Add the context.PrepareSleep platform callback.

It is called only when a task switches to the interruptible sleep, because it
is the only state where the task can sleep for a long time.

PiperOrigin-RevId: 485963460

pkg/sentry/kernel/task_block.go

@@ -189,6 +189,7 @@ func (t *Task) block(C <-chan struct{}, timerChan <-chan struct{}) error {
 // prepareSleep prepares to sleep.
 func (t *Task) prepareSleep() {
 	t.assertTaskGoroutine()
+	t.p.PrepareSleep()
 	t.Deactivate()
 	t.accountTaskGoroutineEnter(TaskGoroutineBlockedInterruptible)
 }

pkg/sentry/platform/kvm/context.go

@@ -126,3 +126,6 @@ func (c *context) FullStateChanged() {}
 
 // PullFullState implements platform.Context.PullFullState.
 func (c *context) PullFullState(as platform.AddressSpace, ac *arch.Context64) {}
+
+// PrepareSleep implements platform.Context.platform.Context.
+func (*context) PrepareSleep() {}

pkg/sentry/platform/platform.go

@@ -244,6 +244,10 @@ type Context interface {
 
 	// Release() releases any resources associated with this context.
 	Release()
+
+	// PrepareSleep() is called when the tread switches to the
+	// interruptible sleep state.
+	PrepareSleep()
 }
 
 var (

pkg/sentry/platform/ptrace/ptrace.go

@@ -199,6 +199,9 @@ func (c *context) FullStateChanged() {}
 // PullFullState implements platform.Context.PullFullState.
 func (c *context) PullFullState(as platform.AddressSpace, ac *arch.Context64) {}
 
+// PrepareSleep implements platform.Context.platform.PrepareSleep.
+func (*context) PrepareSleep() {}
+
 // PTrace represents a collection of ptrace subprocesses.
 type PTrace struct {
 	platform.MMapMinAddr