snap-failure: set symlink before calling snapd

cmd/snap-failure/cmd_snapd.go

@@ -145,6 +145,16 @@ func (c *cmdSnapd) Execute(args []string) error {
 	}
 
 	logger.Noticef("restoring invoking snapd from: %v", snapdPath)
+	if prevRev != "0" {
+		// if prevRev was "0" it means we did *not* find a
+		// previous revision and we would obey the current
+		// symlink. So we overwrite the symlink only if
+		// prevRev != "0".
+		currentSymlink := filepath.Join(dirs.SnapMountDir, "snapd", "current")
+		if err := osutil.AtomicSymlink(prevRev, currentSymlink); err != nil {
+			return fmt.Errorf("cannot create symlink %s: %v", currentSymlink, err)
+		}
+	}
 	// start previous snapd
 	cmd := runCmd(snapdPath, nil, []string{"SNAPD_REVERT_TO_REV=" + prevRev, "SNAPD_DEBUG=1"})
 	if err = cmd.Run(); err != nil {

cmd/snap-failure/cmd_snapd_test.go

@@ -21,6 +21,7 @@ package main_test
 
 import (
 	"encoding/json"
+	"fmt"
 	"os"
 	"path/filepath"
 	"time"
@@ -72,9 +73,15 @@ func (r *failureSuite) TestCallPrevSnapdFromSnap(c *C) {
 		{Revision: snap.R(123)},
 	})
 
+	mockScript := `
+set -eu
+
+[ -L '%[1]s/snapd/current' ]
+[ "$(readlink '%[1]s/snapd/current')" = 100 ]
+[ "${SNAPD_REVERT_TO_REV}" = 100 ]
+`
 	// mock snapd command from 'previous' revision
-	snapdCmd := testutil.MockCommand(c, filepath.Join(dirs.SnapMountDir, "snapd", "100", "/usr/lib/snapd/snapd"),
-		`test "$SNAPD_REVERT_TO_REV" = "100"`)
+	snapdCmd := testutil.MockCommand(c, filepath.Join(dirs.SnapMountDir, "snapd", "100", "/usr/lib/snapd/snapd"), fmt.Sprintf(mockScript, dirs.SnapMountDir))
 	defer snapdCmd.Restore()
 
 	systemctlCmd := testutil.MockCommand(c, "systemctl", "")