agent: command.update handler + updater package (Linux + Windows)

internal/agent/updater/updater_unix.go

@@ -0,0 +1,73 @@
+//go:build !windows
+
+package updater
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"time"
+)
+
+// Update fetches the new binary, swaps it in, then exits so systemd
+// restarts the process under the new binary. The caller should close
+// the WS connection cleanly (so the server transitions the host to
+// disconnected immediately rather than waiting for the heartbeat
+// sweep) before invoking.
+//
+// Service-user assumption: the agent runs as root under the
+// systemd-shipped unit, which can write the binary path directly.
+// If the agent ever moves to a non-root service user, this breaks —
+// would need a setuid helper or an out-of-process update service.
+func Update(ctx context.Context, serverURL string) error {
+	binPath, err := resolveOwnBinary()
+	if err != nil {
+		return err
+	}
+	stage, err := fetch(ctx, serverURL, binPath)
+	if err != nil {
+		return err
+	}
+	if err := swap(stage, binPath); err != nil {
+		return err
+	}
+	slog.Info("agent self-update: binary swapped, exiting for systemd restart",
+		"binary", binPath)
+	// Give logger / WS close-frame a moment to flush, then exit.
+	time.Sleep(200 * time.Millisecond)
+	os.Exit(0)
+	return nil // unreachable
+}
+
+// swap copies the running binary to <bin>.old (M1 — keep one revision
+// back for hand-rolled rollback), then atomic-renames the staged
+// binary into place. Linux supports rename-while-open so this works
+// even though the running process holds the source open.
+func swap(stagePath, binPath string) error {
+	src, err := os.Open(binPath)
+	if err != nil {
+		return fmt.Errorf("open running binary: %w", err)
+	}
+	defer func() { _ = src.Close() }()
+	dst, err := os.OpenFile(binPath+".old", os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o755)
+	if err != nil {
+		return fmt.Errorf("open .old: %w", err)
+	}
+	if _, err := io.Copy(dst, src); err != nil {
+		_ = dst.Close()
+		return fmt.Errorf("copy to .old: %w", err)
+	}
+	if err := dst.Sync(); err != nil {
+		_ = dst.Close()
+		return err
+	}
+	if err := dst.Close(); err != nil {
+		return err
+	}
+	if err := os.Rename(stagePath, binPath); err != nil {
+		return fmt.Errorf("rename .new over running binary: %w", err)
+	}
+	return nil
+}