// Copyright 2017 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 maintner mirrors, searches, syncs, and serves Git, Github,
// and Gerrit metadata.
//
// Maintner is short for "Maintainer". This package is intended for
// use by many tools. The name of the daemon that serves the maintner
// data to other tools is "maintnerd".
package maintner

import (
	"context"
	"errors"
	"fmt"
	"log"
	"regexp"
	"sync"
	"time"

	"golang.org/x/build/maintner/maintpb"
	"golang.org/x/sync/errgroup"
	"golang.org/x/time/rate"
)

// Corpus holds all of a project's metadata.
type Corpus struct {
	mutationLogger MutationLogger // non-nil when this is a self-updating corpus
	mutationSource MutationSource // from Initialize
	verbose        bool
	dataDir        string
	sawErrSplit    bool

	mu sync.RWMutex // guards all following fields
	// corpus state:
	didInit   bool // true after Initialize completes successfully
	debug     bool
	strIntern map[string]string // interned strings, including binary githashes

	// pubsub:
	activityChans map[string]chan struct{} // keyed by topic

	// github-specific
	github             *GitHub
	gerrit             *Gerrit
	watchedGithubRepos []watchedGithubRepo
	watchedGerritRepos []watchedGerritRepo
	githubLimiter      *rate.Limiter

	// git-specific:
	lastGitCount  time.Time // last time of log spam about loading status
	pollGitDirs   []polledGitCommits
	gitPeople     map[string]*GitPerson
	gitCommit     map[GitHash]*GitCommit
	gitCommitTodo map[GitHash]bool          // -> true
	gitOfHg       map[string]GitHash        // hg hex hash -> git hash
	zoneCache     map[string]*time.Location // "+0530" => location
}

// RLock grabs the corpus's read lock. Grabbing the read lock prevents
// any concurrent writes from mutating the corpus. This is only
// necessary if the application is querying the corpus and calling its
// Update method concurrently.
func (c *Corpus) RLock() { c.mu.RLock() }

// RUnlock unlocks the corpus's read lock.
func (c *Corpus) RUnlock() { c.mu.RUnlock() }

type polledGitCommits struct {
	repo *maintpb.GitRepo
	dir  string
}

// EnableLeaderMode prepares c to be the leader. This should only be
// called by the maintnerd process.
//
// The provided scratchDir will store git checkouts.
func (c *Corpus) EnableLeaderMode(logger MutationLogger, scratchDir string) {
	c.mutationLogger = logger
	c.dataDir = scratchDir
}

// SetVerbose enables or disables verbose logging.
func (c *Corpus) SetVerbose(v bool) { c.verbose = v }

func (c *Corpus) getDataDir() string {
	if c.dataDir == "" {
		panic("getDataDir called before Corpus.EnableLeaderMode")
	}
	return c.dataDir
}

// GitHub returns the corpus's github data.
func (c *Corpus) GitHub() *GitHub {
	if c.github != nil {
		return c.github
	}
	return new(GitHub)
}

// Gerrit returns the corpus's Gerrit data.
func (c *Corpus) Gerrit() *Gerrit {
	if c.gerrit != nil {
		return c.gerrit
	}
	return new(Gerrit)
}

// Check verifies the internal structure of the Corpus data structures.
// It is intended for tests and debugging.
func (c *Corpus) Check() error {
	if err := c.Gerrit().check(); err != nil {
		return fmt.Errorf("gerrit: %v", err)
	}

	for hash, gc := range c.gitCommit {
		if gc.Committer == placeholderCommitter {
			return fmt.Errorf("corpus git commit %v has placeholder committer", hash)
		}
		if gc.Hash != hash {
			return fmt.Errorf("git commit for key %q had GitCommit.Hash %q", hash, gc.Hash)
		}
		for _, pc := range gc.Parents {
			if _, ok := c.gitCommit[pc.Hash]; !ok {
				return fmt.Errorf("git commit %q exists but its parent %q does not", gc.Hash, pc.Hash)
			}
		}
	}

	return nil
}

// requires c.mu be held for writing
func (c *Corpus) str(s string) string {
	if v, ok := c.strIntern[s]; ok {
		return v
	}
	if c.strIntern == nil {
		c.strIntern = make(map[string]string)
	}
	c.strIntern[s] = s
	return s
}

func (c *Corpus) strb(b []byte) string {
	if v, ok := c.strIntern[string(b)]; ok {
		return v
	}
	return c.str(string(b))
}

func (c *Corpus) SetDebug() {
	c.debug = true
}

func (c *Corpus) debugf(format string, v ...interface{}) {
	if c.debug {
		log.Printf(format, v...)
	}
}

