ingest/ledgerbackend: Decouple meta pipe buffering in CaptiveStellarCore backend

ingest/ledgerbackend/buffered_meta_pipe_reader.go

@@ -0,0 +1,162 @@
+package ledgerbackend
+
+import (
+	"bufio"
+	"io"
+	"time"
+
+	"github.com/pkg/errors"
+	"github.com/stellar/go/support/log"
+	"github.com/stellar/go/xdr"
+)
+
+const (
+	// The constants below define sizes of metaPipeBufferSize (binary) and
+	// ledgerReadAheadBufferSize (in ledgers). In general:
+	//
+	//   metaPipeBufferSize >=
+	//    ledgerReadAheadBufferSize * max over networks (average ledger size in bytes)
+	//
+	// so that meta pipe buffer always have binary data that can be unmarshaled into
+	// ledger buffer.
+	// After checking a few latest ledgers in pubnet and testnet the average size
+	// is: 100,000 and 5,000 bytes respectively.
+
+	// metaPipeBufferSize defines the meta pipe buffer size. We need at least
+	// a couple MB to ensure there are at least a few ledgers captive core can
+	// unmarshal into read-ahead buffer while waiting for client to finish
+	// processing previous ledgers.
+	metaPipeBufferSize = 10 * 1024 * 1024
+	// ledgerReadAheadBufferSize defines the size (in ledgers) of read ahead
+	// buffer that stores unmarshalled ledgers. This is especially important in
+	// an online mode when GetLedger calls are not blocking. In such case, clients
+	// usually wait for a specific time duration before checking if the ledger is
+	// available. When catching up and small buffer this can increase the overall
+	// time because ledgers are not available.
+	ledgerReadAheadBufferSize = 20
+)
+
+type metaResult struct {
+	*xdr.LedgerCloseMeta
+	err error
+}
+
+// bufferedLedgerMetaReader is responsible for buffering meta pipe data in a
+// fast and safe manner and unmarshaling it into XDR objects.
+//
+// It solves the following issues:
+//
+//   * Decouples buffering from stellarCoreRunner so it can focus on running core.
+//   * Decouples unmarshalling and buffering of LedgerCloseMeta's from CaptiveCore.
+//   * By adding buffering it allows unmarshaling the ledgers available in Stellar-Core
+//     while previous ledger are being processed.
+//   * Limits memory usage in case of large ledgers are closed by the network.
+//
+// Internally, it keeps two buffers: bufio.Reader with binary ledger data and
+// buffered channel with unmarshaled xdr.LedgerCloseMeta objects ready for
+// processing. The first buffer removes overhead time connected to reading from
+// a file. The second buffer allows unmarshaling binary data into XDR objects
+// (which can be a bottleneck) while clients are processing previous ledgers.
+//
+// Finally, when a large ledger (larger than binary buffer) is closed it waits
+// until xdr.LedgerCloseMeta objects channel is empty. This prevents memory
+// exhaustion when network closes a series a large ledgers.
+type bufferedLedgerMetaReader struct {
+	r      *bufio.Reader
+	c      chan metaResult
+	runner stellarCoreRunnerInterface
+}
+
+// newBufferedLedgerMetaReader creates a new meta reader that will shutdown
+// when stellar-core terminates.
+func newBufferedLedgerMetaReader(runner stellarCoreRunnerInterface) bufferedLedgerMetaReader {
+	return bufferedLedgerMetaReader{
+		c:      make(chan metaResult, ledgerReadAheadBufferSize),
+		r:      bufio.NewReaderSize(runner.getMetaPipe(), metaPipeBufferSize),
+		runner: runner,
+	}
+}
+
+// readLedgerMetaFromPipe unmarshalls the next ledger from meta pipe.
+// It can block for two reasons:
+//   * Meta pipe buffer is full so it will wait until it refills.
+//   * The next ledger available in the buffer exceeds the meta pipe buffer size.
+//     In such case the method will block until LedgerCloseMeta buffer is empty.
+func (b *bufferedLedgerMetaReader) readLedgerMetaFromPipe() (*xdr.LedgerCloseMeta, error) {
+	frameLength, err := xdr.ReadFrameLength(b.r)
+	if err != nil {
+		select {
+		case <-b.runner.getProcessExitChan():
+			return nil, errors.New("stellar-core process not-running")
+		default:
+			return nil, errors.Wrap(err, "error reading frame length")
+		}
+	}
+
+	for frameLength > metaPipeBufferSize && len(b.c) > 0 {
+		// Wait for LedgerCloseMeta buffer to be cleared to minimize memory usage.
+		select {
+		case <-b.runner.getProcessExitChan():
+			return nil, errors.New("stellar-core process not-running")
+		case <-time.After(time.Second):
+		}
+	}
+
+	var xlcm xdr.LedgerCloseMeta
+	_, err = xdr.Unmarshal(b.r, &xlcm)
+	if err != nil {
+		if err == io.EOF {
+			err = errors.Wrap(err, "got EOF from subprocess")
+		}
+		err = errors.Wrap(err, "unmarshalling framed LedgerCloseMeta")
+
+		select {
+		case <-b.runner.getProcessExitChan():
+			return nil, errors.New("stellar-core process not-running")
+		default:
+			return nil, err
+		}
+	}
+	return &xlcm, nil
+}
+
+func (b *bufferedLedgerMetaReader) GetChannel() <-chan metaResult {
+	return b.c
+}
+
+func (b *bufferedLedgerMetaReader) Start(untilSequence uint32) {
+	printBufferOccupation := time.NewTicker(5 * time.Second)
+	defer printBufferOccupation.Stop()
+	for {
+		select {
+		case <-b.runner.getProcessExitChan():
+			return
+		case <-printBufferOccupation.C:
+			log.Debug("captive core read-ahead buffer occupation:", len(b.c))
+		default:
+		}
+
+		meta, err := b.readLedgerMetaFromPipe()
+		if err != nil {
+			// When `GetLedger` sees the error it will close the backend. We shouldn't
+			// close it now because there may be some ledgers in a buffer.
+			select {
+			case b.c <- metaResult{nil, err}:
+			case <-b.runner.getProcessExitChan():
+			}
+			return
+		}
+		select {
+		case b.c <- metaResult{meta, nil}:
+		case <-b.runner.getProcessExitChan():
+			return
+		}
+
+		if untilSequence != 0 {
+			if meta.LedgerSequence() >= untilSequence {
+				// we are done
+				return
+			}
+		}
+	}
+}