linux: Prevent deadlock in reaper.WaitPid()

A deadlock can occurs if `WaitPid()` is called twice before the process
dies.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

linux/shim/service.go

@@ -88,7 +88,7 @@ func (s *Service) Create(ctx context.Context, r *shimapi.CreateTaskRequest) (*sh
 	s.processes[r.ID] = process
 	s.mu.Unlock()
 	cmd := &reaper.Cmd{
-		ExitCh: make(chan int, 1),
+		ExitCh: make(chan struct{}),
 	}
 	reaper.Default.Register(pid, cmd)
 	s.events <- &eventsapi.TaskCreate{
@@ -126,7 +126,7 @@ func (s *Service) Start(ctx context.Context, r *shimapi.StartRequest) (*shimapi.
 	} else {
 		pid := p.Pid()
 		cmd := &reaper.Cmd{
-			ExitCh: make(chan int, 1),
+			ExitCh: make(chan struct{}),
 		}
 		reaper.Default.Register(pid, cmd)
 		go s.waitExit(p, pid, cmd)
@@ -366,7 +366,7 @@ func (s *Service) Update(ctx context.Context, r *shimapi.UpdateTaskRequest) (*go
 }
 
 func (s *Service) waitExit(p process, pid int, cmd *reaper.Cmd) {
-	status := <-cmd.ExitCh
+	status, _ := reaper.Default.WaitPid(pid)
 	p.SetExited(status)
 
 	reaper.Default.Delete(pid)

reaper/reaper.go

@@ -4,7 +4,6 @@ package reaper
 
 import (
 	"bytes"
-	"fmt"
 	"os/exec"
 	"sync"
 
@@ -12,6 +11,10 @@ import (
 	"github.com/pkg/errors"
 )
 
+var (
+	ErrNoSuchProcess = errors.New("no such process")
+)
+
 // Reap should be called when the process receives an SIGCHLD.  Reap will reap
 // all exited processes and close their wait channels
 func Reap() error {
@@ -30,7 +33,8 @@ func Reap() error {
 			// pipes are closed and finalizers are run on the process
 			c.c.Wait()
 		}
-		c.ExitCh <- e.Status
+		c.exitStatus = e.Status
+		close(c.ExitCh)
 	}
 	return err
 }
@@ -68,7 +72,7 @@ func (m *Monitor) CombinedOutput(c *exec.Cmd) ([]byte, error) {
 func (m *Monitor) Start(c *exec.Cmd) error {
 	rc := &Cmd{
 		c:      c,
-		ExitCh: make(chan int, 1),
+		ExitCh: make(chan struct{}),
 	}
 	// start the process
 	m.Lock()
@@ -105,7 +109,8 @@ func (m *Monitor) RegisterNL(pid int, c *Cmd) {
 	if status, ok := m.unknown[pid]; ok {
 		delete(m.unknown, pid)
 		m.cmds[pid] = c
-		c.ExitCh <- status
+		c.exitStatus = status
+		close(c.ExitCh)
 		return
 	}
 	m.cmds[pid] = c
@@ -116,13 +121,13 @@ func (m *Monitor) WaitPid(pid int) (int, error) {
 	rc, ok := m.cmds[pid]
 	m.Unlock()
 	if !ok {
-		return 255, fmt.Errorf("process does not exist")
+		return 255, errors.Wrapf(ErrNoSuchProcess, "pid %d", pid)
 	}
-	ec := <-rc.ExitCh
+	<-rc.ExitCh
-	if ec != 0 {
+	if rc.exitStatus != 0 {
-		return ec, errors.Errorf("exit status %d", ec)
+		return rc.exitStatus, errors.Errorf("exit status %d", rc.exitStatus)
 	}
-	return ec, nil
+	return rc.exitStatus, nil
 }
 
 // Command returns the registered pid for the command created
@@ -139,6 +144,7 @@ func (m *Monitor) Delete(pid int) {
 }
 
 type Cmd struct {
-	c      *exec.Cmd
+	c          *exec.Cmd
-	ExitCh chan int
+	ExitCh     chan struct{}
+	exitStatus int
 }