// 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 worker

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"net/http"
	"time"

	"github.com/go-redis/redis/v8"
	"golang.org/x/pkgsite/internal/complete"
	"golang.org/x/pkgsite/internal/database"
	"golang.org/x/pkgsite/internal/derrors"
	"golang.org/x/pkgsite/internal/log"
)

const popularCutoff = 50

// handleUpdateRedisIndexes scans recently modified search documents, and
// updates redis auto completion indexes with data from these documents.
func (s *Server) handleUpdateRedisIndexes(w http.ResponseWriter, r *http.Request) error {
	ctx := r.Context()
	err := updateRedisIndexes(ctx, s.db.Underlying(), s.redisHAClient, popularCutoff)
	if err != nil {
		return err
	}
	fmt.Fprint(w, "OK")
	return nil
}

// updateRedisIndexes updates redisClient with autocompletion data from db.
// cutoff specifies the number of importers at which a package is considered
// popular, and is passed-in as an argument to facilitate testing.
func updateRedisIndexes(ctx context.Context, db *database.DB, redisClient *redis.Client, cutoff int) (err error) {
	defer derrors.Wrap(&err, "updateRedisIndexes")
	if redisClient == nil {
		return errors.New("redis HA client is nil")
	}

	// For autocompletion, we track two separate "indexes" (sorted sets of
	// package path suffixes): one for popular packages, and one for the
	// remainder, as defined by the popularCutoff const.  This allows us to
	// suggest popular completions, even when the user input is short (i.e. we
	// want to suggest 'fmt' when the user types 'f', but don't want to scan all
	// completions that start with the letter 'f').
	//
	// This function scans search documents in the database and builds up a
	// pipeline that writes these two sorted sets to Redis, using timestamped
	// temporary keys, and then renames them to the keys used by the frontend for
	// autocompletion.
	//
	// See https://redis.io/commands/rename for more information on renaming:
	// it's unclear whether renaming is atomic, but we don't really care.
	// Populating these indexes currently takes 1-2 minutes, and renaming takes
	// 1-2 seconds. Even if completions are broken during this 1-2 seconds, it's
	// preferable to them being broken for 1-2 minutes. We could do something
	// more clever, such as updating the completion data in place using
	// ZREMRANGEBYLEX followed by ZADD, but that would be significantly more
	// complicated.
	//
	// One additional concern of this operation is that we temporary double the
	// size of our redis database while we're staging the new completion data.
	// That's fine, but it's dangerous if we ever have a bug and this operation
	// was either not cleaned up properly, or run concurrently. In light of this,
	// we first look for evidence of another update operation currently running,
	// by scanning Redis for keys that match the temporary key pattern.

	// Check for an ongoing update operation, as described above.
	tempKeyPattern := fmt.Sprintf("%s*-*", complete.KeyPrefix)
	existing, _, err := redisClient.Scan(ctx, 0, tempKeyPattern, 1).Result()
	if err != nil {
		return fmt.Errorf(`redis error: Scan(%q): %v`, tempKeyPattern, err)
	}
	if len(existing) > 0 {
		return fmt.Errorf("found existing in-progress completion index: %v", existing[0])
	}

	// Use temporary timestamped keys while we write the completion data, as it
	// can take ~minutes.
	keyPop := fmt.Sprintf("%s-%s", complete.PopularKey, time.Now().Format(time.RFC3339))
	keyRem := fmt.Sprintf("%s-%s", complete.RemainingKey, time.Now().Format(time.RFC3339))

	// Always clean up: DEL succeeds even if the keys have been renamed.
	defer func() {
		if _, err := redisClient.Del(ctx, keyPop).Result(); err != nil {
			log.Errorf(ctx, "redisClient.Del(%q): %v", keyPop, err)
		}
		if _, err := redisClient.Del(ctx, keyRem).Result(); err != nil {
			log.Errorf(ctx, "redisClient.Del(%q): %v", keyRem, err)
		}
	}()

	// pipeSize tracks the number of ZADD statements in the pipe.
	pipeSize := 0
	pipe := redisClient.Pipeline()
	defer pipe.Close()
	// flush executes the current pipeline and resets its state.
	flush := func() error {
		log.Infof(ctx, "Writing completion data pipeline of size %d.", pipeSize)
		if _, err := pipe.Exec(ctx); err != nil {
			return fmt.Errorf("redis error: pipe.Exec: %v", err)
		}
		// As of writing this is unnecessary as ExecContext resets the pipeline
		// commands, but since this is not documented functionality we explicitly
		// Discard.
		pipe.Discard()
		pipeSize = 0
		return nil
	}
	// Track whether or not we have any entries in the popular or remaining
	// indexes. This is an edge case, but if we don't insert any entries for a
	// given index the key won't exist and we'll get an error when renaming.
	var (
		haveRemaining bool
		havePopular   bool
	)
	// As of writing there were around 5M entries in our index, so writing in
	// batches of 1M should result in ~6 batches.
	const batchSize = 1e6
	// processRow builds up a Redis pipeline as we scan the search_documents
	// table.
	processRow := func(rows *sql.Rows) error {
		var partial complete.Completion
		if err := rows.Scan(&partial.PackagePath, &partial.ModulePath, &partial.Version, &partial.Importers); err != nil {
			return fmt.Errorf("rows.Scan: %v", err)
		}
		cmpls := complete.PathCompletions(partial)
		var zs []*redis.Z
		for _, cmpl := range cmpls {
			zs = append(zs, &redis.Z{Member: cmpl.Encode()})
		}
		switch {
		case partial.Importers >= cutoff:
			havePopular = true
			pipe.ZAdd(ctx, keyPop, zs...)
		default:
			haveRemaining = true
			pipe.ZAdd(ctx, keyRem, zs...)
		}
		pipeSize += len(zs)
		if pipeSize > batchSize {
			if err := flush(); err != nil {
				return err
			}
		}
		return nil
	}

	// Here we use the *database.DB rather than a function on postgres.DB,
	// so that we can write to our redis pipeline while we stream results from
	// the DB. Otherwise, we would have to:
	//  - add a method on postgres.DB for the trivial query above
	//  - add a type (or reuse SearchResult) to hold the subset of search
	//    document data used here.
	//  - hold two copies of all search results in memory while building the
	//    redis pipeline below.
	query := `
		SELECT package_path, module_path, version, imported_by_count
		FROM search_documents`
	if err := db.RunQuery(ctx, query, processRow); err != nil {
		return err
	}
	if err := flush(); err != nil {
		return err
	}
	pipe.Close()
	if havePopular {
		log.Infof(ctx, "Renaming %q to %q", keyPop, complete.PopularKey)
		if _, err := redisClient.Rename(ctx, keyPop, complete.PopularKey).Result(); err != nil {
			return fmt.Errorf(`redis error: Rename(%q, %q): %v`, keyPop, complete.PopularKey, err)
		}
	}
	if haveRemaining {
		log.Infof(ctx, "Renaming %q to %q", keyRem, complete.RemainingKey)
		if _, err := redisClient.Rename(ctx, keyRem, complete.RemainingKey).Result(); err != nil {
			return fmt.Errorf(`redis error: Rename(%q, %q): %v`, keyRem, complete.RemainingKey, err)
		}
	}
	return nil
}