Cherry-pick #21835 to 7.x: Use symlink path for reexecutions

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -17,6 +17,7 @@
 - Fix issue where inputs without processors defined would panic {pull}21628[21628]
 - Prevent reporting ecs version twice {pull}21616[21616]
 - Partial extracted beat result in failure to spawn beat {issue}21718[21718]
+- Use symlink path for reexecutions {pull}21835[21835]
 - Use ML_SYSTEM to detect if agent is running as a service {pull}21884[21884]
 - Use local temp instead of system one {pull}21883[21883]
 - Fix issue with named pipes on Windows 7 {pull}21931[21931]

x-pack/elastic-agent/pkg/agent/cmd/run.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"os"
 	"os/signal"
+	"path/filepath"
 	"syscall"
 
 	"github.com/spf13/cobra"
@@ -26,6 +27,10 @@ import (
 	"github.com/elastic/beats/v7/x-pack/elastic-agent/pkg/release"
 )
 
+const (
+	agentName = "elastic-agent"
+)
+
 func newRunCommandWithArgs(flags *globalFlags, _ []string, streams *cli.IOStreams) *cobra.Command {
 	return &cobra.Command{
 		Use:   "run",
@@ -87,7 +92,7 @@ func run(flags *globalFlags, streams *cli.IOStreams) error { // Windows: Mark se
 		logger.Warn("Artifact has been build with security disabled. Elastic Agent will not verify signatures of used artifacts.")
 	}
 
-	execPath, err := os.Executable()
+	execPath, err := reexecPath()
 	if err != nil {
 		return err
 	}
@@ -146,3 +151,16 @@ func run(flags *globalFlags, streams *cli.IOStreams) error { // Windows: Mark se
 	rex.ShutdownComplete()
 	return err
 }
+
+func reexecPath() (string, error) {
+	// set executable path to symlink instead of binary
+	// in case of updated symlinks we should spin up new agent
+	potentialReexec := filepath.Join(paths.Top(), agentName)
+
+	// in case it does not exists fallback to executable
+	if _, err := os.Stat(potentialReexec); os.IsNotExist(err) {
+		return os.Executable()
+	}
+
+	return potentialReexec, nil
+}