// Copyright 2019 The Go 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 dcensus provides functionality for debug instrumentation.
package dcensus

import (
	"context"
	"fmt"
	"net/http"
	"strings"
	"time"

	"contrib.go.opencensus.io/exporter/prometheus"
	"contrib.go.opencensus.io/exporter/stackdriver"
	"go.opencensus.io/plugin/ochttp"
	"go.opencensus.io/stats/view"
	"go.opencensus.io/tag"
	"go.opencensus.io/trace"
	"go.opencensus.io/zpages"
	"golang.org/x/pkgsite/internal/config"
	"golang.org/x/pkgsite/internal/derrors"
	"golang.org/x/pkgsite/internal/log"
	mrpb "google.golang.org/genproto/googleapis/api/monitoredres"
)

// RouteTagger is a func that can be used to derive a dynamic route tag for an
// incoming request.
type RouteTagger func(route string, r *http.Request) string

// Router is an http multiplexer that instruments per-handler debugging
// information and census instrumentation.
type Router struct {
	http.Handler
	mux    *http.ServeMux
	tagger RouteTagger
}

// NewRouter creates a new Router, using tagger to tag incoming requests in
// monitoring. If tagger is nil, a default route tagger is used.
func NewRouter(tagger RouteTagger) *Router {
	if tagger == nil {
		tagger = func(route string, r *http.Request) string {
			return strings.Trim(route, "/")
		}
	}
	mux := http.NewServeMux()
	return &Router{
		mux:     mux,
		Handler: &ochttp.Handler{Handler: mux},
		tagger:  tagger,
	}
}

// Handle registers handler with the given route. It has the same routing
// semantics as http.ServeMux.
func (r *Router) Handle(route string, handler http.Handler) {
	r.mux.HandleFunc(route, func(w http.ResponseWriter, req *http.Request) {
		tag := r.tagger(route, req)
		ochttp.WithRouteTag(handler, tag).ServeHTTP(w, req)
	})
}

// HandleFunc is a wrapper around Handle for http.HandlerFuncs.
func (r *Router) HandleFunc(route string, handler http.HandlerFunc) {
	r.Handle(route, handler)
}

const debugPage = `
<html>
<p><a href="/tracez">/tracez</a> - trace spans</p>
<p><a href="/statsz">/statz</a> - prometheus metrics page</p>
`

// Init configures tracing and aggregation according to the given Views. If
// running on GCP, Init also configures exporting to StackDriver.
func Init(cfg *config.Config, views ...*view.View) error {
	// The default trace sampler samples with probability 1e-4. That's too
	// infrequent for our traffic levels. In the future we may want to decrease
	// this sampling rate.
	trace.ApplyConfig(trace.Config{DefaultSampler: trace.ProbabilitySampler(0.01)})
	if err := view.Register(views...); err != nil {
		return fmt.Errorf("dcensus.Init(views): view.Register: %v", err)
	}
	exportToStackdriver(context.Background(), cfg)
	return nil
}

// NewServer creates a new http.Handler for serving debug information.
func NewServer() (http.Handler, error) {
	pe, err := prometheus.NewExporter(prometheus.Options{})
	if err != nil {
		return nil, fmt.Errorf("dcensus.NewServer: prometheus.NewExporter: %v", err)
	}
	mux := http.NewServeMux()
	zpages.Handle(mux, "/")
	mux.Handle("/statsz", pe)
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprint(w, debugPage)
	})

	return mux, nil
}

// monitoredResource wraps a *mrpb.MonitoredResource to implement the
// monitoredresource.MonitoredResource interface.
type monitoredResource mrpb.MonitoredResource

func (r *monitoredResource) MonitoredResource() (resType string, labels map[string]string) {
	return r.Type, r.Labels
}

// ExportToStackdriver checks to see if the process is running in a GCP
// environment, and if so configures exporting to stackdriver.
func exportToStackdriver(ctx context.Context, cfg *config.Config) {
	if cfg.ProjectID == "" {
		log.Infof(ctx, "Not exporting to StackDriver: GOOGLE_CLOUD_PROJECT is unset.")
		return
	}

