Bound durable outbox storage and enforce process writer exclusion

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a140c4-354a-7823-8f0d-8f717397f786

internal/emission/lock_linux.go

@@ -0,0 +1,44 @@
+//go:build linux
+
+package emission
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+func acquireWriterLock(path string) (*os.File, error) {
+	file, err := openOwnedFile(path, os.O_CREATE|os.O_RDWR, 0600)
+	if err != nil {
+		return nil, err
+	}
+	if err := syscall.Flock(int(file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
+		file.Close()
+		return nil, errors.New("outbox already has a writer or filesystem lock unavailable")
+	}
+	return file, nil
+}
+func infoSys(info os.FileInfo) (*syscall.Stat_t, bool) {
+	if info == nil {
+		return nil, false
+	}
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	return stat, ok
+}
+
+// Shared journal/cursor aliases must not evade directory-based exclusion.
+// Nonblocking open also rejects unexpected FIFOs/devices without waiting.
+func openOwnedFile(path string, flags int, mode os.FileMode) (*os.File, error) {
+	file, err := os.OpenFile(path, flags|syscall.O_NOFOLLOW|syscall.O_NONBLOCK, mode)
+	if err != nil {
+		return nil, err
+	}
+	info, err := file.Stat()
+	stat, ok := infoSys(info)
+	if err != nil || !ok || !info.Mode().IsRegular() || info.Mode().Perm()&0007 != 0 || stat.Nlink != 1 || stat.Uid != uint32(os.Geteuid()) || (info.Mode().Perm()&0070 != 0 && stat.Gid != uint32(os.Getegid())) {
+		file.Close()
+		return nil, errors.New("outbox file must be a single-link owner/current-group regular file")
+	}
+	return file, nil
+}