Add entrypoint override

cmd/orchestrator/main.go

@@ -19,6 +19,7 @@ import (
 	"github.com/circleci/runner-init/cmd/setup"
 	initialize "github.com/circleci/runner-init/init"
 	"github.com/circleci/runner-init/task"
+	"github.com/circleci/runner-init/task/entrypoint"
 	"github.com/circleci/runner-init/task/taskerrors"
 )
 
@@ -39,6 +40,7 @@ type initCmd struct {
 type runTaskCmd struct {
 	TerminationGracePeriod time.Duration `default:"10s" help:"How long the agent will wait for the task to complete if interrupted."`
 	HealthCheckAddr        string        `default:":7623" help:"Address for the health check API to listen on."`
+	EntrypointOverride     []string      `hidden:"true"`
 
 	// Task environment configuration should be injected through a Kubernetes Secret
 	Config task.Config `required:"" hidden:"-"`
@@ -102,9 +104,13 @@ func run(version, date string) (err error) {
 	return sys.Run(ctx, cli.ShutdownDelay)
 }
 
-func runSetup(ctx context.Context, cli cli, version string, sys *system.System) (*task.Orchestrator, error) {
+func runSetup(ctx context.Context, cli cli, version string, sys *system.System) (Runner, error) {
 	c := cli.RunTask
 
+	if len(c.EntrypointOverride) > 0 && os.Getpid() == 1 {
+		return entrypoint.New(c.EntrypointOverride), nil
+	}
+
 	// Strip the orchestrator configuration from the environment
 	_ = os.Unsetenv("CIRCLECI_GOAT_CONFIG")
 
@@ -130,3 +136,7 @@ func runSetup(ctx context.Context, cli cli, version string, sys *system.System)
 
 	return o, nil
 }
+
+type Runner interface {
+	Run(ctx context.Context) error
+}

task/cmd/cmd.go

@@ -10,7 +10,6 @@ import (
 	"os/signal"
 	"os/user"
 	"strconv"
-	"strings"
 	"sync/atomic"
 	"syscall"
 
@@ -106,17 +105,7 @@ func newCmd(ctx context.Context, argv []string, user string, stderrSaver *prefix
 	//#nosec:G204 // this is intentionally setting up a command
 	cmd := exec.CommandContext(ctx, argv[0], argv[1:]...)
 
-	for _, env := range os.Environ() {
-		if strings.HasPrefix(env, "CIRCLECI_GOAT") {
-			// Prevent internal configuration from being injected in the command environment
-			continue
-		}
-		cmd.Env = append(cmd.Env, env)
-	}
-	if env != nil {
-		cmd.Env = append(cmd.Env, env...)
-	}
-
+	cmd.Env = Environ(env...)
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = io.MultiWriter(os.Stderr, stderrSaver)
 

task/cmd/environ.go

@@ -0,0 +1,21 @@
+package cmd
+
+import (
+	"os"
+	"strings"
+)
+
+func Environ(extraEnv ...string) (environ []string) {
+	for _, env := range os.Environ() {
+		if strings.HasPrefix(env, "CIRCLECI_GOAT") {
+			// Prevent internal configuration from being unintentionally injected in the command environment
+			continue
+		}
+		environ = append(environ, env)
+	}
+	if extraEnv != nil {
+		environ = append(environ, extraEnv...)
+	}
+
+	return environ
+}

task/entrypoint/entrypoint.go

@@ -0,0 +1,35 @@
+package entrypoint
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"syscall"
+
+	"github.com/circleci/ex/o11y"
+)
+
+type Entrypoint struct {
+	args []string
+}
+
+func New(args []string) Entrypoint {
+	return Entrypoint{args}
+}
+
+func (e Entrypoint) Run(ctx context.Context) (err error) {
+	_, span := o11y.StartSpan(ctx, "override-entrypoint")
+	defer o11y.End(span, &err)
+
+	args := os.Args
+	if len(e.args) > 1 {
+		args = append(e.args[1:], os.Args...)
+	}
+
+	//#nosec:G204 // this is intentionally setting up a command
+	if err := syscall.Exec(e.args[0], args, os.Environ()); err != nil {
+		return fmt.Errorf("error execing entrypoint overide: %w", err)
+	}
+
+	return nil
+}