Adjust Elastic Agent 7.17 to launch upgrade watcher version (elastic#…

…40452)

* Adjust so 7.17 agent starts latest watcher.

* Add changelog entry.

* Fix code review.

CHANGELOG.next.asciidoc

@@ -31,6 +31,10 @@ Fix CVE-2023-25173 by updating github.com/containerd/containerd to v1.5.18 {pull
 
 *Functionbeat*
 
+*Elastic Agent*
+
+- Fixes the issue where upgrading from 7.17 to a 8.x+ version only runs the 7.17 watcher for the latest 8.x release.
+
 ==== Bugfixes
 
 *Affecting all Beats*

x-pack/elastic-agent/pkg/agent/application/paths/common.go

@@ -169,6 +169,13 @@ func SetInstall(path string) {
 	installPath = path
 }
 
+// TopBinaryPath returns the path to the Elastic Agent binary that is inside the Top directory.
+//
+// This always points to the symlink that points to the latest Elastic Agent version.
+func TopBinaryPath() string {
+	return filepath.Join(Top(), BinaryName)
+}
+
 // initialTop returns the initial top-level path for the binary
 //
 // When nested in top-level/data/elastic-agent-${hash}/ the result is top-level/.

x-pack/elastic-agent/pkg/agent/application/upgrade/rollback.go

@@ -104,8 +104,7 @@ func InvokeWatcher(log *logger.Logger) error {
 		return nil
 	}
 
-	versionedHome := paths.VersionedHome(paths.Top())
-	cmd := invokeCmd(versionedHome)
+	cmd := invokeCmd()
 	defer func() {
 		if cmd.Process != nil {
 			log.Debugf("releasing watcher %v", cmd.Process.Pid)

x-pack/elastic-agent/pkg/agent/application/upgrade/service.go

@@ -200,10 +200,8 @@ func (p *noopPidProvider) Name() string { return "noop" }
 
 func (p *noopPidProvider) PID(ctx context.Context) (int, error) { return 0, nil }
 
-func invokeCmd(topPath string) *exec.Cmd {
-	homeExePath := filepath.Join(topPath, agentName)
-
-	cmd := exec.Command(homeExePath, watcherSubcommand,
+func invokeCmd() *exec.Cmd {
+	cmd := exec.Command(paths.TopBinaryPath(), watcherSubcommand,
 		"--path.config", paths.Config(),
 		"--path.home", paths.Top(),
 	)

x-pack/elastic-agent/pkg/agent/application/upgrade/service_darwin.go

@@ -14,7 +14,6 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
-	"path/filepath"
 	"regexp"
 	"strconv"
 	"strings"
@@ -114,10 +113,8 @@ func (p *darwinPidProvider) piderFromCmd(ctx context.Context, name string, args
 	}
 }
 
-func invokeCmd(topPath string) *exec.Cmd {
-	homeExePath := filepath.Join(topPath, agentName)
-
-	cmd := exec.Command(homeExePath, watcherSubcommand,
+func invokeCmd() *exec.Cmd {
+	cmd := exec.Command(paths.TopBinaryPath(), watcherSubcommand,
 		"--path.config", paths.Config(),
 		"--path.home", paths.Top(),
 	)

x-pack/elastic-agent/pkg/agent/application/upgrade/service_windows.go

@@ -10,7 +10,6 @@ package upgrade
 import (
 	"context"
 	"os/exec"
-	"path/filepath"
 	"time"
 
 	"golang.org/x/sys/windows/svc/mgr"
@@ -62,10 +61,8 @@ func (p *pidProvider) PID(ctx context.Context) (int, error) {
 	return int(status.ProcessId), nil
 }
 
-func invokeCmd(topPath string) *exec.Cmd {
-	homeExePath := filepath.Join(topPath, agentName)
-
-	cmd := exec.Command(homeExePath, watcherSubcommand,
+func invokeCmd() *exec.Cmd {
+	cmd := exec.Command(paths.TopBinaryPath(), watcherSubcommand,
 		"--path.config", paths.Config(),
 		"--path.home", paths.Top(),
 	)

x-pack/elastic-agent/pkg/agent/application/upgrade/upgrade.go

@@ -168,6 +168,7 @@ func (u *Upgrader) Upgrade(ctx context.Context, a Action, reexecNow bool, skipVe
 		return nil, err
 	}
 
+	// InvokeWatcher invokes the watcher using the symlink that is rotated above with ChangeSymlink
 	if err := InvokeWatcher(u.log); err != nil {
 		rollbackInstall(ctx, newHash)
 		return nil, errors.New("failed to invoke rollback watcher", err)