// Copyright 2020 The Monogon Project Authors.
//
// SPDX-License-Identifier: Apache-2.0
//
// 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 supervisor

// The service supervision library allows for writing of reliable,
// service-style software within a Metropolis node.  It builds upon the
// Erlang/OTP supervision tree system, adapted to be more Go-ish.  For detailed
// design see go/supervision.

import (
	"context"
	"io"
	"sync"

	"source.monogon.dev/metropolis/pkg/logtree"
)

// A Runnable is a function that will be run in a goroutine, and supervised
// throughout its lifetime. It can in turn start more runnables as its
// children, and those will form part of a supervision tree.
// The context passed to a runnable is very important and needs to be handled
// properly. It will be live (non-errored) as long as the runnable should be
// running, and canceled (ctx.Err() will be non-nil) when the supervisor wants
// it to exit. This means this context is also perfectly usable for performing
// any blocking operations.
type Runnable func(ctx context.Context) error

// RunGroup starts a set of runnables as a group. These runnables will run
// together, and if any one of them quits unexpectedly, the result will be
// canceled and restarted.
// The context here must be an existing Runnable context, and the spawned
// runnables will run under the node that this context represents.
func RunGroup(ctx context.Context, runnables map[string]Runnable) error {
	node, unlock := fromContext(ctx)
	defer unlock()
	return node.runGroup(runnables)
}

// Run starts a single runnable in its own group.
func Run(ctx context.Context, name string, runnable Runnable) error {
	return RunGroup(ctx, map[string]Runnable{
		name: runnable,
	})
}

// Signal tells the supervisor that the calling runnable has reached a certain
// state of its lifecycle. All runnables should SignalHealthy when they are
// ready with set up, running other child runnables and are now 'serving'.
func Signal(ctx context.Context, signal SignalType) {
	node, unlock := fromContext(ctx)
	defer unlock()
	node.signal(signal)
}

type SignalType int

const (
	// The runnable is healthy, done with setup, done with spawning more
	// Runnables, and ready to serve in a loop.  The runnable needs to check
	// the parent context and ensure that if that context is done, the runnable
	// exits.
	SignalHealthy SignalType = iota
	// The runnable is done - it does not need to run any loop. This is useful
	// for Runnables that only set up other child runnables. This runnable will
	// be restarted if a related failure happens somewhere in the supervision
	// tree.
	SignalDone
)

// supervisor represents and instance of the supervision system. It keeps track
// of a supervision tree and a request channel to its internal processor
// goroutine.
type supervisor struct {
	// mu guards the entire state of the supervisor.
	mu sync.RWMutex
	// root is the root node of the supervision tree, named 'root'. It
	// represents the Runnable started with the supervisor.New call.
	root *node
	// logtree is the main logtree exposed to runnables and used internally.
	logtree *logtree.LogTree
	// ilogger is the internal logger logging to "supervisor" in the logtree.
	ilogger logtree.LeveledLogger

	// pReq is an interface channel to the lifecycle processor of the
	// supervisor.
	pReq chan *processorRequest

	// propagate panics, ie. don't catch them.
	propagatePanic bool
}

// SupervisorOpt are runtime configurable options for the supervisor.
type SupervisorOpt func(s *supervisor)

var (
	// WithPropagatePanic prevents the Supervisor from catching panics in
	// runnables and treating them as failures. This is useful to enable for
	// testing and local debugging.
	WithPropagatePanic = func(s *supervisor) {
		s.propagatePanic = true
	}
)

func WithExistingLogtree(lt *logtree.LogTree) SupervisorOpt {
	return func(s *supervisor) {
		s.logtree = lt
	}
}

// New creates a new supervisor with its root running the given root runnable.
// The given context can be used to cancel the entire supervision tree.
func New(ctx context.Context, rootRunnable Runnable, opts ...SupervisorOpt) *supervisor {
	sup := &supervisor{
		logtree: logtree.New(),
		pReq:    make(chan *processorRequest),
	}

	for _, o := range opts {
		o(sup)
	}

	sup.ilogger = sup.logtree.MustLeveledFor("supervisor")
	sup.root = newNode("root", rootRunnable, sup, nil)

	go sup.processor(ctx)

	sup.pReq <- &processorRequest{
		schedule: &processorRequestSchedule{dn: "root"},
	}

	return sup
}

func Logger(ctx context.Context) logtree.LeveledLogger {
	node, unlock := fromContext(ctx)
	defer unlock()
	return node.sup.logtree.MustLeveledFor(logtree.DN(node.dn()))
}

func RawLogger(ctx context.Context) io.Writer {
	node, unlock := fromContext(ctx)
	defer unlock()
	return node.sup.logtree.MustRawFor(logtree.DN(node.dn()))
}