Deflake SendFileTest_Shutdown.

The sendfile syscall's backing doSplice contained a race with regard to
blocking. If the first attempt failed with syserror.ErrWouldBlock and then
the blocking file became ready before registering a waiter, we would just
return the ErrWouldBlock (even if we were supposed to block).

PiperOrigin-RevId: 254114432

pkg/sentry/syscalls/linux/sys_splice.go

@@ -48,12 +48,12 @@ func doSplice(t *kernel.Task, outFile, inFile *fs.File, opts fs.SpliceOpts, nonB
 		if ch == nil {
 			ch = make(chan struct{}, 1)
 		}
-		if !inW && inFile.Readiness(EventMaskRead) == 0 && !inFile.Flags().NonBlocking {
+		if !inW && !inFile.Flags().NonBlocking {
 			w, _ := waiter.NewChannelEntry(ch)
 			inFile.EventRegister(&w, EventMaskRead)
 			defer inFile.EventUnregister(&w)
 			inW = true // Registered.
-		} else if !outW && outFile.Readiness(EventMaskWrite) == 0 && !outFile.Flags().NonBlocking {
+		} else if !outW && !outFile.Flags().NonBlocking {
 			w, _ := waiter.NewChannelEntry(ch)
 			outFile.EventRegister(&w, EventMaskWrite)
 			defer outFile.EventUnregister(&w)
@@ -65,6 +65,11 @@ func doSplice(t *kernel.Task, outFile, inFile *fs.File, opts fs.SpliceOpts, nonB
 			break
 		}
 
+		if (!inW || inFile.Readiness(EventMaskRead) != 0) && (!outW || outFile.Readiness(EventMaskWrite) != 0) {
+			// Something became ready, try again without blocking.
+			continue
+		}
+
 		// Block until there's data.
 		if err = t.Block(ch); err != nil {
 			break