Retry when WAL segment has not been found

[Debezium](https://github.com/debezium/debezium/blob/db0cfea550308e7386413db25115d531a4c0a61c/debezium-connector-postgres/src/main/java/io/debezium/connector/postgresql/connection/PostgresReplicationConnection.java#L584) also does something similar. Awaiting confirmation from Postgres experts on this but so far what we have seen from the field indicates that this error is OK.

This PR also does some minor refactors and cleaning up of error messages that looked very similar.

flow/activities/flowable.go

@@ -271,7 +271,7 @@ func (a *FlowableActivity) StartFlow(ctx context.Context,
 		err = errGroup.Wait()
 		if err != nil {
 			a.Alerter.LogFlowError(ctx, flowName, err)
-			return nil, fmt.Errorf("failed to pull records: %w", err)
+			return nil, fmt.Errorf("failed in pull records when: %w", err)
 		}
 		slog.InfoContext(ctx, "no records to push")
 		syncResponse := &model.SyncResponse{}
@@ -615,7 +615,7 @@ func (a *FlowableActivity) replicateQRepPartition(ctx context.Context,
 		recordBatch, err := srcConn.PullQRepRecords(config, partition)
 		if err != nil {
 			a.Alerter.LogFlowError(ctx, config.FlowJobName, err)
-			return fmt.Errorf("failed to pull records: %w", err)
+			return fmt.Errorf("failed to pull qrep records: %w", err)
 		}
 		numRecords := int64(recordBatch.NumRecords)
 		slog.InfoContext(ctx, fmt.Sprintf("pulled %d records\n", len(recordBatch.Records)))
@@ -955,7 +955,7 @@ func (a *FlowableActivity) ReplicateXminPartition(ctx context.Context,
 		numRecords, currentSnapshotXmin, pullErr = pgConn.PullXminRecordStream(config, partition, stream)
 		if pullErr != nil {
 			a.Alerter.LogFlowError(ctx, config.FlowJobName, err)
-			slog.InfoContext(ctx, fmt.Sprintf("failed to pull records: %v", err))
+			slog.InfoContext(ctx, fmt.Sprintf("[xmin] failed to pull records: %v", err))
 			return err
 		}
 

flow/connectors/postgres/cdc.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log/slog"
+	"regexp"
 	"time"
 
 	"github.com/PeerDB-io/peer-flow/connectors/utils"
@@ -44,6 +45,8 @@ type PostgresCDCSource struct {
 	// for storing chema delta audit logs to catalog
 	catalogPool *pgxpool.Pool
 	flowJobName string
+
+	walSegmentRemovedRegex *regexp.Regexp
 }
 
 type PostgresCDCConfig struct {
@@ -59,13 +62,22 @@ type PostgresCDCConfig struct {
 	SetLastOffset          func(int64) error
 }
 
+type startReplicationOpts struct {
+	conn            *pgconn.PgConn
+	startLSN        pglogrepl.LSN
+	replicationOpts pglogrepl.StartReplicationOptions
+}
+
 // Create a new PostgresCDCSource
 func NewPostgresCDCSource(cdcConfig *PostgresCDCConfig, customTypeMap map[uint32]string) (*PostgresCDCSource, error) {
 	childToParentRelIDMap, err := getChildToParentRelIDMap(cdcConfig.AppContext, cdcConfig.Connection)
 	if err != nil {
 		return nil, fmt.Errorf("error getting child to parent relid map: %w", err)
 	}
 
+	pattern := "requested WAL segment .* has already been removed.*"
+	regex := regexp.MustCompile(pattern)
+
 	flowName, _ := cdcConfig.AppContext.Value(shared.FlowNameKey).(string)
 	return &PostgresCDCSource{
 		ctx:                       cdcConfig.AppContext,
@@ -83,6 +95,7 @@ func NewPostgresCDCSource(cdcConfig *PostgresCDCConfig, customTypeMap map[uint32
 		logger:                    *slog.With(slog.String(string(shared.FlowNameKey), flowName)),
 		catalogPool:               cdcConfig.CatalogPool,
 		flowJobName:               cdcConfig.FlowJobName,
+		walSegmentRemovedRegex:    regex,
 	}, nil
 }
 
@@ -120,21 +133,11 @@ func getChildToParentRelIDMap(ctx context.Context, pool *pgxpool.Pool) (map[uint
 
