systemd: fix incorrect err handling in ensureMountUnitFile()

The ensureMountUnitFile() was not handling errors from osutil.EnsureFileState() correctly. This commit fixes this and adds a unit test. 

2 files changed, 21 insertions, 2 deletions

diff --git a/systemd/systemd.go b/systemd/systemd.go
index 7bc92a46ca..48ca5f997e 100644
--- a/systemd/systemd.go
+++ b/systemd/systemd.go
@@ -1429,8 +1429,8 @@ func ensureMountUnitFile(u *MountUnitOptions) (mountUnitName string, modified mo
 
	if stateErr == osutil.ErrSameState {
	modified = mountUnchanged
-	} else if err != nil {
-	return "", mountUnchanged, err
+	} else if stateErr != nil {
+	return "", mountUnchanged, stateErr
	}
 
	return filepath.Base(mu), modified, nil
diff --git a/systemd/systemd_test.go b/systemd/systemd_test.go
index f177094f34..93e795417e 100644
--- a/systemd/systemd_test.go
+++ b/systemd/systemd_test.go
@@ -2411,3 +2411,22 @@ func (s *systemdErrorSuite) TestErrorStringNoSystemctl(c *C) {
	_, err := Version()
	c.Check(err, ErrorMatches, `systemctl command \[--version\] failed with: exec: "systemctl": executable file not found in \$PATH`)
 }
+
+func (s *systemdErrorSuite) TestEnsureMountUnitFileEnsureFileStateErr(c *C) {
+	dirs.SetRootDir(c.MkDir())
+	defer dirs.SetRootDir("")
+
+	restore := osutil.MockMountInfo("")
+	defer restore()
+
+	mockSnapPath := filepath.Join(dirs.GlobalRootDir, "/var/lib/snappy/snaps/foo_1.0.snap")
+	makeMockFile(c, mockSnapPath)
+
+	// trigger an error below by creating a directory with the
+	// same name as the mount unit file
+	err := os.MkdirAll(filepath.Join(dirs.GlobalRootDir, "/etc/systemd/system/snap-snapname-123.mount"), 0755)
+	c.Assert(err, IsNil)
+
+	_, err = New(SystemMode, nil).EnsureMountUnitFile("foo", "42", mockSnapPath, "/snap/snapname/123", "squashfs")
+	c.Assert(err, ErrorMatches, `rename .*/etc/systemd/system/snap-snapname-123.mount.* .*/etc/systemd/system/snap-snapname-123.mount: file exists`)
+}