bug: fix creating multiple scans for the same shard

allgroup.go

@@ -38,7 +38,7 @@ type AllGroup struct {
 
 // Start is a blocking operation which will loop and attempt to find new
 // shards on a regular cadence.
-func (g *AllGroup) Start(ctx context.Context, shardc chan types.Shard) error {
+func (g *AllGroup) Start(ctx context.Context, shardC chan types.Shard) error {
 	// Note: while ticker is a rather naive approach to this problem,
 	// it actually simplifies a few things. I.e. If we miss a new shard
 	// while AWS is resharding, we'll pick it up max 30 seconds later.
@@ -51,7 +51,7 @@ func (g *AllGroup) Start(ctx context.Context, shardc chan types.Shard) error {
 	var ticker = time.NewTicker(30 * time.Second)
 
 	for {
-		if err := g.findNewShards(ctx, shardc); err != nil {
+		if err := g.findNewShards(ctx, shardC); err != nil {
 			ticker.Stop()
 			return err
 		}
@@ -94,7 +94,7 @@ func waitForCloseChannel(ctx context.Context, c <-chan struct{}) bool {
 // findNewShards pulls the list of shards from the Kinesis API
 // and uses a local cache to determine if we are already processing
 // a particular shard.
-func (g *AllGroup) findNewShards(ctx context.Context, shardc chan types.Shard) error {
+func (g *AllGroup) findNewShards(ctx context.Context, shardC chan types.Shard) error {
 	g.shardMu.Lock()
 	defer g.shardMu.Unlock()
 
@@ -110,14 +110,17 @@ func (g *AllGroup) findNewShards(ctx context.Context, shardc chan types.Shard) e
 	// channels before we start using any of them.  It's highly probable
 	// that Kinesis provides us the shards in dependency order (parents
 	// before children), but it doesn't appear to be a guarantee.
+	newShards := make(map[string]types.Shard)
 	for _, shard := range shards {
 		if _, ok := g.shards[*shard.ShardId]; ok {
 			continue
 		}
 		g.shards[*shard.ShardId] = shard
 		g.shardsClosed[*shard.ShardId] = make(chan struct{})
+		newShards[*shard.ShardId] = shard
 	}
-	for _, shard := range shards {
+	// only new shards need to be checked for parent dependencies
+	for _, shard := range newShards {
 		shard := shard // Shadow shard, since we use it in goroutine
 		var parent1, parent2 <-chan struct{}
 		if shard.ParentShardId != nil {
@@ -133,7 +136,7 @@ func (g *AllGroup) findNewShards(ctx context.Context, shardc chan types.Shard) e
 			// but when splits or joins happen, we need to process all parents prior
 			// to processing children or that ordering guarantee is not maintained.
 			if waitForCloseChannel(ctx, parent1) && waitForCloseChannel(ctx, parent2) {
-				shardc <- shard
+				shardC <- shard
 			}
 		}()
 	}

consumer.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"log"
 	"sync"
 	"time"
@@ -38,7 +38,7 @@ func New(streamName string, opts ...Option) (*Consumer, error) {
 		store:                    &noopStore{},
 		counter:                  &noopCounter{},
 		logger: &noopLogger{
-			logger: log.New(ioutil.Discard, "", log.LstdFlags),
+			logger: log.New(io.Discard, "", log.LstdFlags),
 		},
 		scanInterval: 250 * time.Millisecond,
 		maxRecords:   10000,
@@ -102,25 +102,35 @@ func (c *Consumer) Scan(ctx context.Context, fn ScanFunc) error {
 	defer cancel()
 
 	var (
-		errc   = make(chan error, 1)
-		shardc = make(chan types.Shard, 1)
+		errC   = make(chan error, 1)
+		shardC = make(chan types.Shard, 1)
 	)
 
 	go func() {
-		err := c.group.Start(ctx, shardc)
+		err := c.group.Start(ctx, shardC)
 		if err != nil {
-			errc <- fmt.Errorf("error starting scan: %w", err)
+			errC <- fmt.Errorf("error starting scan: %w", err)
 			cancel()
 		}
 		<-ctx.Done()
-		close(shardc)
+		close(shardC)
 	}()
 
 	wg := new(sync.WaitGroup)
 	// process each of the shards
-	for shard := range shardc {
+	shardsInProcess := make(map[string]struct{})
+	for shard := range shardC {
+		shardId := aws.ToString(shard.ShardId)
+		if _, ok := shardsInProcess[shardId]; ok {
+			// safetynet: if shard already in process by another goroutine, just skipping the request
+			continue
+		}
 		wg.Add(1)
 		go func(shardID string) {
+			shardsInProcess[shardID] = struct{}{}
+			defer func() {
+				delete(shardsInProcess, shardID)
+			}()
 			defer wg.Done()
 			var err error
 			if err = c.ScanShard(ctx, shardID, fn); err != nil {
@@ -132,20 +142,20 @@ func (c *Consumer) Scan(ctx context.Context, fn ScanFunc) error {
 			}
 			if err != nil {
 				select {
-				case errc <- fmt.Errorf("shard %s error: %w", shardID, err):
+				case errC <- fmt.Errorf("shard %s error: %w", shardID, err):
 					cancel()
 				default:
 				}
 			}
-		}(aws.ToString(shard.ShardId))
+		}(shardId)
 	}
 
 	go func() {
 		wg.Wait()
-		close(errc)
+		close(errC)
 	}()
 
-	return <-errc
+	return <-errC
 }
 
 // ScanShard loops over records on a specific shard, calls the callback func