 // PullRecords pulls records from the cdc stream
 func (p *PostgresCDCSource) PullRecords(req *model.PullRecordsRequest) error {
-	// setup options
-	pluginArguments := []string{
-		"proto_version '1'",
-	}
-
-	if p.publication != "" {
-		pubOpt := fmt.Sprintf("publication_names '%s'", p.publication)
-		pluginArguments = append(pluginArguments, pubOpt)
-	} else {
-		return fmt.Errorf("publication name is not set")
+	replicationOpts, err := p.replicationOptions()
+	if err != nil {
+		return fmt.Errorf("error getting replication options: %w", err)
 	}
 
-	replicationOpts := pglogrepl.StartReplicationOptions{PluginArgs: pluginArguments}
-	replicationSlot := p.slot
-
 	// create replication connection
 	replicationConn, err := p.replPool.Acquire(p.ctx)
 	if err != nil {
@@ -146,13 +149,6 @@ func (p *PostgresCDCSource) PullRecords(req *model.PullRecordsRequest) error {
 	pgConn := replicationConn.Conn().PgConn()
 	p.logger.Info("created replication connection")
 
-	sysident, err := pglogrepl.IdentifySystem(p.ctx, pgConn)
-	if err != nil {
-		return fmt.Errorf("IdentifySystem failed: %w", err)
-	}
-	p.logger.Debug(fmt.Sprintf("SystemID: %s, Timeline: %d, XLogPos: %d, DBName: %s",
-		sysident.SystemID, sysident.Timeline, sysident.XLogPos, sysident.DBName))
-
 	// start replication
 	var clientXLogPos, startLSN pglogrepl.LSN
 	if req.LastOffset > 0 {
@@ -161,15 +157,48 @@ func (p *PostgresCDCSource) PullRecords(req *model.PullRecordsRequest) error {
 		startLSN = clientXLogPos + 1
 	}
 
-	err = pglogrepl.StartReplication(p.ctx, pgConn, replicationSlot, startLSN, replicationOpts)
+	opts := startReplicationOpts{
+		conn:            pgConn,
+		startLSN:        startLSN,
+		replicationOpts: *replicationOpts,
+	}
+
+	err = p.startReplication(opts)
 	if err != nil {
-		return fmt.Errorf("error starting replication at startLsn - %d: %w", startLSN, err)
+		return fmt.Errorf("error starting replication: %w", err)
 	}
+
 	p.logger.Info(fmt.Sprintf("started replication on slot %s at startLSN: %d", p.slot, startLSN))
 
 	return p.consumeStream(pgConn, req, clientXLogPos, req.RecordStream)
 }
 
+func (p *PostgresCDCSource) startReplication(opts startReplicationOpts) error {
+	err := pglogrepl.StartReplication(p.ctx, opts.conn, p.slot, opts.startLSN, opts.replicationOpts)
+	if err != nil {
+		p.logger.Error("error starting replication", slog.Any("error", err))
+		return fmt.Errorf("error starting replication at startLsn - %d: %w", opts.startLSN, err)
+	}
+
+	p.logger.Info(fmt.Sprintf("started replication on slot %s at startLSN: %d", p.slot, opts.startLSN))
+	return nil
+}
+
+func (p *PostgresCDCSource) replicationOptions() (*pglogrepl.StartReplicationOptions, error) {
+	pluginArguments := []string{
+		"proto_version '1'",
+	}
+
+	if p.publication != "" {
+		pubOpt := fmt.Sprintf("publication_names '%s'", p.publication)
+		pluginArguments = append(pluginArguments, pubOpt)
+	} else {
+		return nil, fmt.Errorf("publication name is not set")
+	}
+
+	return &pglogrepl.StartReplicationOptions{PluginArgs: pluginArguments}, nil
+}
+
 // start consuming the cdc stream
 func (p *PostgresCDCSource) consumeStream(
 	conn *pgconn.PgConn,
@@ -324,6 +353,13 @@ func (p *PostgresCDCSource) consumeStream(
 		}
 
 		if errMsg, ok := rawMsg.(*pgproto3.ErrorResponse); ok {
+			if errMsg.Severity == "ERROR" && errMsg.Code == "XX000" {
+				if p.walSegmentRemovedRegex.MatchString(errMsg.Message) {
+					p.logger.Warn(fmt.Sprintf("WAL segment removed, restarting replication retrying in 30 seconds..."))
+					time.Sleep(30 * time.Second)
+					continue
+				}
+			}
 			return fmt.Errorf("received Postgres WAL error: %+v", errMsg)
 		}