/*
 *
 * Copyright 2021 gRPC authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

// Package googledirectpath implements a resolver that configures xds to make
// cloud to prod directpath connection.
//
// It's a combo of DNS and xDS resolvers. It delegates to DNS if
// - not on GCE, or
// - xDS bootstrap env var is set (so this client needs to do normal xDS, not
// direct path, and clients with this scheme is not part of the xDS mesh).
package googledirectpath

import (
	"encoding/json"
	"fmt"
	"math/rand"
	"net/url"
	"time"

	"google.golang.org/grpc/grpclog"
	"google.golang.org/grpc/internal/envconfig"
	"google.golang.org/grpc/internal/googlecloud"
	internalgrpclog "google.golang.org/grpc/internal/grpclog"
	"google.golang.org/grpc/internal/xds/bootstrap"
	"google.golang.org/grpc/resolver"

	_ "google.golang.org/grpc/xds" // To register xds resolvers and balancers.
)

const (
	c2pScheme    = "google-c2p"
	c2pAuthority = "traffic-director-c2p.xds.googleapis.com"

	tdURL                   = "dns:///directpath-pa.googleapis.com"
	zoneURL                 = "http://metadata.google.internal/computeMetadata/v1/instance/zone"
	ipv6URL                 = "http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/ipv6s"
	ipv6CapableMetadataName = "TRAFFICDIRECTOR_DIRECTPATH_C2P_IPV6_CAPABLE"
	httpReqTimeout          = 10 * time.Second

	logPrefix        = "[google-c2p-resolver]"
	dnsName, xdsName = "dns", "xds"
)

// For overriding in unittests.
var (
	onGCE   = googlecloud.OnGCE
	randInt = rand.Int
	logger  = internalgrpclog.NewPrefixLogger(grpclog.Component("directpath"), logPrefix)
)

func init() {
	resolver.Register(c2pResolverBuilder{})
}

type c2pResolverBuilder struct{}

func (c2pResolverBuilder) Build(t resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (resolver.Resolver, error) {
	if t.URL.Host != "" {
		return nil, fmt.Errorf("google-c2p URI scheme does not support authorities")
	}

	if !runDirectPath() {
		// If not xDS, fallback to DNS.
		t.URL.Scheme = dnsName
		return resolver.Get(dnsName).Build(t, cc, opts)
	}

	// Note that the following calls to getZone() and getIPv6Capable() does I/O,
	// and has 10 seconds timeout each.
	//
	// This should be fine in most of the cases. In certain error cases, this
	// could block Dial() for up to 10 seconds (each blocking call has its own
	// goroutine).
	zoneCh, ipv6CapableCh := make(chan string), make(chan bool)
	go func() { zoneCh <- getZone(httpReqTimeout) }()
	go func() { ipv6CapableCh <- getIPv6Capable(httpReqTimeout) }()

	xdsServerURI := envconfig.C2PResolverTestOnlyTrafficDirectorURI
	if xdsServerURI == "" {
		xdsServerURI = tdURL
	}

	nodeCfg := newNodeConfig(<-zoneCh, <-ipv6CapableCh)
	xdsServerCfg := newXdsServerConfig(xdsServerURI)
	authoritiesCfg := newAuthoritiesConfig(xdsServerCfg)

	cfg := map[string]any{
		"xds_servers": []any{xdsServerCfg},
		"client_default_listener_resource_name_template": "%s",
		"authorities": authoritiesCfg,
		"node":        nodeCfg,
	}
	cfgJSON, err := json.Marshal(cfg)
	if err != nil {
		return nil, fmt.Errorf("failed to marshal bootstrap configuration: %v", err)
	}
	if err := bootstrap.SetFallbackBootstrapConfig(cfgJSON); err != nil {
		return nil, fmt.Errorf("failed to set fallback bootstrap configuration: %v", err)
	}

	t = resolver.Target{
		URL: url.URL{
			Scheme: xdsName,
			Host:   c2pAuthority,
			Path:   t.URL.Path,
		},
	}
	return resolver.Get(xdsName).Build(t, cc, opts)
}

func (b c2pResolverBuilder) Scheme() string {
	return c2pScheme
}

func newNodeConfig(zone string, ipv6Capable bool) map[string]any {
	node := map[string]any{
		"id":       fmt.Sprintf("C2P-%d", randInt()),
		"locality": map[string]any{"zone": zone},
	}
	if ipv6Capable {
		node["metadata"] = map[string]any{ipv6CapableMetadataName: true}
	}
	return node
}

func newAuthoritiesConfig(serverCfg map[string]any) map[string]any {
	return map[string]any{
		c2pAuthority: map[string]any{"xds_servers": []any{serverCfg}},
	}
}

func newXdsServerConfig(uri string) map[string]any {
	return map[string]any{
		"server_uri":      uri,
		"channel_creds":   []map[string]any{{"type": "google_default"}},
		"server_features": []any{"ignore_resource_deletion"},
	}
}

// runDirectPath returns whether this resolver should use direct path.
//
// direct path is enabled if this client is running on GCE.
func runDirectPath() bool {
	return onGCE()
}