// Copyright 2020 The Prometheus 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 discovery

import (
	"context"
	"reflect"

	"github.com/go-kit/log"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/common/config"

	"github.com/prometheus/prometheus/discovery/targetgroup"
)

// Discoverer provides information about target groups. It maintains a set
// of sources from which TargetGroups can originate. Whenever a discovery provider
// detects a potential change, it sends the TargetGroup through its channel.
//
// Discoverer does not know if an actual change happened.
// It does guarantee that it sends the new TargetGroup whenever a change happens.
//
// Discoverers should initially send a full set of all discoverable TargetGroups.
type Discoverer interface {
	// Run hands a channel to the discovery provider (Consul, DNS, etc.) through which
	// it can send updated target groups. It must return when the context is canceled.
	// It should not close the update channel on returning.
	Run(ctx context.Context, up chan<- []*targetgroup.Group)
}

// DiscovererOptions provides options for a Discoverer.
type DiscovererOptions struct {
	Logger log.Logger

	// A registerer for the Discoverer's metrics.
	// Some Discoverers may ignore this registerer and use the global one instead.
	// For now this will work, because the Prometheus `main` function uses the global registry.
	// However, in the future the Prometheus `main` function will be updated to not use the global registry.
	// Hence, if a discoverer wants its metrics to be visible via the Prometheus executable's
	// `/metrics` endpoint, it should use this explicit registerer.
	// TODO(ptodev): Update this comment once the Prometheus `main` function does not use the global registry.
	Registerer prometheus.Registerer

	// Extra HTTP client options to expose to Discoverers. This field may be
	// ignored; Discoverer implementations must opt-in to reading it.
	HTTPClientOptions []config.HTTPClientOption
}

// A Config provides the configuration and constructor for a Discoverer.
type Config interface {
	// Name returns the name of the discovery mechanism.
	Name() string

	// NewDiscoverer returns a Discoverer for the Config
	// with the given DiscovererOptions.
	NewDiscoverer(DiscovererOptions) (Discoverer, error)
}

// Configs is a slice of Config values that uses custom YAML marshaling and unmarshaling
// to represent itself as a mapping of the Config values grouped by their types.
type Configs []Config

// SetDirectory joins any relative file paths with dir.
func (c *Configs) SetDirectory(dir string) {
	for _, c := range *c {
		if v, ok := c.(config.DirectorySetter); ok {
			v.SetDirectory(dir)
		}
	}
}

// UnmarshalYAML implements yaml.Unmarshaler.
func (c *Configs) UnmarshalYAML(unmarshal func(interface{}) error) error {
	cfgTyp := getConfigType(configsType)
	cfgPtr := reflect.New(cfgTyp)
	cfgVal := cfgPtr.Elem()

	if err := unmarshal(cfgPtr.Interface()); err != nil {
		return replaceYAMLTypeError(err, cfgTyp, configsType)
	}

	var err error
	*c, err = readConfigs(cfgVal, 0)
	return err
}

// MarshalYAML implements yaml.Marshaler.
func (c Configs) MarshalYAML() (interface{}, error) {
	cfgTyp := getConfigType(configsType)
	cfgPtr := reflect.New(cfgTyp)
	cfgVal := cfgPtr.Elem()

	if err := writeConfigs(cfgVal, c); err != nil {
		return nil, err
	}

	return cfgPtr.Interface(), nil
}

// A StaticConfig is a Config that provides a static list of targets.
type StaticConfig []*targetgroup.Group

// Name returns the name of the service discovery mechanism.
func (StaticConfig) Name() string { return "static" }

// NewDiscoverer returns a Discoverer for the Config.
func (c StaticConfig) NewDiscoverer(DiscovererOptions) (Discoverer, error) {
	return staticDiscoverer(c), nil
}

type staticDiscoverer []*targetgroup.Group

func (c staticDiscoverer) Run(ctx context.Context, up chan<- []*targetgroup.Group) {
	// TODO: existing implementation closes up chan, but documentation explicitly forbids it...?
	defer close(up)
	select {
	case <-ctx.Done():
	case up <- c:
	}
}