Confine cleanup and use held session locks

internal/artifacts/lock_unix.go

@@ -0,0 +1,22 @@
+//go:build linux || darwin
+
+package artifacts
+
+import (
+	"errors"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func acquireHeldFileLock(file *os.File) error {
+	err := unix.Flock(int(file.Fd()), unix.LOCK_EX|unix.LOCK_NB)
+	if errors.Is(err, unix.EWOULDBLOCK) || errors.Is(err, unix.EAGAIN) {
+		return errHeldLockConflict
+	}
+	return err
+}
+
+func releaseHeldFileLock(file *os.File) error {
+	return unix.Flock(int(file.Fd()), unix.LOCK_UN)
+}