	// Report statistics every minutes, due to stackdriver limitations described at
	// https://cloud.google.com/monitoring/custom-metrics/creating-metrics#writing-ts
	view.SetReportingPeriod(time.Minute)

	viewExporter, err := NewViewExporter(cfg)
	if err != nil {
		log.Fatalf(ctx, "error creating view exporter: %v", err)
	}
	view.RegisterExporter(viewExporter)

	// We want traces to be associated with the *app*, not the instance.
	// TraceSpansBufferMaxBytes is increased from the default of 8MiB, though we
	// can't increase *too* much because this is still running in GAE, which is
	// relatively memory-constrained.
	traceExporter, err := stackdriver.NewExporter(stackdriver.Options{
		ProjectID:                cfg.ProjectID,
		MonitoredResource:        (*monitoredResource)(cfg.MonitoredResource),
		TraceSpansBufferMaxBytes: 32 * 1024 * 1024, // 32 MiB
		DefaultMonitoringLabels:  stackdriverLabels(cfg),
	})
	if err != nil {
		log.Fatalf(ctx, "error creating trace exporter: %v", err)
	}
	trace.RegisterExporter(traceExporter)
}

// NewViewExporter creates a StackDriver exporter for stats.
func NewViewExporter(cfg *config.Config) (_ *stackdriver.Exporter, err error) {
	defer derrors.Wrap(&err, "NewViewExporter()")

	// Views must be associated with the instance, else we run into overlapping
	// timeseries problems. Note that generic_task is used because the
	// gae_instance resource type is not supported for metrics:
	// https://cloud.google.com/monitoring/custom-metrics/creating-metrics#which-resource
	return stackdriver.NewExporter(stackdriver.Options{
		ProjectID: cfg.ProjectID,
		MonitoredResource: &monitoredResource{
			Type: "generic_task",
			Labels: map[string]string{
				"project_id": cfg.ProjectID,
				"location":   cfg.LocationID,
				"job":        cfg.ServiceID,
				"namespace":  "go-discovery",
				"task_id":    cfg.InstanceID,
			},
		},
		DefaultMonitoringLabels: stackdriverLabels(cfg),
	})
}

func stackdriverLabels(cfg *config.Config) *stackdriver.Labels {
	labels := &stackdriver.Labels{}
	labels.Set("version", cfg.AppVersionLabel(), "Version label of the running binary")
	labels.Set("env", cfg.DeploymentEnvironment(), "deployment environment")
	labels.Set("app", cfg.Application(), "application name")
	return labels
}

// Customizations of ochttp views. Views are updated as follows:
//  + ClientHost and ServerRoute are added to resp. client and server metrics.
//    Since these are bounded cardinality in our metrics, they are useful to
//    add additional context.
//  + Method tags are removed. We don't have any routes that accept more than
//    one HTTP method.
var (
	ServerRequestCount = &view.View{
		Name:        "go-discovery/http/server/request_count",
		Description: "Count of HTTP requests started by Method",
		TagKeys:     []tag.Key{ochttp.Method},
		Measure:     ochttp.ServerRequestCount,
		Aggregation: view.Count(),
	}
	ServerResponseCount = &view.View{
		Name:        "go-discovery/http/server/response_count",
		Description: "Server response count by status code and route",
		TagKeys:     []tag.Key{ochttp.StatusCode, ochttp.KeyServerRoute},
		Measure:     ochttp.ServerLatency,
		Aggregation: view.Count(),
	}
	ServerLatency = &view.View{
		Name:        "go-discovery/http/server/response_latency",
		Description: "Server response distribution by status code and route",
		TagKeys:     []tag.Key{ochttp.KeyServerRoute},
		Measure:     ochttp.ServerLatency,
		Aggregation: ochttp.DefaultLatencyDistribution,
	}
	ServerResponseBytes = &view.View{
		Name:        "go-discovery/http/server/response_bytes",
		Description: "Size distribution of HTTP response body",
		TagKeys:     []tag.Key{ochttp.KeyServerRoute},
		Measure:     ochttp.ServerResponseBytes,
		Aggregation: ochttp.DefaultSizeDistribution,
	}
	ServerViews = []*view.View{
		ServerRequestCount,
		ServerResponseCount,
		ServerLatency,
		ServerResponseBytes,
	}
)