sandbox/cgroup: support killing running snap apps

This is an alternative implmentation to the userd approach
in canonical#14160. The userd approach will not work in the absense
of user session daemon (i.e. ubuntu-core) or if a snap is
started as root.

The alternative algorithm kills the running apps in snapd
directly in their cgroups.

- many: address review comments (thanks @zyga)
  - add contexts support to FreezeSnapProcesses
  - add comment on possible race when killing processes in a v2 cgroup
  - for cgroups V2, only thaw on error
  - for cgroups V1, add comment explaining why we always need to thaw
  - update KillSnapProcesses to only send SIGKILL
  - replace deprecated os.IsNotExist with errors.Is
- trim whitespace when reading freezer.state

Signed-off-by: Zeyad Gouda <zeyad.gouda@canonical.com>

cmd/snap-update-ns/common.go

@@ -20,6 +20,7 @@
 package main
 
 import (
+	"context"
 	"fmt"
 
 	"github.com/snapcore/snapd/cmd/snaplock"
@@ -79,7 +80,8 @@ func (upCtx *CommonProfileUpdateContext) Lock() (func(), error) {
 	// introduce a symlink that would cause us to mount something other
 	// than what we expected).
 	logger.Debugf("freezing processes of snap %q", instanceName)
-	if err := cgroup.FreezeSnapProcesses(instanceName); err != nil {
+	// TODO: Ideally we should use signal.NotifyContext
+	if err := cgroup.FreezeSnapProcesses(context.TODO(), instanceName); err != nil {
 		// If we cannot freeze the processes we should drop the lock.
 		lock.Close()
 		return nil, err

cmd/snap-update-ns/common_test.go

@@ -21,6 +21,7 @@ package main_test
 
 import (
 	"bytes"
+	"context"
 	"os"
 	"path/filepath"
 
@@ -55,7 +56,7 @@ func (s *commonSuite) TestInstanceName(c *C) {
 func (s *commonSuite) TestLock(c *C) {
 	// Mock away real freezer code, allowing test code to return an error when freezing.
 	var freezingError error
-	restore := cgroup.MockFreezing(func(string) error { return freezingError }, func(string) error { return nil })
+	restore := cgroup.MockFreezing(func(context.Context, string) error { return freezingError }, func(string) error { return nil })
 	defer restore()
 	// Mock system directories, we use the lock directory.
 	dirs.SetRootDir(s.dir)

cmd/snap-update-ns/system_test.go

@@ -21,6 +21,7 @@ package main_test
 
 import (
 	"bytes"
+	"context"
 	"os"
 	"path/filepath"
 
@@ -46,7 +47,7 @@ func (s *systemSuite) TestLockCgroup(c *C) {
 
 	var frozen []string
 	var thawed []string
-	happyFreeze := func(snapName string) error {
+	happyFreeze := func(ctx context.Context, snapName string) error {
 		frozen = append(frozen, snapName)
 		return nil
 	}

overlord/snapstate/backend_test.go

@@ -1295,6 +1295,7 @@ func (f *fakeSnappyBackend) StartServices(svcs []*snap.AppInfo, disabledSvcs *wr
 }
 
 func (f *fakeSnappyBackend) StopServices(svcs []*snap.AppInfo, reason snap.ServiceStopReason, meter progress.Meter, tm timings.Measurer) error {
+	meter.Notify("stop-services")
 	f.appendOp(&fakeOp{
 		op:   fmt.Sprintf("stop-snap-services:%s", reason),
 		path: svcSnapMountDir(svcs),

sandbox/cgroup/cgroup.go

@@ -73,9 +73,13 @@ func init() {
 func pickVersionSpecificImpl() {
 	switch probeVersion {
 	case V1:
-		pickFreezerV1Impl()
+		FreezeSnapProcesses = freezeSnapProcessesImplV1
+		ThawSnapProcesses = thawSnapProcessesImplV1
+		KillSnapProcesses = killSnapProcessesImplV1
 	case V2:
-		pickFreezerV2Impl()
+		FreezeSnapProcesses = freezeSnapProcessesImplV2
+		ThawSnapProcesses = thawSnapProcessesImplV2
+		KillSnapProcesses = killSnapProcessesImplV2
 	}
 }
 

sandbox/cgroup/export_test.go

@@ -20,6 +20,7 @@ package cgroup
 
 import (
 	"context"
+	"syscall"
 	"time"
 
 	"github.com/godbus/dbus"
@@ -135,3 +136,31 @@ func (iw *inotifyWatcher) MonitorDelete(folders []string, name string, channel c
 }
 
 var NewInotifyWatcher = newInotifyWatcher
+
+func MockFreezeSnapProcessesImplV1(fn func(ctx context.Context, snapName string) error) (restore func()) {
+	return testutil.Mock(&freezeSnapProcessesImplV1, fn)
+}
+
+func MockThawSnapProcessesImplV1(fn func(snapName string) error) (restore func()) {
+	return testutil.Mock(&thawSnapProcessesImplV1, fn)
+}
+
+func MockSyscallKill(fn func(pid int, sig syscall.Signal) error) (restore func()) {
+	return testutil.Mock(&syscallKill, fn)
+}
+
+func MockFreezeOneV2(fn func(ctx context.Context, dir string) error) (restore func()) {
+	return testutil.Mock(&freezeOneV2, fn)
+}
+
+func MockThawOneV2(fn func(dir string) error) (restore func()) {
+	return testutil.Mock(&thawOneV2, fn)
+}
+
+func MockFreezePulseDelay(t time.Duration) (restore func()) {
+	return testutil.Mock(&freezePulseDelay, t)
+}
+
+func MockOsReadFile(fn func(name string) ([]byte, error)) (restore func()) {
+	return testutil.Mock(&osReadFile, fn)
+}