doctor: add ts_omit_doctor support

Updates #12614

Change-Id: I84c166c4b99ca75d70abe4087e5ff3f7d90d4bcc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/doctor/doctor.go

@@ -0,0 +1,95 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// The doctor package registers the "doctor" problem diagnosis support into the
+// rest of Tailscale.
+package doctor
+
+import (
+	"context"
+	"fmt"
+	"html"
+	"net/http"
+	"time"
+
+	"tailscale.com/doctor"
+	"tailscale.com/doctor/ethtool"
+	"tailscale.com/doctor/permissions"
+	"tailscale.com/doctor/routetable"
+	"tailscale.com/ipn/ipnlocal"
+	"tailscale.com/net/tsaddr"
+	"tailscale.com/types/logger"
+)
+
+func init() {
+	ipnlocal.HookDoctor.Set(visitDoctor)
+	ipnlocal.RegisterPeerAPIHandler("/v0/doctor", handleServeDoctor)
+}
+
+func handleServeDoctor(h ipnlocal.PeerAPIHandler, w http.ResponseWriter, r *http.Request) {
+	if !h.CanDebug() {
+		http.Error(w, "denied; no debug access", http.StatusForbidden)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	fmt.Fprintln(w, "<h1>Doctor Output</h1>")
+
+	fmt.Fprintln(w, "<pre>")
+
+	b := h.LocalBackend()
+	visitDoctor(r.Context(), b, func(format string, args ...any) {
+		line := fmt.Sprintf(format, args...)
+		fmt.Fprintln(w, html.EscapeString(line))
+	})
+
+	fmt.Fprintln(w, "</pre>")
+}
+
+func visitDoctor(ctx context.Context, b *ipnlocal.LocalBackend, logf logger.Logf) {
+	// We can write logs too fast for logtail to handle, even when
+	// opting-out of rate limits. Limit ourselves to at most one message
+	// per 20ms and a burst of 60 log lines, which should be fast enough to
+	// not block for too long but slow enough that we can upload all lines.
+	logf = logger.SlowLoggerWithClock(ctx, logf, 20*time.Millisecond, 60, b.Clock().Now)
+
+	var checks []doctor.Check
+	checks = append(checks,
+		permissions.Check{},
+		routetable.Check{},
+		ethtool.Check{},
+	)
+
+	// Print a log message if any of the global DNS resolvers are Tailscale
+	// IPs; this can interfere with our ability to connect to the Tailscale
+	// controlplane.
+	checks = append(checks, doctor.CheckFunc("dns-resolvers", func(_ context.Context, logf logger.Logf) error {
+		nm := b.NetMap()
+		if nm == nil {
+			return nil
+		}
+
+		for i, resolver := range nm.DNS.Resolvers {
+			ipp, ok := resolver.IPPort()
+			if ok && tsaddr.IsTailscaleIP(ipp.Addr()) {
+				logf("resolver %d is a Tailscale address: %v", i, resolver)
+			}
+		}
+		for i, resolver := range nm.DNS.FallbackResolvers {
+			ipp, ok := resolver.IPPort()
+			if ok && tsaddr.IsTailscaleIP(ipp.Addr()) {
+				logf("fallback resolver %d is a Tailscale address: %v", i, resolver)
+			}
+		}
+		return nil
+	}))
+
+	// TODO(andrew): more
+
+	numChecks := len(checks)
+	checks = append(checks, doctor.CheckFunc("numchecks", func(_ context.Context, log logger.Logf) error {
+		log("%d checks", numChecks)
+		return nil
+	}))
+
+	doctor.RunChecks(ctx, logf, checks...)
+}