{client/web},{ipn/ipnlocal}: replace localapi debug-web-client endpoint

This change removes the existing debug-web-client localapi endpoint
and replaces it with functions passed directly to the web.ServerOpts
when constructing a web.ManageServerMode client.

The debug-web-client endpoint previously handled making noise
requests to the control server via the /machine/webclient/ endpoints.
The noise requests must be made from tailscaled, which has the noise
connection open. But, now that the full client is served from
tailscaled, we no longer need to proxy this request over the localapi.

Updates tailscale/corp#14335

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

ipn/ipnlocal/web_client.go

@@ -7,8 +7,10 @@ package ipnlocal
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"net"
 	"net/http"
 	"net/netip"
@@ -19,6 +21,7 @@ import (
 	"tailscale.com/client/web"
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/net/netutil"
+	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/mak"
 )
@@ -67,6 +70,8 @@ func (b *LocalBackend) webClientGetOrInit() (s *web.Server, err error) {
 		Mode:        web.ManageServerMode,
 		LocalClient: b.webClient.lc,
 		Logf:        b.logf,
+		NewAuthURL:  b.newWebClientAuthURL,
+		WaitAuthURL: b.waitWebClientAuthURL,
 	}); err != nil {
 		return nil, fmt.Errorf("web.NewServer: %w", err)
 	}
@@ -144,3 +149,57 @@ func (b *LocalBackend) newWebClientListener(ctx context.Context, ap netip.AddrPo
 		bo:      backoff.NewBackoff("webclient-listener", logf, 30*time.Second),
 	}
 }
+
+// newWebClientAuthURL talks to the control server to create a new auth
+// URL that can be used to validate a browser session to manage this
+// tailscaled instance via the web client.
+func (b *LocalBackend) newWebClientAuthURL(ctx context.Context, src tailcfg.NodeID) (*tailcfg.WebClientAuthResponse, error) {
+	return b.doWebClientNoiseRequest(ctx, "", src)
+}
+
+// waitWebClientAuthURL connects to the control server and blocks
+// until the associated auth URL has been completed by its user,
+// or until ctx is canceled.
+func (b *LocalBackend) waitWebClientAuthURL(ctx context.Context, id string, src tailcfg.NodeID) (*tailcfg.WebClientAuthResponse, error) {
+	return b.doWebClientNoiseRequest(ctx, id, src)
+}
+
+// doWebClientNoiseRequest handles making the "/machine/webclient"
+// noise requests to the control server for web client user auth.
+//
+// It either creates a new control auth URL or waits for an existing
+// one to be completed, based on the presence or absence of the
+// provided id value.
+func (b *LocalBackend) doWebClientNoiseRequest(ctx context.Context, id string, src tailcfg.NodeID) (*tailcfg.WebClientAuthResponse, error) {
+	nm := b.NetMap()
+	if nm == nil || !nm.SelfNode.Valid() {
+		return nil, errors.New("[unexpected] no self node")
+	}
+	dst := nm.SelfNode.ID()
+	var noiseURL string
+	if id != "" {
+		noiseURL = fmt.Sprintf("https://unused/machine/webclient/wait/%d/to/%d/%s", src, dst, id)
+	} else {
+		noiseURL = fmt.Sprintf("https://unused/machine/webclient/init/%d/to/%d", src, dst)
+	}
+
+	req, err := http.NewRequestWithContext(ctx, "POST", noiseURL, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := b.DoNoiseRequest(req)
+	if err != nil {
+		return nil, err
+	}
+
+	body, _ := io.ReadAll(resp.Body)
+	resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("failed request: %s", body)
+	}
+	var authResp *tailcfg.WebClientAuthResponse
+	if err := json.Unmarshal(body, &authResp); err != nil {
+		return nil, err
+	}
+	return authResp, nil
+}