Implement `fcntl` options `F_GETSIG` and `F_SETSIG`.

These options allow overriding the signal that gets sent to the process when
I/O operations are available on the file descriptor, rather than the default
`SIGIO` signal. Doing so also populates `siginfo` to contain extra information
about which file descriptor caused the event (`si_fd`) and what events happened
on it (`si_band`). The logic around which FD is populated within `si_fd`
matches Linux's, which means it has some weird edge cases where that value may
not actually refer to a file descriptor that is still valid.

This CL also ports extra S/R logic regarding async handler in VFS2.
Without this, async I/O handlers aren't properly re-registered after S/R.

PiperOrigin-RevId: 345436598

2 files changed, 20 insertions, 11 deletions

diff --git a/pkg/waiter/waiter.go b/pkg/waiter/waiter.go
index 08519d986..83d4f893a 100644
--- a/pkg/waiter/waiter.go
+++ b/pkg/waiter/waiter.go
@@ -119,7 +119,10 @@ type EntryCallback interface {
 	// The callback is supposed to perform minimal work, and cannot call
 	// any method on the queue itself because it will be locked while the
 	// callback is running.
-	Callback(e *Entry)
+	//
+	// The mask indicates the events that occurred and that the entry is
+	// interested in.
+	Callback(e *Entry, mask EventMask)
 }
 
 // Entry represents a waiter that can be add to the a wait queue. It can
@@ -140,7 +143,7 @@ type channelCallback struct {
 }
 
 // Callback implements EntryCallback.Callback.
-func (c *channelCallback) Callback(*Entry) {
+func (c *channelCallback) Callback(*Entry, EventMask) {
 	select {
 	case c.ch <- struct{}{}:
 	default:
@@ -193,8 +196,8 @@ func (q *Queue) EventUnregister(e *Entry) {
 func (q *Queue) Notify(mask EventMask) {
 	q.mu.RLock()
 	for e := q.list.Front(); e != nil; e = e.Next() {
-		if mask&e.mask != 0 {
-			e.Callback.Callback(e)
+		if m := mask & e.mask; m != 0 {
+			e.Callback.Callback(e, m)
 		}
 	}
 	q.mu.RUnlock()
diff --git a/pkg/waiter/waiter_test.go b/pkg/waiter/waiter_test.go
index c1b94a4f3..6928f28b4 100644
--- a/pkg/waiter/waiter_test.go
+++ b/pkg/waiter/waiter_test.go
@@ -20,12 +20,12 @@ import (
 )
 
 type callbackStub struct {
-	f func(e *Entry)
+	f func(e *Entry, m EventMask)
 }
 
 // Callback implements EntryCallback.Callback.
-func (c *callbackStub) Callback(e *Entry) {
-	c.f(e)
+func (c *callbackStub) Callback(e *Entry, m EventMask) {
+	c.f(e, m)
 }
 
 func TestEmptyQueue(t *testing.T) {
@@ -36,7 +36,7 @@ func TestEmptyQueue(t *testing.T) {
 
 	// Register then unregister a waiter, then notify the queue.
 	cnt := 0
-	e := Entry{Callback: &callbackStub{func(*Entry) { cnt++ }}}
+	e := Entry{Callback: &callbackStub{func(*Entry, EventMask) { cnt++ }}}
 	q.EventRegister(&e, EventIn)
 	q.EventUnregister(&e)
 	q.Notify(EventIn)
@@ -49,7 +49,7 @@ func TestMask(t *testing.T) {
 	// Register a waiter.
 	var q Queue
 	var cnt int
-	e := Entry{Callback: &callbackStub{func(*Entry) { cnt++ }}}
+	e := Entry{Callback: &callbackStub{func(*Entry, EventMask) { cnt++ }}}
 	q.EventRegister(&e, EventIn|EventErr)
 
 	// Notify with an overlapping mask.
@@ -101,11 +101,14 @@ func TestConcurrentRegistration(t *testing.T) {
 	for i := 0; i < concurrency; i++ {
 		go func() {
 			var e Entry
-			e.Callback = &callbackStub{func(entry *Entry) {
+			e.Callback = &callbackStub{func(entry *Entry, mask EventMask) {
 				cnt++
 				if entry != &e {
 					t.Errorf("entry = %p, want %p", entry, &e)
 				}
+				if mask != EventIn {
+					t.Errorf("mask = %#x want %#x", mask, EventIn)
+				}
 			}}
 
 			// Wait for notification, then register.
@@ -158,11 +161,14 @@ func TestConcurrentNotification(t *testing.T) {
 	// Register waiters.
 	for i := 0; i < waiterCount; i++ {
 		var e Entry
-		e.Callback = &callbackStub{func(entry *Entry) {
+		e.Callback = &callbackStub{func(entry *Entry, mask EventMask) {
 			atomic.AddInt32(&cnt, 1)
 			if entry != &e {
 				t.Errorf("entry = %p, want %p", entry, &e)
 			}
+			if mask != EventIn {
+				t.Errorf("mask = %#x want %#x", mask, EventIn)
+			}
 		}}
 
 		q.EventRegister(&e, EventIn|EventErr)