/*
Copyright 2020 The Crossplane 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 controller provides utilties for working with controllers.
package controller

import (
	"context"
	"sync"

	"k8s.io/client-go/rest"
	"sigs.k8s.io/controller-runtime/pkg/cache"
	"sigs.k8s.io/controller-runtime/pkg/client"
	"sigs.k8s.io/controller-runtime/pkg/client/apiutil"
	"sigs.k8s.io/controller-runtime/pkg/controller"
	"sigs.k8s.io/controller-runtime/pkg/handler"
	"sigs.k8s.io/controller-runtime/pkg/manager"
	"sigs.k8s.io/controller-runtime/pkg/predicate"
	"sigs.k8s.io/controller-runtime/pkg/source"

	"github.com/crossplane/crossplane-runtime/pkg/errors"
)

// Error strings.
const (
	errCreateCache      = "cannot create new cache"
	errCreateController = "cannot create new controller"
	errCrashCache       = "cache error"
	errCrashController  = "controller error"
	errWatch            = "cannot setup watch"
)

// A NewCacheFn creates a new controller-runtime cache.
type NewCacheFn func(cfg *rest.Config, o cache.Options) (cache.Cache, error)

// A NewControllerFn creates a new controller-runtime controller.
type NewControllerFn func(name string, m manager.Manager, o controller.Options) (controller.Controller, error)

// The default new cache and new controller functions.
//
//nolint:gochecknoglobals // We treat these as constants.
var (
	DefaultNewCacheFn      NewCacheFn      = cache.New
	DefaultNewControllerFn NewControllerFn = controller.NewUnmanaged
)

// An Engine manages the lifecycles of controller-runtime controllers (and their
// caches). The lifecycles of the controllers are not coupled to lifecycle of
// the engine, nor to the lifecycle of the controller manager it uses.
type Engine struct {
	mgr manager.Manager

	started map[string]context.CancelFunc
	errors  map[string]error
	mx      sync.RWMutex

	newCache NewCacheFn
	newCtrl  NewControllerFn
}

// An EngineOption configures an Engine.
type EngineOption func(*Engine)

// WithNewCacheFn may be used to configure a different cache implementation.
// DefaultNewCacheFn is used by default.
func WithNewCacheFn(fn NewCacheFn) EngineOption {
	return func(e *Engine) {
		e.newCache = fn
	}
}

// WithNewControllerFn may be used to configure a different controller
// implementation. DefaultNewControllerFn is used by default.
func WithNewControllerFn(fn NewControllerFn) EngineOption {
	return func(e *Engine) {
		e.newCtrl = fn
	}
}

// NewEngine produces a new Engine.
func NewEngine(mgr manager.Manager, o ...EngineOption) *Engine {
	e := &Engine{
		mgr: mgr,

		started: make(map[string]context.CancelFunc),
		errors:  make(map[string]error),

		newCache: DefaultNewCacheFn,
		newCtrl:  DefaultNewControllerFn,
	}

	for _, eo := range o {
		eo(e)
	}

	return e
}

// IsRunning indicates whether the named controller is running - i.e. whether it
// has been started and does not appear to have crashed.
func (e *Engine) IsRunning(name string) bool {
	e.mx.RLock()
	defer e.mx.RUnlock()

	_, running := e.started[name]
	return running
}

// Err returns any error encountered by the named controller. The returned error
// is always nil if the named controller is running.
func (e *Engine) Err(name string) error {
	e.mx.RLock()
	defer e.mx.RUnlock()

	return e.errors[name]
}

// Stop the named controller.
func (e *Engine) Stop(name string) {
	e.done(name, nil)
}

func (e *Engine) done(name string, err error) {
	e.mx.Lock()
	defer e.mx.Unlock()

	stop, ok := e.started[name]
	if ok {
		stop()
		delete(e.started, name)
	}

	// Don't overwrite the first error if done is called multiple times.
	if e.errors[name] != nil {
		return
	}
	e.errors[name] = err
}

// Watch an object.
type Watch struct {
	// one of the two:
	kind         client.Object
	customSource source.Source

	handler    handler.EventHandler
	predicates []predicate.Predicate
}

// For returns a Watch for the supplied kind of object. Events will be handled
// by the supplied EventHandler, and may be filtered by the supplied predicates.
func For(kind client.Object, h handler.EventHandler, p ...predicate.Predicate) Watch {
	return Watch{kind: kind, handler: h, predicates: p}
}

