Calling HeadObjectWithContext() concurrently to check the existence of a module (#1844)

* Using HeadObject to check the existence of a module

* fixing build

* calling cancel before closing channel

* fixing test

* using waitgroup

* calling cancel

* calling wg.Done

* only return the first error

* Revert "only return the first error"

This reverts commit c0aa18b522.

* clean err if not exist

pkg/storage/s3/checker.go

@@ -2,13 +2,14 @@ package s3
 
 import (
 	"context"
-	"fmt"
+	errs "errors"
+	"sync"
 
 	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/aws/awserr"
 	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/gomods/athens/pkg/config"
 	"github.com/gomods/athens/pkg/errors"
-	"github.com/gomods/athens/pkg/log"
 	"github.com/gomods/athens/pkg/observ"
 )
 
@@ -19,27 +20,39 @@ func (s *Storage) Exists(ctx context.Context, module, version string) (bool, err
 	ctx, span := observ.StartSpan(ctx, op.String())
 	defer span.End()
 
-	lsParams := &s3.ListObjectsInput{
-		Bucket: aws.String(s.bucket),
-		Prefix: aws.String(fmt.Sprintf("%s/@v", module)),
+	files := []string{"info", "mod", "zip"}
+	errChan := make(chan error, len(files))
+	defer close(errChan)
+	cancelingCtx, cancel := context.WithCancel(ctx)
+	var wg sync.WaitGroup
+	for _, file := range files {
+		wg.Add(1)
+		go func(file string) {
+			defer wg.Done()
+			_, err := s.s3API.HeadObjectWithContext(
+				cancelingCtx,
+				&s3.HeadObjectInput{
+					Bucket: aws.String(s.bucket),
+					Key:    aws.String(config.PackageVersionedName(module, version, file)),
+				})
+			errChan <- err
+		}(file)
 	}
-	found := make(map[string]struct{}, 3)
-	err := s.s3API.ListObjectsPagesWithContext(ctx, lsParams, func(loo *s3.ListObjectsOutput, lastPage bool) bool {
-		for _, o := range loo.Contents {
-			if _, exists := found[*o.Key]; exists {
-				log.EntryFromContext(ctx).Warnf("duplicate key in prefix %q: %q", *lsParams.Prefix, *o.Key)
-				continue
-			}
-			if *o.Key == config.PackageVersionedName(module, version, "info") ||
-				*o.Key == config.PackageVersionedName(module, version, "mod") ||
-				*o.Key == config.PackageVersionedName(module, version, "zip") {
-				found[*o.Key] = struct{}{}
-			}
+	exists := true
+	var err error
+	for range files {
+		err = <-errChan
+		if err == nil {
+			continue
 		}
-		return len(found) < 3
-	})
-	if err != nil {
-		return false, errors.E(op, err, errors.M(module), errors.V(version))
+		var aerr awserr.Error
+		if errs.As(err, &aerr) && aerr.Code() == "NotFound" {
+			err = nil
+			exists = false
+		}
+		break
 	}
-	return len(found) == 3, nil
+	cancel()
+	wg.Wait()
+	return exists, err
 }