - this triggers the bug
+        unsafe {
+            libc::close(raw_fd);
+        }
+
+        drop(async_fd);
+    }
+
+    // Let things settle
+    tokio::task::yield_now().await;
+    let baseline = allocated_bytes();
+
+    // Phase 2: Run more iterations and check for growth
+    for _ in 0..ITERATIONS {
+        let (fd_a, _fd_b) = socket::socketpair(
+            AddressFamily::Unix,
+            SockType::Stream,
+            None,
+            SockFlag::empty(),
+        )
+        .unwrap();
+
+        let raw_fd = fd_a.as_raw_fd();
+        set_nonblocking(raw_fd);
+        std::mem::forget(fd_a);
+
+        let wrapper = Arc::new(RawFdWrapper { fd: raw_fd });
+        let async_fd = AsyncFd::new(ArcFd(wrapper)).unwrap();
+
+        unsafe {
+            libc::close(raw_fd);
+        }
+
+        drop(async_fd);
+    }
+
+    tokio::task::yield_now().await;
+    let after_phase2 = allocated_bytes();
+
+    // Phase 3: Run even more iterations
+    for _ in 0..ITERATIONS {
+        let (fd_a, _fd_b) = socket::socketpair(
+            AddressFamily::Unix,
+            SockType::Stream,
+            None,
+            SockFlag::empty(),
+        )
+        .unwrap();
+
+        let raw_fd = fd_a.as_raw_fd();
+        set_nonblocking(raw_fd);
+        std::mem::forget(fd_a);
+
+        let wrapper = Arc::new(RawFdWrapper { fd: raw_fd });
+        let async_fd = AsyncFd::new(ArcFd(wrapper)).unwrap();
+
+        unsafe {
+            libc::close(raw_fd);
+        }
+
+        drop(async_fd);
+    }
+
+    tokio::task::yield_now().await;
+    let after_phase3 = allocated_bytes();
+
+    let growth_phase2 = after_phase2.saturating_sub(baseline);
+    let growth_phase3 = after_phase3.saturating_sub(after_phase2);
+
+    // If there's a leak, each phase adds ~250KB (1000 * ~256 bytes per ScheduledIo)
+    // If fixed, memory should stabilize (minimal growth between phases)
+    // Allow 64KB tolerance for normal allocation variance
+    let threshold = 64 * 1024; // 64KB
+
+    assert!(
+        growth_phase2 < threshold || growth_phase3 < threshold,
+        "Memory leak detected: allocations keep growing without stabilizing. \
+         Phase 1->2: +{growth_phase2} bytes, Phase 2->3: +{growth_phase3} bytes. \
+         (baseline: {baseline} bytes, phase2: {after_phase2} bytes, phase3: {after_phase3} bytes). \
+         Expected at least one phase with <{threshold} bytes growth.",
+    );
+}