prober: library to build healthchecking probers.

Signed-off-by: David Anderson <danderson@tailscale.com>

prober/tcp.go

@@ -0,0 +1,30 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package prober
+
+import (
+	"context"
+	"fmt"
+	"net"
+)
+
+// TCP returns a Probe that healthchecks a TCP endpoint.
+//
+// The Probe reports whether it can successfully connect to addr.
+func TCP(addr string) Probe {
+	return func(ctx context.Context) error {
+		return probeTCP(ctx, addr)
+	}
+}
+
+func probeTCP(ctx context.Context, addr string) error {
+	var d net.Dialer
+	conn, err := d.DialContext(ctx, "tcp", addr)
+	if err != nil {
+		return fmt.Errorf("dialing %q: %v", addr, err)
+	}
+	conn.Close()
+	return nil
+}