// gerritProjNameRx is the pattern describing a Gerrit project name.
// TODO: figure out if this is accurate.
var gerritProjNameRx = regexp.MustCompile(`^[a-z0-9]+[a-z0-9\-\_]*$`)

// TrackGoGitRepo registers a git directory to have its metadata slurped into the corpus.
// The goRepo is a name like "go" or "net". The dir is a path on disk.
func (c *Corpus) TrackGoGitRepo(goRepo, dir string) {
	if c.mutationLogger == nil {
		panic("can't TrackGoGitRepo in non-leader mode")
	}
	if !gerritProjNameRx.MatchString(goRepo) {
		panic(fmt.Sprintf("bogus goRepo value %q", goRepo))
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	c.pollGitDirs = append(c.pollGitDirs, polledGitCommits{
		repo: &maintpb.GitRepo{GoRepo: goRepo},
		dir:  dir,
	})
}

// A MutationSource yields a log of mutations that will catch a corpus
// back up to the present.
type MutationSource interface {
	// GetMutations returns a channel of mutations or related events.
	// The channel will never be closed.
	// All sends on the returned channel should select
	// on the provided context.
	GetMutations(context.Context) <-chan MutationStreamEvent
}

// MutationStreamEvent represents one of three possible events while
// reading mutations from disk or another source.
// An event is either a mutation, an error, or reaching the current
// end of the log. Exactly one of the three fields will be non-zero.
type MutationStreamEvent struct {
	Mutation *maintpb.Mutation

	// Err is a fatal error reading the log. No other events will
	// follow an Err.
	Err error

	// End, if true, means that all mutations have been sent and
	// the next event might take some time to arrive (it might not
	// have occurred yet). The End event is not a terminal state
	// like Err. There may be multiple Ends.
	End bool
}

// Initialize populates the Corpus using the data from the
// MutationSource. It returns once it's up-to-date. To incrementally
// update it later, use the Update method.
func (c *Corpus) Initialize(ctx context.Context, src MutationSource) error {
	if c.mutationSource != nil {
		panic("duplicate call to Initialize")
	}
	c.mutationSource = src
	log.Printf("Loading data from log %T ...", src)
	return c.update(ctx, nil)
}

// ErrSplit is returned when the client notices the leader's
// mutation log has changed. This can happen if the leader restarts
// with uncommitted transactions. (The leader only commits mutations
// periodically.)
var ErrSplit = errors.New("maintner: leader server's history split, process out of sync")

// Update incrementally updates the corpus from its current state to
// the latest state from the MutationSource passed earlier to
// Initialize. It does not return until there's either a new change or
// the context expires.
// If Update returns ErrSplit, the corpus can no longer be updated.
//
// Update must not be called concurrently with any other Update calls. If
// reading the corpus concurrently while the corpus is updating, you must hold
// the read lock using Corpus.RLock.
func (c *Corpus) Update(ctx context.Context) error {
	if c.mutationSource == nil {
		panic("Update called without call to Initialize")
	}
	if c.sawErrSplit {
		panic("Update called after previous call returned ErrSplit")
	}
	log.Printf("Updating data from log %T ...", c.mutationSource)
	err := c.update(ctx, nil)
	if err == ErrSplit {
		c.sawErrSplit = true
	}
	return err
}

// UpdateWithLocker behaves just like Update, but holds lk when processing
// mutation events.
func (c *Corpus) UpdateWithLocker(ctx context.Context, lk sync.Locker) error {
	if c.mutationSource == nil {
		panic("UpdateWithLocker called without call to Initialize")
	}
	if c.sawErrSplit {
		panic("UpdateWithLocker called after previous call returned ErrSplit")
	}
	log.Printf("Updating data from log %T ...", c.mutationSource)
	err := c.update(ctx, lk)
	if err == ErrSplit {
		c.sawErrSplit = true
	}
	return err
}