// TriggeredBy returns a custom watch for secondary resources triggering the
// controller. source.Kind can be used to create a source for a secondary cache.
// Events will be handled by the supplied EventHandler, and may be filtered by
// the supplied predicates.
func TriggeredBy(source source.Source, h handler.EventHandler, p ...predicate.Predicate) Watch {
	return Watch{customSource: source, handler: h, predicates: p}
}

// Start the named controller. Each controller is started with its own cache
// whose lifecycle is coupled to the controller. The controller is started with
// the supplied options, and configured with the supplied watches. Start does
// not block.
func (e *Engine) Start(name string, o controller.Options, w ...Watch) error {
	c, err := e.Create(name, o, w...)
	if err != nil {
		return err
	}
	return c.Start(context.Background())
}

// NamedController is a controller that's not yet started. It gives access to
// the underlying cache, which may be used e.g. to add indexes.
type NamedController interface {
	Start(ctx context.Context) error
	GetCache() cache.Cache
}

type namedController struct {
	name string
	e    *Engine
	ca   cache.Cache
	ctrl controller.Controller
}

// Create the named controller. Each controller gets its own cache
// whose lifecycle is coupled to the controller. The controller is created with
// the supplied options, and configured with the supplied watches. It is not
// started yet.
func (e *Engine) Create(name string, o controller.Options, w ...Watch) (NamedController, error) {
	// Each controller gets its own cache for the GVKs it owns. This cache is
	// wrapped by a GVKRoutedCache that routes requests to other GVKs to the
	// manager's cache. This way we can share informers for composed resources
	// (that's where this is primarily used) with other controllers, but get
	// control about the lifecycle of the owned GVKs' informers.
	ca, err := e.newCache(e.mgr.GetConfig(), cache.Options{Scheme: e.mgr.GetScheme(), Mapper: e.mgr.GetRESTMapper()})
	if err != nil {
		return nil, errors.Wrap(err, errCreateCache)
	}

	// Wrap the existing manager to use our cache for the GVKs of this controller.
	rc := NewGVKRoutedCache(e.mgr.GetScheme(), e.mgr.GetCache())
	rm := &routedManager{
		Manager: e.mgr,
		client: &cachedRoutedClient{
			Client: e.mgr.GetClient(),
			scheme: e.mgr.GetScheme(),
			cache:  rc,
		},
		cache: rc,
	}

	ctrl, err := e.newCtrl(name, rm, o)
	if err != nil {
		return nil, errors.Wrap(err, errCreateController)
	}

	for _, wt := range w {
		if wt.customSource != nil {
			if err := ctrl.Watch(wt.customSource, wt.handler, wt.predicates...); err != nil {
				return nil, errors.Wrap(err, errWatch)
			}
			continue
		}

		// route cache and client (read) requests to our cache for this GVK.
		gvk, err := apiutil.GVKForObject(wt.kind, e.mgr.GetScheme())
		if err != nil {
			return nil, errors.Wrapf(err, "failed to get GVK for type %T", wt.kind)
		}
		rc.AddDelegate(gvk, ca)

		if err := ctrl.Watch(source.Kind(ca, wt.kind), wt.handler, wt.predicates...); err != nil {
			return nil, errors.Wrap(err, errWatch)
		}
	}

	return &namedController{name: name, e: e, ca: ca, ctrl: ctrl}, nil
}

// Start the named controller. Start does not block.
func (c *namedController) Start(ctx context.Context) error {
	if c.e.IsRunning(c.name) {
		return nil
	}

	ctx, stop := context.WithCancel(ctx)
	c.e.mx.Lock()
	c.e.started[c.name] = stop
	c.e.errors[c.name] = nil
	c.e.mx.Unlock()

	go func() {
		<-c.e.mgr.Elected()
		c.e.done(c.name, errors.Wrap(c.ca.Start(ctx), errCrashCache))
	}()
	go func() {
		<-c.e.mgr.Elected()
		if synced := c.ca.WaitForCacheSync(ctx); !synced {
			c.e.done(c.name, errors.New(errCrashCache))
			return
		}
		c.e.done(c.name, errors.Wrap(c.ctrl.Start(ctx), errCrashController))
	}()

	return nil
}

// GetCache returns the cache used by the named controller.
func (c *namedController) GetCache() cache.Cache {
	return c.ca
}