Merge pull request #3808 from alexlarsson/execdriver-get-pids-for-container

execdriver: Make GetPidsForContainer() a driver call

execdriver/chroot/driver.go

@@ -95,3 +95,7 @@ func (d *driver) Info(id string) execdriver.Info {
 func (d *driver) Name() string {
 	return fmt.Sprintf("%s-%s", DriverName, Version)
 }
+
+func (d *driver) GetPidsForContainer(id string) ([]int, error) {
+	return nil, fmt.Errorf("Not supported")
+}

execdriver/driver.go

@@ -60,9 +60,10 @@ type Info interface {
 type Driver interface {
 	Run(c *Command, startCallback StartCallback) (int, error) // Run executes the process and blocks until the process exits and returns the exit code
 	Kill(c *Command, sig int) error
-	Restore(c *Command) error // Wait and try to re-attach on an out of process command
-	Name() string             // Driver name
-	Info(id string) Info      // "temporary" hack (until we move state from core to plugins)
+	Restore(c *Command) error                     // Wait and try to re-attach on an out of process command
+	Name() string                                 // Driver name
+	Info(id string) Info                          // "temporary" hack (until we move state from core to plugins)
+	GetPidsForContainer(id string) ([]int, error) // Returns a list of pids for the given container.
 }
 
 // Network settings of the container

execdriver/lxc/driver.go

@@ -3,12 +3,14 @@ package lxc
 import (
 	"fmt"
 	"github.com/dotcloud/docker/execdriver"
+	"github.com/dotcloud/docker/pkg/cgroups"
 	"github.com/dotcloud/docker/utils"
 	"io/ioutil"
 	"log"
 	"os"
 	"os/exec"
 	"path"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"syscall"
@@ -281,6 +283,45 @@ func (d *driver) Info(id string) execdriver.Info {
 	}
 }
 
+func (d *driver) GetPidsForContainer(id string) ([]int, error) {
+	pids := []int{}
+
+	// memory is chosen randomly, any cgroup used by docker works
+	subsystem := "memory"
+
+	cgroupRoot, err := cgroups.FindCgroupMountpoint(subsystem)
+	if err != nil {
+		return pids, err
+	}
+
+	cgroupDir, err := cgroups.GetThisCgroupDir(subsystem)
+	if err != nil {
+		return pids, err
+	}
+
+	filename := filepath.Join(cgroupRoot, cgroupDir, id, "tasks")
+	if _, err := os.Stat(filename); os.IsNotExist(err) {
+		// With more recent lxc versions use, cgroup will be in lxc/
+		filename = filepath.Join(cgroupRoot, cgroupDir, "lxc", id, "tasks")
+	}
+
+	output, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return pids, err
+	}
+	for _, p := range strings.Split(string(output), "\n") {
+		if len(p) == 0 {
+			continue
+		}
+		pid, err := strconv.Atoi(p)
+		if err != nil {
+			return pids, fmt.Errorf("Invalid pid '%s': %s", p, err)
+		}
+		pids = append(pids, pid)
+	}
+	return pids, nil
+}
+
 func linkLxcStart(root string) error {
 	sourcePath, err := exec.LookPath("lxc-start")
 	if err != nil {