2 files changed, 132 insertions, 0 deletions
diff --git a/pkg/sentry/kernel/time/BUILD b/pkg/sentry/kernel/time/BUILD
index 7ba7dc50c..2817aa3ba 100644
--- a/pkg/sentry/kernel/time/BUILD
+++ b/pkg/sentry/kernel/time/BUILD
@@ -6,6 +6,7 @@ go_library(
     name = "time",
     srcs = [
         "context.go",
+        "tcpip.go",
         "time.go",
     ],
     visibility = ["//pkg/sentry:internal"],
diff --git a/pkg/sentry/kernel/time/tcpip.go b/pkg/sentry/kernel/time/tcpip.go
new file mode 100644
index 000000000..c4474c0cf
--- /dev/null
+++ b/pkg/sentry/kernel/time/tcpip.go
@@ -0,0 +1,131 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package time
+
+import (
+	"sync"
+	"time"
+)
+
+// TcpipAfterFunc waits for duration to elapse according to clock then runs fn.
+// The timer is started immediately and will fire exactly once.
+func TcpipAfterFunc(clock Clock, duration time.Duration, fn func()) *TcpipTimer {
+	timer := &TcpipTimer{
+		clock: clock,
+	}
+	timer.notifier = functionNotifier{
+		fn: func() {
+			// tcpip.Timer.Stop() explicitly states that the function is called in a
+			// separate goroutine that Stop() does not synchronize with.
+			// Timer.Destroy() synchronizes with calls to TimerListener.Notify().
+			// This is semantically meaningful because, in the former case, it's
+			// legal to call tcpip.Timer.Stop() while holding locks that may also be
+			// taken by the function, but this isn't so in the latter case. Most
+			// immediately, Timer calls TimerListener.Notify() while holding
+			// Timer.mu. A deadlock occurs without spawning a goroutine:
+			//   T1: (Timer expires)
+			//     => Timer.Tick()           <- Timer.mu.Lock() called
+			//     => TimerListener.Notify()
+			//     => Timer.Stop()
+			//     => Timer.Destroy()        <- Timer.mu.Lock() called, deadlock!
+			//
+			// Spawning a goroutine avoids the deadlock:
+			//   T1: (Timer expires)
+			//     => Timer.Tick()           <- Timer.mu.Lock() called
+			//     => TimerListener.Notify() <- Launches T2
+			//   T2:
+			//     => Timer.Stop()
+			//     => Timer.Destroy()        <- Timer.mu.Lock() called, blocks
+			//   T1:
+			//     => (returns)              <- Timer.mu.Unlock() called
+			//   T2:
+			//     => (continues)            <- No deadlock!
+			go func() {
+				timer.Stop()
+				fn()
+			}()
+		},
+	}
+	timer.Reset(duration)
+	return timer
+}
+
+// TcpipTimer is a resettable timer with variable duration expirations.
+// Implements tcpip.Timer, which does not define a Destroy method; instead, all
+// resources are released after timer expiration and calls to Timer.Stop.
+//
+// Must be created by AfterFunc.
+type TcpipTimer struct {
+	// clock is the time source. clock is immutable.
+	clock Clock
+
+	// notifier is called when the Timer expires. notifier is immutable.
+	notifier functionNotifier
+
+	// mu protects t.
+	mu sync.Mutex
+
+	// t stores the latest running Timer. This is replaced whenever Reset is
+	// called since Timer cannot be restarted once it has been Destroyed by Stop.
+	//
+	// This field is nil iff Stop has been called.
+	t *Timer
+}
+
+// Stop implements tcpip.Timer.Stop.
+func (r *TcpipTimer) Stop() bool {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	if r.t == nil {
+		return false
+	}
+	_, lastSetting := r.t.Swap(Setting{})
+	r.t.Destroy()
+	r.t = nil
+	return lastSetting.Enabled
+}
+
+// Reset implements tcpip.Timer.Reset.
+func (r *TcpipTimer) Reset(d time.Duration) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	if r.t == nil {
+		r.t = NewTimer(r.clock, &r.notifier)
+	}
+
+	r.t.Swap(Setting{
+		Enabled: true,
+		Period:  0,
+		Next:    r.clock.Now().Add(d),
+	})
+}
+
+// functionNotifier is a TimerListener that runs a function.
+//
+// functionNotifier cannot be saved or loaded.
+type functionNotifier struct {
+	fn func()
+}
+
+// Notify implements ktime.TimerListener.Notify.
+func (f *functionNotifier) Notify(uint64, Setting) (Setting, bool) {
+	f.fn()
+	return Setting{}, false
+}
+
+// Destroy implements ktime.TimerListener.Destroy.
+func (f *functionNotifier) Destroy() {}