prober: library to build healthchecking probers.

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

prober/http.go

@@ -0,0 +1,62 @@
+// 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 (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+const maxHTTPBody = 4 << 20 // MiB
+
+// HTTP returns a Probe that healthchecks an HTTP URL.
+//
+// The Probe sends a GET request for url, expects an HTTP 200
+// response, and verifies that want is present in the response
+// body. If the URL is HTTPS, the probe further checks that the TLS
+// certificate is good for at least the next 7 days.
+func HTTP(url, wantText string) Probe {
+	return func(ctx context.Context) error {
+		return probeHTTP(ctx, url, []byte(wantText))
+	}
+}
+
+func probeHTTP(ctx context.Context, url string, want []byte) error {
+	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
+	if err != nil {
+		return fmt.Errorf("constructing request: %w", err)
+	}
+
+	// Get a completely new transport each time, so we don't reuse a
+	// past connection.
+	tr := http.DefaultTransport.(*http.Transport).Clone()
+	defer tr.CloseIdleConnections()
+	c := &http.Client{
+		Transport: tr,
+	}
+
+	resp, err := c.Do(req)
+	if err != nil {
+		return fmt.Errorf("fetching %q: %w", url, err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		return fmt.Errorf("fetching %q: status code %d, want 200", url, resp.StatusCode)
+	}
+
+	bs, err := io.ReadAll(&io.LimitedReader{resp.Body, maxHTTPBody})
+	if err != nil {
+		return fmt.Errorf("reading body of %q: %w", url, err)
+	}
+
+	if !bytes.Contains(bs, want) {
+		return fmt.Errorf("body of %q does not contain %q", url, want)
+	}
+
+	return nil
+}