feat(health): add TCP connectivity probe

internal/health/health.go

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+
+// Package health performs post-apply connectivity checks.
+package health
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"time"
+)
+
+// DialTCP attempts a TCP connection to addr within timeout.
+func DialTCP(ctx context.Context, addr string, timeout time.Duration) error {
+	d := net.Dialer{Timeout: timeout}
+	conn, err := d.DialContext(ctx, "tcp", addr)
+	if err != nil {
+		return fmt.Errorf("connect %s: %w", addr, err)
+	}
+	return conn.Close()
+}

internal/health/health_test.go

@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: MIT
+
+package health
+
+import (
+	"context"
+	"net"
+	"testing"
+	"time"
+)
+
+func TestDialTCPSuccess(t *testing.T) {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("Listen: %v", err)
+	}
+	defer ln.Close()
+	go func() {
+		for {
+			conn, err := ln.Accept()
+			if err != nil {
+				return
+			}
+			_ = conn.Close()
+		}
+	}()
+
+	if err := DialTCP(context.Background(), ln.Addr().String(), time.Second); err != nil {
+		t.Fatalf("DialTCP to a live listener failed: %v", err)
+	}
+}
+
+func TestDialTCPFailure(t *testing.T) {
+	// Reserve a port and close it so the dial is refused.
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("Listen: %v", err)
+	}
+	addr := ln.Addr().String()
+	ln.Close()
+
+	if err := DialTCP(context.Background(), addr, 500*time.Millisecond); err == nil {
+		t.Fatal("expected a dial error to a closed port")
+	}
+}