devicestate: keep log from install-mode on installed system

This commit ensures that we capture the full log of the
ephemeral install mode on the writable disk when installing
the system.

This is useful for debugging if install mode worked as expected
or to get logs about why things are different than they should
have been.

overlord/devicestate/devicestate_install_mode_test.go

@@ -85,6 +85,9 @@ func (s *deviceMgrInstallModeSuite) SetUpTest(c *C) {
 	s.state.Lock()
 	defer s.state.Unlock()
 	s.state.Set("seeded", true)
+
+	fakeJournalctl := testutil.MockCommand(c, "journalctl", "")
+	s.AddCleanup(fakeJournalctl.Restore)
 }
 
 func (s *deviceMgrInstallModeSuite) makeMockInstalledPcGadget(c *C, grade, gadgetDefaultsYaml string) *asserts.Model {

overlord/devicestate/handlers_install.go

@@ -22,6 +22,7 @@ package devicestate
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 
 	"gopkg.in/tomb.v2"
@@ -89,6 +90,28 @@ func writeModel(model *asserts.Model, where string) error {
 	return asserts.NewEncoder(f).Encode(model)
 }
 
+func writeLogs(rootdir string) error {
+	// XXX: would be great to use native journal format but it's tied
+	//      to machine-id, we could journal -o export but there
+	//      is no systemd-journal-remote on core{,18,20}
+	//
+	// XXX: or only log if persistent journal is enabled?
+	logPath := filepath.Join(rootdir, "var/log/install-mode.log")
+	if err := os.MkdirAll(filepath.Dir(logPath), 0755); err != nil {
+		return err
+	}
+
+	f, err := os.Create(logPath)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	cmd := exec.Command("journalctl")
+	cmd.Stdout = f
+	return cmd.Run()
+}
+
 func (m *DeviceManager) doSetupRunSystem(t *state.Task, _ *tomb.Tomb) error {
 	st := t.State()
 	st.Lock()
@@ -212,6 +235,12 @@ func (m *DeviceManager) doSetupRunSystem(t *state.Task, _ *tomb.Tomb) error {
 		return fmt.Errorf("cannot make run system bootable: %v", err)
 	}
 
+	// store install-mode log into ubuntu-data partition when
+	// persistent journal is selected
+	if err := writeLogs(boot.InstallHostWritableDir); err != nil {
+		logger.Noticef("cannot write logs: %v", err)
+	}
+
 	// request a restart as the last action after a successful install
 	logger.Noticef("request system restart")
 	st.RequestRestart(state.RestartSystemNow)

tests/nested/manual/core20-early-config/task.yaml

@@ -71,6 +71,10 @@ execute: |
 
     check_core20_early_config
 
+    # check that we go the install log after the transition to run mode
+    nested_exec "test -e /var/log/install-mode.log"
+    xxx
+
     # now transition to recover mode and check it again
     boot_id="$(nested_get_boot_id)"
     # TODO:UC20: shouldn't need to specify the label here, fix when snap reboot