feature/featuretags: add build tag to remove captive portal detection

This doesn't yet fully pull it out into a feature/captiveportal package.
This is the usual first step, moving the code to its own files within
the same packages.

Updates #17254

Change-Id: Idfaec839debf7c96f51ca6520ce36ccf2f8eec92
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netcheck/captiveportal.go

@@ -0,0 +1,55 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_captiveportal
+
+package netcheck
+
+import (
+	"context"
+	"time"
+
+	"tailscale.com/net/captivedetection"
+	"tailscale.com/tailcfg"
+)
+
+func init() {
+	hookStartCaptivePortalDetection.Set(startCaptivePortalDetection)
+}
+
+func startCaptivePortalDetection(ctx context.Context, rs *reportState, dm *tailcfg.DERPMap, preferredDERP int) (done <-chan struct{}, stop func()) {
+	c := rs.c
+
+	// NOTE(andrew): we can't simply add this goroutine to the
+	// `NewWaitGroupChan` below, since we don't wait for that
+	// waitgroup to finish when exiting this function and thus get
+	// a data race.
+	ch := make(chan struct{})
+
+	tmr := time.AfterFunc(c.captivePortalDelay(), func() {
+		defer close(ch)
+		d := captivedetection.NewDetector(c.logf)
+		found := d.Detect(ctx, c.NetMon, dm, preferredDERP)
+		rs.report.CaptivePortal.Set(found)
+	})
+
+	stop = func() {
+		// Don't cancel our captive portal check if we're
+		// explicitly doing a verbose netcheck.
+		if c.Verbose {
+			return
+		}
+
+		if tmr.Stop() {
+			// Stopped successfully; need to close the
+			// signal channel ourselves.
+			close(ch)
+			return
+		}
+
+		// Did not stop; do nothing and it'll finish by itself
+		// and close the signal channel.
+	}
+
+	return ch, stop
+}