the definitive tun offloads branch (#1704)

overlay/tio/blockon_linux.go

@@ -0,0 +1,44 @@
+//go:build linux && !android
+// +build linux,!android
+
+package tio
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// blockOn parks the calling goroutine until fd is ready or shutdownFd signals teardown.
+// (events is POLLIN for reads, POLLOUT for writes)
+// It builds the pollfd array on the stack every call, so concurrent callers on the same Queue never share Revents storage.
+//
+// Returns os.ErrClosed when shutdown was signaled (POLLIN on shutdownFd)
+// or either fd reported a problem condition (POLLHUP|POLLNVAL|POLLERR).
+func blockOn(fd, shutdownFd int32, events int16) error {
+	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
+	pfds := [2]unix.PollFd{
+		{Fd: fd, Events: events},
+		{Fd: shutdownFd, Events: unix.POLLIN},
+	}
+	var err error
+	for {
+		_, err = unix.Poll(pfds[:], -1)
+		if err != unix.EINTR {
+			break
+		}
+	}
+	tunEvents := pfds[0].Revents
+	shutdownEvents := pfds[1].Revents
+	// Check err before trusting the potentially bogus bits we just got.
+	if err != nil {
+		return err
+	}
+	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
+		return os.ErrClosed
+	}
+	if tunEvents&problemFlags != 0 {
+		return os.ErrClosed
+	}
+	return nil
+}