control.go

// SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

package runtime

import (
	"fmt"
	"os"
	"path/filepath"
	"strconv"

	snapshotprotocol "github.com/ai-dynamo/dynamo/deploy/snapshot/protocol"
)

// WriteControlSentinel writes a sentinel file into the workload container's
// snapshot-control volume at SnapshotControlMountPath/<name>, accessed through
// the agent's /host/proc/<pid>/root view of the container's mount namespace.
//
// hostPID must be a PID inside the container's mount namespace (the container
// task PID is the canonical choice). The sentinel is observed by the workload
// via inotify on the control directory; it replaces the SIGUSR1/SIGCONT
// agent-to-workload signals that previously required the workload to run as
// PID 1.
//
// The write uses create-then-rename so the workload never observes a partial
// file.
func WriteControlSentinel(hostPID int, name string) error {
	if hostPID <= 0 {
		return fmt.Errorf("invalid host PID %d for control sentinel %q", hostPID, name)
	}
	dir := filepath.Join(HostProcPath, strconv.Itoa(hostPID), "root", snapshotprotocol.SnapshotControlMountPath)
	return writeSentinelInDir(dir, name)
}

func writeSentinelInDir(dir, name string) error {
	tmpPath := filepath.Join(dir, "."+name+".tmp")
	finalPath := filepath.Join(dir, name)
	if err := os.WriteFile(tmpPath, []byte("done\n"), 0o644); err != nil {
		return fmt.Errorf("write temp sentinel %s: %w", tmpPath, err)
	}
	if err := os.Rename(tmpPath, finalPath); err != nil {
		_ = os.Remove(tmpPath)
		return fmt.Errorf("rename sentinel %s -> %s: %w", tmpPath, finalPath, err)
	}
	return nil
}