cmd/snap-failure: do not attempt to revert snapd when reexec isn't su…

…pported by the systemd

The snap-failure helper is used as a general failure handler for failures of the
snapd.service unit. Its purpose is to trigger a revert of snapd snap to the
previous revision whenever a failure occurs. The logic asserting whether a snapd
refresh was in progress is privy to the actual snapd binary, however any attempt
to execute snapd binary from the snapd snap makes no sense if the system does
not support running snapd directly from the snap in the first place.

Signed-off-by: Maciej Borzecki <[email protected]>

cmd/snap-failure/cmd_snapd.go

@@ -31,6 +31,8 @@ import (
 	"github.com/snapcore/snapd/dirs"
 	"github.com/snapcore/snapd/logger"
 	"github.com/snapcore/snapd/osutil"
+	"github.com/snapcore/snapd/release"
+	"github.com/snapcore/snapd/snapdtool"
 )
 
 func init() {
@@ -112,6 +114,16 @@ var (
 )
 
 func (c *cmdSnapd) Execute(args []string) error {
+	if release.OnClassic {
+		// snap failure was invoked in a classic system, while there are
+		// scenarios in which it may make sense, they are limited to a
+		// case when snapd is being reexec'd from the snapd snap
+		if !snapdtool.DistroSupportsReExec() || !snapdtool.IsReexecEnabled() {
+			logger.Noticef("re-exec unsupported or disabled")
+			return nil
+		}
+	}
+
 	var snapdPath string
 	// find previous the snapd snap
 	prevRev, err := prevRevision("snapd")

cmd/snap-failure/cmd_snapd_test.go

@@ -31,6 +31,7 @@ import (
 	failure "github.com/snapcore/snapd/cmd/snap-failure"
 	"github.com/snapcore/snapd/dirs"
 	"github.com/snapcore/snapd/osutil"
+	"github.com/snapcore/snapd/release"
 	"github.com/snapcore/snapd/snap"
 	"github.com/snapcore/snapd/testutil"
 )
@@ -438,3 +439,38 @@ func (r *failureSuite) TestStickySnapdSocket(c *C) {
 	// make sure the socket file was deleted
 	c.Assert(osutil.FileExists(dirs.SnapdSocket), Equals, false)
 }
+
+func (r *failureSuite) testNoReexec(c *C) {
+	origArgs := os.Args
+	defer func() { os.Args = origArgs }()
+
+	writeSeqFile(c, "snapd", snap.R(100), []*snap.SideInfo{
+		{Revision: snap.R(99)},
+		{Revision: snap.R(100)},
+	})
+
+	// mock snapd command from 'previous' revision
+	snapdCmd := testutil.MockCommand(c, filepath.Join(dirs.SnapMountDir, "snapd", "99", "/usr/lib/snapd/snapd"), "exit 1")
+	defer snapdCmd.Restore()
+
+	os.Args = []string{"snap-failure", "snapd"}
+	err := failure.Run()
+	c.Check(err, IsNil)
+
+	c.Check(snapdCmd.Calls(), HasLen, 0)
+	c.Check(r.systemctlCmd.Calls(), HasLen, 0)
+	c.Check(r.log.String(), testutil.Contains, "re-exec unsupported or disabled")
+}
+
+func (r *failureSuite) TestReexecDisabled(c *C) {
+	os.Setenv("SNAP_REEXEC", "0")
+	defer os.Unsetenv("SNAP_REEXEC")
+	r.testNoReexec(c)
+
+}
+
+func (r *failureSuite) TestReexecUnsupported(c *C) {
+	r.AddCleanup(release.MockReleaseInfo(&release.OS{ID: "fedora"}))
+	dirs.SetRootDir(r.rootdir)
+	r.testNoReexec(c)
+}

cmd/snap-failure/main_test.go

@@ -28,6 +28,7 @@ import (
 	failure "github.com/snapcore/snapd/cmd/snap-failure"
 	"github.com/snapcore/snapd/dirs"
 	"github.com/snapcore/snapd/logger"
+	"github.com/snapcore/snapd/release"
 	"github.com/snapcore/snapd/testutil"
 )
 
@@ -59,6 +60,8 @@ func (r *failureSuite) SetUpTest(c *C) {
 	failure.Stderr = r.stderr
 	failure.Stdout = r.stdout
 
+	r.AddCleanup(release.MockReleaseInfo(&release.OS{ID: "ubuntu"}))
+
 	r.rootdir = c.MkDir()
 	dirs.SetRootDir(r.rootdir)
 	r.AddCleanup(func() { dirs.SetRootDir("/") })