Add qemu API to stream journal, use it in devshell

This came up in coreos/ignition-dracut#146
and since then we've been doing more "ad hoc unit writing to virtio"
in mantle, but let's add a general API that streams the journal.

This is just better for what devshell wants - we can more precisely
watch for sshd starting.  And more code in e.g. `testiso.go` could
use it too which can come later.

The immediate motivation here is I may add another kola test
which could use this.

mantle/cmd/kola/devshell.go

@@ -113,32 +113,13 @@ func runDevShellSSH(builder *platform.QemuBuilder, conf *v3types.Config) error {
 	}
 	sshPubKey := v3types.SSHAuthorizedKey(strings.TrimSpace(string(sshPubKeyBuf)))
 
-	readinessSignalChan := "coreos.devshellready"
-	signalReadyUnit := fmt.Sprintf(`[Unit]
-Requires=dev-virtio\\x2dports-%s.device
-OnFailure=emergency.target
-OnFailureJobMode=isolate
-After=systemd-user-sessions.service
-After=sshd.service
-[Service]
-Type=oneshot
-RemainAfterExit=yes
-ExecStart=/bin/echo ready
-StandardOutput=file:/dev/virtio-ports/%[1]s
-[Install]
-RequiredBy=multi-user.target`, readinessSignalChan)
-	signalPoweroffUnit := fmt.Sprintf(`[Unit]
-Requires=dev-virtio\\x2dports-%s.device
-OnFailure=emergency.target
-OnFailureJobMode=isolate
-Conflicts=reboot.target
-[Service]
-Type=oneshot
-RemainAfterExit=yes
-ExecStop=/bin/echo poweroff
-StandardOutput=file:/dev/virtio-ports/%[1]s
-[Install]
-WantedBy=multi-user.target`, readinessSignalChan)
+	// Await sshd startup
+	journalConf, journalPipe, err := builder.VirtioJournal("-u sshd MESSAGE_ID=7d4958e842da4a758f6c1cdc7b36dcc5")
+	if err != nil {
+		return err
+	}
+	confm := v3.Merge(*conf, *journalConf)
+	conf = &confm
 
 	devshellConfig := v3types.Config{
 		Ignition: v3types.Ignition{
@@ -154,29 +135,11 @@ WantedBy=multi-user.target`, readinessSignalChan)
 				},
 			},
 		},
-		Systemd: v3types.Systemd{
-			Units: []v3types.Unit{
-				{
-					Name:     "coreos-devshell-signal-ready.service",
-					Contents: &signalReadyUnit,
-					Enabled:  util.BoolToPtr(true),
-				},
-				{
-					Name:     "coreos-devshell-signal-poweroff.service",
-					Contents: &signalPoweroffUnit,
-					Enabled:  util.BoolToPtr(true),
-				},
-			},
-		},
 	}
-	confm := v3.Merge(*conf, devshellConfig)
+	confm = v3.Merge(*conf, devshellConfig)
 	conf = &confm
 
-	readyChan, err := builder.VirtioChannelRead(readinessSignalChan)
-	if err != nil {
-		return err
-	}
-	readyReader := bufio.NewReader(readyChan)
+	readyReader := bufio.NewReader(journalPipe)
 
 	builder.SetConfig(*conf, kola.Options.IgnitionVersion == "v2")
 
@@ -230,8 +193,8 @@ WantedBy=multi-user.target`, readinessSignalChan)
 		if err != nil {
 			errchan <- err
 		}
-		if readyMsg != "ready" {
-			errchan <- fmt.Errorf("Unexpected ready message: %s", readyMsg)
+		if !strings.Contains(readyMsg, "Starting OpenSSH server daemon") {
+			errchan <- fmt.Errorf("Unexpected journal message: %s", readyMsg)
 		}
 		var s struct{}
 		readychan <- s
@@ -263,7 +226,7 @@ loop:
 			// Caught SIGINT, we're done
 			return fmt.Errorf("Caught SIGINT before successful login")
 		case _ = <-readychan:
-			fmt.Printf("\033[2K\rvirtio: connected\n")
+			fmt.Printf("\033[2K\rvirtio journal connected - sshd started\n")
 			break loop
 		}
 	}

mantle/platform/qemu.go

@@ -900,6 +900,43 @@ func (builder *QemuBuilder) SerialPipe() (*os.File, error) {
 	return r, nil
 }
 
+// VirtioJournal configures the OS and VM to stream the systemd journal
+// (post-switchroot) over a virtio-serial channel.  The first return value
+// is an Ignition fragment that should be included in the target config,
+// and the file stream will be newline-separated JSON.  The optional
+// queryArguments filters the stream - see `man journalctl` for more information.
+func (builder *QemuBuilder) VirtioJournal(queryArguments string) (*v3types.Config, *os.File, error) {
+	stream, err := builder.VirtioChannelRead("mantlejournal")
+	if err != nil {
+		return nil, nil, err
+	}
+	var streamJournalUnit = fmt.Sprintf(`[Unit]
+	Requires=dev-virtio\\x2dports-mantlejournal.device
+	[Service]
+	Type=simple
+	StandardOutput=file:/dev/virtio-ports/mantlejournal
+	ExecStart=/usr/bin/journalctl -q -b -f -o json --no-tail %s
+	[Install]
+	RequiredBy=multi-user.target
+	`, queryArguments)
+
+	conf := v3types.Config{
+		Ignition: v3types.Ignition{
+			Version: "3.0.0",
+		},
+		Systemd: v3types.Systemd{
+			Units: []v3types.Unit{
+				{
+					Name:     "mantle-virtio-journal-stream.service",
+					Contents: &streamJournalUnit,
+					Enabled:  util.BoolToPtr(true),
+				},
+			},
+		},
+	}
+	return &conf, stream, nil
+}
+
 func (builder *QemuBuilder) Exec() (*QemuInstance, error) {
 	builder.finalize()
 	var err error