Add pivot.service as a host API

Add a new systemd service that simply executes `pivot`. But note that we
don't have a `pivot.path` unit to auto-trigger it: this needs to be
triggered explicitly by whatever drives pivot. Making it a service and
standizing on a path in `/etc` means that we can trigger a pivot on boot
from Ignition (patch to preset enable the service is pending), as well
as from the MCD by directly starting the unit.

Related: openshift/machine-config-operator#314

.gitignore

@@ -13,3 +13,5 @@ pivot
 *.out
 # editor
 .vscode
+
+systemd/pivot.service

Makefile

@@ -8,6 +8,7 @@ LDFLAGS := -X main.version=${VERSION} -X main.commitHash=${COMMIT_HASH} -X main.
 PREFIX ?= /usr
 CONFIG_DIR ?= /etc
 BIN_DIR ?= ${PREFIX}/bin
+SYSTEMD_UNIT_DIR ?= ${PREFIX}/lib/systemd/system
 
 .PHONY: help build clean deps install lint static test
 
@@ -32,11 +33,14 @@ help:
 changelog:
 	git log --format="- %s" `git tag | tail -n 1`..HEAD
 
+systemd/pivot.service: systemd/pivot.service.in
+	sed "s,@@PIVOT_BINARY_PATH@@,${BIN_DIR}/pivot,g" < systemd/pivot.service.in > systemd/pivot.service
+
 pivot: Gopkg.* *.go cmd/*.go utils/*.go types/*.go
 	go build -ldflags '${LDFLAGS}' -o pivot main.go
 	strip pivot
 
-build: pivot
+build: pivot systemd/pivot.service
 
 static: clean
 	CGO_ENABLED=0 go build -ldflags '${LDFLAGS} -w -extldflags "-static"' -a -o pivot main.go
@@ -51,6 +55,8 @@ deps:
 install: build
 	install -d ${DESTDIR}${BIN_DIR}
 	install --mode 755 pivot ${DESTDIR}${BIN_DIR}/pivot
+	install -d ${DESTDIR}${SYSTEMD_UNIT_DIR}
+	install --mode 664 systemd/pivot.service ${DESTDIR}${SYSTEMD_UNIT_DIR}
 