type noopLocker struct{}

func (noopLocker) Lock()   {}
func (noopLocker) Unlock() {}

// lk optionally specifies a locker to use while processing mutations.
func (c *Corpus) update(ctx context.Context, lk sync.Locker) error {
	src := c.mutationSource
	ch := src.GetMutations(ctx)
	done := ctx.Done()
	c.mu.Lock()
	defer c.mu.Unlock()
	if lk == nil {
		lk = noopLocker{}
	}
	for {
		select {
		case <-done:
			err := ctx.Err()
			log.Printf("Context expired while loading data from log %T: %v", src, err)
			return err
		case e := <-ch:
			if e.Err != nil {
				log.Printf("Corpus GetMutations: %v", e.Err)
				return e.Err
			}
			if e.End {
				c.didInit = true
				lk.Lock()
				c.finishProcessing()
				lk.Unlock()
				log.Printf("Reloaded data from log %T.", src)
				return nil
			}
			lk.Lock()
			c.processMutationLocked(e.Mutation)
			lk.Unlock()
		}
	}
}

// addMutation adds a mutation to the log and immediately processes it.
func (c *Corpus) addMutation(m *maintpb.Mutation) {
	if c.verbose {
		log.Printf("mutation: %v", m)
	}
	c.mu.Lock()
	c.processMutationLocked(m)
	c.finishProcessing()
	c.mu.Unlock()

	if c.mutationLogger == nil {
		return
	}
	err := c.mutationLogger.Log(m)
	if err != nil {
		// TODO: handle errors better? failing is only safe option.
		log.Fatalf("could not log mutation %v: %v\n", m, err)
	}
}

// c.mu must be held.
func (c *Corpus) processMutationLocked(m *maintpb.Mutation) {
	if im := m.GithubIssue; im != nil {
		c.processGithubIssueMutation(im)
	}
	if gm := m.Github; gm != nil {
		c.processGithubMutation(gm)
	}
	if gm := m.Git; gm != nil {
		c.processGitMutation(gm)
	}
	if gm := m.Gerrit; gm != nil {
		c.processGerritMutation(gm)
	}
}

// finishProcessing fixes up invariants and data structures before
// returning the Corpus from the Update loop back to the user.
//
// c.mu must be held.
func (c *Corpus) finishProcessing() {
	c.gerrit.finishProcessing()
}

// SyncLoop runs forever (until an error or context expiration) and
// updates the corpus as the tracked sources change.
func (c *Corpus) SyncLoop(ctx context.Context) error {
	return c.sync(ctx, true)
}

// Sync updates the corpus from its tracked sources.
func (c *Corpus) Sync(ctx context.Context) error {
	return c.sync(ctx, false)
}

func (c *Corpus) sync(ctx context.Context, loop bool) error {
	if _, ok := c.mutationSource.(*netMutSource); ok {
		return errors.New("maintner: can't run Corpus.Sync on a Corpus using NetworkMutationSource (did you mean Update?)")
	}

	group, ctx := errgroup.WithContext(ctx)
	for _, w := range c.watchedGithubRepos {
		gr, token := w.gr, w.token
		group.Go(func() error {
			log.Printf("Polling %v ...", gr.id)
			for {
				err := gr.sync(ctx, token, loop)
				if loop && isTempErr(err) {
					log.Printf("Temporary error from github %v: %v", gr.ID(), err)
					time.Sleep(30 * time.Second)
					continue
				}
				log.Printf("github sync ending for %v: %v", gr.ID(), err)
				return err
			}
		})
	}
	for _, rp := range c.pollGitDirs {
		rp := rp
		group.Go(func() error {
			for {
				err := c.syncGitCommits(ctx, rp, loop)
				if loop && isTempErr(err) {
					log.Printf("Temporary error from git repo %v: %v", rp.dir, err)
					time.Sleep(30 * time.Second)
					continue
				}
				log.Printf("git sync ending for %v: %v", rp.dir, err)
				return err
			}
		})
	}
	for _, w := range c.watchedGerritRepos {
		gp := w.project
		group.Go(func() error {
			log.Printf("Polling gerrit %v ...", gp.proj)
			for {
				err := gp.sync(ctx, loop)
				if loop && isTempErr(err) {
					log.Printf("Temporary error from gerrit %v: %v", gp.proj, err)
					time.Sleep(30 * time.Second)
					continue
				}
				log.Printf("gerrit sync ending for %v: %v", gp.proj, err)
				return err
			}
		})
	}
	return group.Wait()
}

func isTempErr(err error) bool {
	log.Printf("IS TEMP ERROR? %T %v", err, err)
	return true
}