exec: don't panic on rootless raw_exec tasks (#26401)

the executor dies, leaving an orphaned process still running.

the panic fix:
 * don't `panic()`
 * and return an empty, but non-nil, func on cgroup error

feature fix:
 * allow non-root agent to proceed with exec when cgroups are off

.changelog/26401.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+alloc exec: Fixed executor panic when exec-ing a rootless raw_exec task
+```

drivers/shared/executor/executor.go

@@ -5,6 +5,7 @@ package executor
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -49,6 +50,9 @@ var (
 	// The statistics the basic executor exposes
 	ExecutorBasicMeasuredMemStats = []string{"RSS", "Swap"}
 	ExecutorBasicMeasuredCpuStats = []string{"System Mode", "User Mode", "Percent"}
+
+	// ErrCgroupMustBeSet occurs if a cgroup is not provided when expected
+	ErrCgroupMustBeSet = errors.New("cgroup must be set")
 )
 
 // Executor is the interface which allows a driver to launch and supervise
@@ -441,7 +445,7 @@ func (e *UniversalExecutor) Exec(deadline time.Time, name string, args []string)
 	defer cancel()
 
 	if cleanup, err := e.setSubCmdCgroup(&e.childCmd, e.command.StatsCgroup()); err != nil {
-		return nil, 0, err
+		return nil, 0, fmt.Errorf("Exec: %w", err)
 	} else {
 		defer cleanup()
 	}
@@ -533,7 +537,7 @@ func (e *UniversalExecutor) ExecStreaming(ctx context.Context, command []string,
 			}
 			cgroup := e.command.StatsCgroup()
 			if cleanup, err := e.setSubCmdCgroup(cmd, cgroup); err != nil {
-				return err
+				return fmt.Errorf("ExecStreaming: %w", err)
 			} else {
 				defer cleanup()
 			}

drivers/shared/executor/executor_linux.go

@@ -7,7 +7,6 @@ package executor
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -766,7 +765,7 @@ func (l *LibcontainerExecutor) configureCgroups(cfg *runc.Config, command *ExecC
 
 	cg := command.StatsCgroup()
 	if cg == "" {
-		return errors.New("cgroup must be set")
+		return fmt.Errorf("configureCgroups: %w", ErrCgroupMustBeSet)
 	}
 
 	// // set the libcontainer hook for writing the PID to cgroup.procs file

drivers/shared/executor/executor_universal_linux.go

@@ -30,27 +30,32 @@ const (
 // setSubCmdCgroup sets the cgroup for non-Task child processes of the
 // executor.Executor (since in cg2 it lives outside the task's cgroup)
 func (e *UniversalExecutor) setSubCmdCgroup(cmd *exec.Cmd, cgroup string) (func(), error) {
+
+	// no extra setup needed for cg v1 or when cgroups are "off"
+	switch cgroupslib.GetMode() {
+	case cgroupslib.OFF, cgroupslib.CG1:
+		return func() {}, nil
+	default:
+		// continue for cg v2
+	}
+
 	if cgroup == "" {
-		panic("cgroup must be set")
+		return nil, fmt.Errorf("error setting up exec subcommand: %w", ErrCgroupMustBeSet)
+	}
+
+	fd, cleanup, err := e.statCG(cgroup)
+	if err != nil {
+		return nil, err
 	}
 
 	// make sure attrs struct has been set
 	if cmd.SysProcAttr == nil {
 		cmd.SysProcAttr = new(syscall.SysProcAttr)
 	}
+	cmd.SysProcAttr.UseCgroupFD = true
+	cmd.SysProcAttr.CgroupFD = fd
 
-	switch cgroupslib.GetMode() {
-	case cgroupslib.CG2:
-		fd, cleanup, err := e.statCG(cgroup)
-		if err != nil {
-			return nil, err
-		}
-		cmd.SysProcAttr.UseCgroupFD = true
-		cmd.SysProcAttr.CgroupFD = fd
-		return cleanup, nil
-	default:
-		return func() {}, nil
-	}
+	return cleanup, nil
 }
 
 func (e *UniversalExecutor) ListProcesses() set.Collection[procstats.ProcessID] {
@@ -93,11 +98,8 @@ func (e *UniversalExecutor) configureResourceContainer(
 ) (runningFunc, cleanupFunc, error) {
 	cgroup := command.StatsCgroup()
 
-	// ensure tasks get the desired oom_score_adj value set
-	if err := e.setOomAdj(command.OOMScoreAdj); err != nil {
-		return nil, nil, err
-	}
-
+	// we specify these return funcs as empty but non-nil,
+	// because callers may call them even if this function errors.
 	// deleteCgroup will be called after the task has been launched
 	// v1: remove the executor process from the task's cgroups
 	// v2: let go of the file descriptor of the task's cgroup
@@ -106,6 +108,11 @@ func (e *UniversalExecutor) configureResourceContainer(
 		moveProcess  = func() error { return nil }
 	)
 
+	// ensure tasks get the desired oom_score_adj value set
+	if err := e.setOomAdj(command.OOMScoreAdj); err != nil {
+		return moveProcess, deleteCgroup, err
+	}
+
 	// manually configure cgroup for cpu / memory constraints
 	switch cgroupslib.GetMode() {
 	case cgroupslib.CG1:
@@ -113,9 +120,10 @@ func (e *UniversalExecutor) configureResourceContainer(
 			return moveProcess, deleteCgroup, err
 		}
 		moveProcess, deleteCgroup = e.enterCG1(cgroup, command.CpusetCgroup())
+
 	case cgroupslib.OFF:
-		deleteCgroup = func() {}
-		moveProcess = func() error { return nil }
+		// do nothing
+
 	default:
 		e.configureCG2(cgroup, command)
 		// configure child process to spawn in the cgroup