// fetchUtxosMain fetches unspent transaction output data about the provided
// set of transactions from the point of view of the end of the main chain at
// the time of the call.
//
// Upon completion of this function, the view will contain an entry for each
// requested transaction.  Fully spent transactions, or those which otherwise
// don't exist, will result in a nil entry in the view.
func (view *UtxoViewpoint) fetchUtxosMain(db database.DB, txSet map[wire.ShaHash]struct{}) error {
	// Nothing to do if there are no requested hashes.
	if len(txSet) == 0 {
		return nil
	}

	// Load the unspent transaction output information for the requested set
	// of transactions from the point of view of the end of the main chain.
	//
	// NOTE: Missing entries are not considered an error here and instead
	// will result in nil entries in the view.  This is intentionally done
	// since other code uses the presence of an entry in the store as a way
	// to optimize spend and unspend updates to apply only to the specific
	// utxos that the caller needs access to.
	return db.View(func(dbTx database.Tx) error {
		for hash := range txSet {
			hashCopy := hash
			entry, err := dbFetchUtxoEntry(dbTx, &hashCopy)
			if err != nil {
				return err
			}

			view.entries[hash] = entry
		}

		return nil
	})
}

// dropBlockIDIndex drops the internal block id index.
func dropBlockIDIndex(db database.DB) error {
	return db.Update(func(dbTx database.Tx) error {
		meta := dbTx.Metadata()
		err := meta.DeleteBucket(idByHashIndexBucketName)
		if err != nil {
			return err
		}

		return meta.DeleteBucket(hashByIDIndexBucketName)
	})
}

// dropIndex drops the passed index from the database.  Since indexes can be
// massive, it deletes the index in multiple database transactions in order to
// keep memory usage to reasonable levels.  It also marks the drop in progress
// so the drop can be resumed if it is stopped before it is done before the
// index can be used again.
func dropIndex(db database.DB, idxKey []byte, idxName string) error {
	// Nothing to do if the index doesn't already exist.
	var needsDelete bool
	err := db.View(func(dbTx database.Tx) error {
		indexesBucket := dbTx.Metadata().Bucket(indexTipsBucketName)
		if indexesBucket != nil && indexesBucket.Get(idxKey) != nil {
			needsDelete = true
		}
		return nil
	})
	if err != nil {
		return err
	}
	if !needsDelete {
		log.Infof("Not dropping %s because it does not exist", idxName)
		return nil
	}

	// Mark that the index is in the process of being dropped so that it
	// can be resumed on the next start if interrupted before the process is
	// complete.
	log.Infof("Dropping all %s entries.  This might take a while...",
		idxName)
	err = db.Update(func(dbTx database.Tx) error {
		indexesBucket := dbTx.Metadata().Bucket(indexTipsBucketName)
		return indexesBucket.Put(indexDropKey(idxKey), idxKey)
	})
	if err != nil {
		return err
	}

	// Since the indexes can be so large, attempting to simply delete
	// the bucket in a single database transaction would result in massive
	// memory usage and likely crash many systems due to ulimits.  In order
	// to avoid this, use a cursor to delete a maximum number of entries out
	// of the bucket at a time.
	const maxDeletions = 2000000
	var totalDeleted uint64
	for numDeleted := maxDeletions; numDeleted == maxDeletions; {
		numDeleted = 0
		err := db.Update(func(dbTx database.Tx) error {
			bucket := dbTx.Metadata().Bucket(idxKey)
			cursor := bucket.Cursor()
			for ok := cursor.First(); ok; ok = cursor.Next() &&
				numDeleted < maxDeletions {

				if err := cursor.Delete(); err != nil {
					return err
				}
				numDeleted++
			}
			return nil
		})
		if err != nil {
			return err
		}

		if numDeleted > 0 {
			totalDeleted += uint64(numDeleted)
			log.Infof("Deleted %d keys (%d total) from %s",
				numDeleted, totalDeleted, idxName)
		}
	}

	// Call extra index specific deinitialization for the transaction index.
	if idxName == txIndexName {
		if err := dropBlockIDIndex(db); err != nil {
			return err
		}
	}

	// Remove the index tip, index bucket, and in-progress drop flag now
	// that all index entries have been removed.
	err = db.Update(func(dbTx database.Tx) error {
		meta := dbTx.Metadata()
		indexesBucket := meta.Bucket(indexTipsBucketName)
		if err := indexesBucket.Delete(idxKey); err != nil {
			return err
		}

		if err := meta.DeleteBucket(idxKey); err != nil {
			return err
		}

		return indexesBucket.Delete(indexDropKey(idxKey))
	})
	if err != nil {
		return err
	}

	log.Infof("Dropped %s", idxName)
	return nil
}

// chainSetup is used to create a new db and chain instance with the genesis
// block already inserted.  In addition to the new chain instnce, it returns
// a teardown function the caller should invoke when done testing to clean up.
func chainSetup(dbName string) (*blockchain.BlockChain, func(), error) {
	if !isSupportedDbType(testDbType) {
		return nil, nil, fmt.Errorf("unsupported db type %v", testDbType)
	}

	// Handle memory database specially since it doesn't need the disk
	// specific handling.
	var db database.DB
	var teardown func()
	if testDbType == "memdb" {
		ndb, err := database.Create(testDbType)
		if err != nil {
			return nil, nil, fmt.Errorf("error creating db: %v", err)
		}
		db = ndb

		// Setup a teardown function for cleaning up.  This function is
		// returned to the caller to be invoked when it is done testing.
		teardown = func() {
			db.Close()
		}
	} else {
		// Create the root directory for test databases.
		if !fileExists(testDbRoot) {
			if err := os.MkdirAll(testDbRoot, 0700); err != nil {
				err := fmt.Errorf("unable to create test db "+
					"root: %v", err)
				return nil, nil, err
			}
		}

		// Create a new database to store the accepted blocks into.
		dbPath := filepath.Join(testDbRoot, dbName)
		_ = os.RemoveAll(dbPath)
		ndb, err := database.Create(testDbType, dbPath, blockDataNet)
		if err != nil {
			return nil, nil, fmt.Errorf("error creating db: %v", err)
		}
		db = ndb

		// Setup a teardown function for cleaning up.  This function is
		// returned to the caller to be invoked when it is done testing.
		teardown = func() {
			db.Close()
			os.RemoveAll(dbPath)
			os.RemoveAll(testDbRoot)
		}
	}

	// Create the main chain instance.
	chain, err := blockchain.New(&blockchain.Config{
		DB:          db,
		ChainParams: &chaincfg.MainNetParams,
	})
	if err != nil {
		teardown()
		err := fmt.Errorf("failed to create chain instance: %v", err)
		return nil, nil, err
	}
	return chain, teardown, nil
}