Add S/R support for FIOASYNC

PiperOrigin-RevId: 215655197
Change-Id: I668b1bc7c29daaf2999f8f759138bcbb09c4de6f

pkg/sentry/fs/file.go

@@ -85,6 +85,9 @@ type File struct {
 	// async handles O_ASYNC notifications.
 	async FileAsync
 
+	// saving indicates that this file is in the process of being saved.
+	saving bool `state:"nosave"`
+
 	// mu is dual-purpose: first, to make read(2) and write(2) thread-safe
 	// in conformity with POSIX, and second, to cancel operations before they
 	// begin in response to interruptions (i.e. signals).
@@ -127,10 +130,15 @@ func (f *File) DecRef() {
 		// Release a reference on the Dirent.
 		f.Dirent.DecRef()
 
+		// Only unregister if we are currently registered. There is nothing
+		// to register if f.async is nil (this happens when async mode is
+		// enabled without setting an owner). Also, we unregister during
+		// save.
 		f.flagsMu.Lock()
-		if f.flags.Async && f.async != nil {
+		if !f.saving && f.flags.Async && f.async != nil {
 			f.async.Unregister(f)
 		}
+		f.async = nil
 		f.flagsMu.Unlock()
 	})
 }

pkg/sentry/fs/file_state.go

@@ -14,7 +14,18 @@
 
 package fs
 
+// beforeSave is invoked by stateify.
+func (f *File) beforeSave() {
+	f.saving = true
+	if f.flags.Async && f.async != nil {
+		f.async.Unregister(f)
+	}
+}
+
 // afterLoad is invoked by stateify.
 func (f *File) afterLoad() {
 	f.mu.Init()
+	if f.flags.Async && f.async != nil {
+		f.async.Register(f)
+	}
 }

pkg/sentry/kernel/fasync/fasync.go

@@ -32,8 +32,10 @@ func New() fs.FileAsync {
 }
 
 // FileAsync sends signals when the registered file is ready for IO.
+//
+// +stateify savable
 type FileAsync struct {
-	mu        sync.Mutex
+	mu        sync.Mutex `state:"nosave"`
 	e         waiter.Entry
 	requester *auth.Credentials
 

pkg/waiter/waiter.go

@@ -113,6 +113,8 @@ type EntryCallback interface {
 // Entry represents a waiter that can be add to the a wait queue. It can
 // only be in one queue at a time, and is added "intrusively" to the queue with
 // no extra memory allocations.
+//
+// +stateify savable
 type Entry struct {
 	// Context stores any state the waiter may wish to store in the entry
 	// itself, which may be used at wake up time.