 lint:
 	go get -u github.com/golang/lint/golint

README.md

@@ -37,6 +37,19 @@ Though normally, one wants to use digests rather than tags, e.g.:
 pivot -r $REGISTRY/os@sha256:fdf70521df4ed1dc135d81fd3c4608574aeca45dc22d1b4e38d16630e9d6f1a7
 ```
 
+It also comes with a systemd unit to provide a "host API". For example:
+
+```
+mkdir -p /etc/pivot
+echo $REGISTRY/os:latest > /etc/pivot/image-pullspec
+touch /run/pivot-reboot-needed
+systemctl start pivot
+```
+
+This will start `pivot`, which will read the file and execute the pivot.
+If the pivot is completed, the file will be deleted. The expected way to
+make use of this is to create the necessary files from Ignition.
+
 See
 ---
 - [openshift/os](https://github.com/openshift/os/)

cmd/root.go

@@ -2,9 +2,9 @@ package cmd
 
 import (
 	"encoding/json"
-	"errors"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"strings"
 
@@ -21,22 +21,20 @@ var reboot bool
 var container string
 var exit_77 bool
 
-// the number of times to retry commands that pull data from the network
-const numRetriesNetCommands = 5
+const (
+	// the number of times to retry commands that pull data from the network
+	numRetriesNetCommands = 5
+	etcPivotFile          = "/etc/pivot/image-pullspec"
+	runPivotRebootFile    = "/run/pivot/reboot-needed"
+)
 
 // RootCmd houses the cobra config for the main command
 var RootCmd = &cobra.Command{
-	Use: "pivot [FLAGS] <IMAGE_PULLSPEC>",
+	Use: "pivot [FLAGS] [IMAGE_PULLSPEC]",
 	DisableFlagsInUseLine: true,
 	Short: "Allows moving from one OSTree deployment to another",
-	//	Long: ``,
-	Args: func(cmd *cobra.Command, args []string) error {
-		if len(args) < 1 {
-			return errors.New("An image name must be provided")
-		}
-		return nil
-	},
-	Run: Execute,
+	Args:  cobra.MaximumNArgs(1),
+	Run:   Execute,
 }
 
 // init executes upon import
@@ -150,9 +148,32 @@ func pullAndRebase(container string) (imgid string, changed bool) {
 
 // Execute runs the command
 func Execute(cmd *cobra.Command, args []string) {
-	container := args[0]
+	var fromFile bool
+	var container string
+	if len(args) > 0 {
+		container = args[0]
+		fromFile = false
+	} else {
+		glog.Infof("Using image pullspec from %s", etcPivotFile)
+		data, err := ioutil.ReadFile(etcPivotFile)
+		if err != nil {
+			glog.Fatalf("Failed to read from %s: %v", etcPivotFile, err)
+		}
+		container = strings.TrimSpace(string(data))
+		fromFile = true
+	}
+
 	imgid, changed := pullAndRebase(container)
 
+	// Delete the file now that we successfully rebased
+	if fromFile {
+		if err := os.Remove(etcPivotFile); err != nil {
+			if !os.IsNotExist(err) {
+				glog.Fatal("Failed to delete %s: %v", etcPivotFile, err)
+			}
+		}
+	}
+
 	// By default, delete the image.
 	if !keep {
 		// Related: https://github.com/containers/libpod/issues/2234
@@ -164,7 +185,7 @@ func Execute(cmd *cobra.Command, args []string) {
 		if exit_77 {
 			os.Exit(77)
 		}
-	} else if reboot {
+	} else if reboot || utils.FileExists(runPivotRebootFile) {
 		// Reboot the machine if asked to do so
 		utils.Run("systemctl", "reboot")
 	}

pivot.spec

@@ -1,8 +1,8 @@
 %define debug_package %{nil}
 
 Name:           pivot
-Version:        0.0.2
-Release:        0.1%{?dist}
+Version:        0.0.3
+Release:        1%{?dist}
 Summary:        allows moving from one OSTree deployment to another
 
 License:        ASL 2.0
@@ -21,7 +21,7 @@ deployment to another with minimal effort.
 %prep
 %autosetup -n %{name}-%{version}
 mkdir -p src/github.com/openshift/%{name}/
-cp -rf cmd  Gopkg.lock  Gopkg.toml  LICENSE  main.go  Makefile  pivot.spec  README.md  types  utils vendor VERSION src/github.com/openshift/%{name}
+cp -rf cmd  Gopkg.lock  Gopkg.toml  LICENSE  main.go  Makefile  pivot.spec  README.md  types  utils vendor VERSION systemd src/github.com/openshift/%{name}
 
 %build
 export GOPATH=`pwd`
@@ -36,9 +36,12 @@ make install DESTDIR=%{buildroot}
 %license LICENSE
 %doc README.md
 %{_bindir}/%{name}
-
+%{_prefix}/lib/systemd/system/pivot.*
 
 %changelog
+* Tue Feb 05 2019 Jonathan Lebon <[email protected]> - 0.0.3-1
+- Add systemd service unit
+
 * Wed Nov 14 2018 Steve Milner <[email protected]> - 0.0.2-0.1
 - Makefile: Add changelog target
 - cmd/root: Print pivoting msg before pull

systemd/pivot.service.in

@@ -0,0 +1,12 @@
+[Unit]
+Description=Pivot Tool
+ConditionPathExists=/etc/pivot/image-pullspec
+After=ignition-firstboot-complete.service
+Before=kubelet.service
+
+[Service]
+Type=simple
+ExecStart=@@PIVOT_BINARY_PATH@@
+
+[Install]
+WantedBy=multi-user.target

utils/fs.go

@@ -0,0 +1,18 @@
+package utils
+
+import (
+	"os"
+
+	"github.com/golang/glog"
+)
+
+// FileExists checks if the file exists, gracefully handling ENOENT.
+func FileExists(path string) bool {
+	if _, err := os.Stat(path); err != nil {
+		if os.IsNotExist(err) {
+			return false
+		}
+		glog.Fatalf("Failed to stat %s: %v", path, err)
+	}
+	return true
+}