VictoriaMetrics/lib/logstorage/pipe_limit.go

package logstorage

import (
	"fmt"
	"sync/atomic"
)

// pipeLimit implements '| limit ...' pipe.
//
// See https://docs.victoriametrics.com/victorialogs/logsql/#limit-pipe
type pipeLimit struct {
	limit uint64
}

func (pl *pipeLimit) String() string {
	return fmt.Sprintf("limit %d", pl.limit)
}

func (pl *pipeLimit) canLiveTail() bool {
	return false
}

func (pl *pipeLimit) updateNeededFields(_, _ fieldsSet) {
	// nothing to do
}

func (pl *pipeLimit) optimize() {
	// nothing to do
}

func (pl *pipeLimit) hasFilterInWithQuery() bool {
	return false
}

func (pl *pipeLimit) initFilterInValues(_ map[string][]string, _ getFieldValuesFunc) (pipe, error) {
	return pl, nil
}

func (pl *pipeLimit) newPipeProcessor(_ int, _ <-chan struct{}, cancel func(), ppNext pipeProcessor) pipeProcessor {
	if pl.limit == 0 {
		// Special case - notify the caller to stop writing data to the returned pipeLimitProcessor
		cancel()
	}
	return &pipeLimitProcessor{
		pl:     pl,
		cancel: cancel,
		ppNext: ppNext,
	}
}

type pipeLimitProcessor struct {
	pl     *pipeLimit
	cancel func()
	ppNext pipeProcessor

	rowsProcessed atomic.Uint64
}

func (plp *pipeLimitProcessor) writeBlock(workerID uint, br *blockResult) {
	if len(br.timestamps) == 0 {
		return
	}

	rowsProcessed := plp.rowsProcessed.Add(uint64(len(br.timestamps)))
	limit := plp.pl.limit
	if rowsProcessed <= limit {
		// Fast path - write all the rows to ppNext.
		plp.ppNext.writeBlock(workerID, br)
		if rowsProcessed == limit {
			plp.cancel()
		}
		return
	}

	// Slow path - overflow. Write the remaining rows if needed.
	rowsProcessed -= uint64(len(br.timestamps))
	if rowsProcessed >= limit {
		// Nothing to write. There is no need in cancel() call, since it has been called by another goroutine.
		return
	}

	// Write remaining rows.
	keepRows := limit - rowsProcessed
	br.truncateRows(int(keepRows))
	plp.ppNext.writeBlock(workerID, br)

	// Notify the caller that it should stop passing more data to writeBlock().
	plp.cancel()
}

func (plp *pipeLimitProcessor) flush() error {
	return nil
}

func parsePipeLimit(lex *lexer) (*pipeLimit, error) {
	if !lex.isKeyword("limit", "head") {
		return nil, fmt.Errorf("expecting 'limit' or 'head'; got %q", lex.token)
	}
	lex.nextToken()

	limit := uint64(10)
	if !lex.isKeyword("|", ")", "") {
		n, err := parseUint(lex.token)
		if err != nil {
			return nil, fmt.Errorf("cannot parse rows limit from %q: %w", lex.token, err)
		}
		lex.nextToken()
		limit = n
	}

	pl := &pipeLimit{
		limit: limit,
	}
	return pl, nil
}