Update s3 checker to iterate through all objects pages (#1802)

pkg/storage/s3/checker.go

@@ -11,10 +11,6 @@ import (
 	"github.com/gomods/athens/pkg/observ"
 )
 
-const (
-	s3ErrorCodeNotFound = "NotFound"
-)
-
 // Exists implements the (./pkg/storage).Checker interface
 // returning true if the module at version exists in storage
 func (s *Storage) Exists(ctx context.Context, module, version string) (bool, error) {
@@ -26,12 +22,8 @@ func (s *Storage) Exists(ctx context.Context, module, version string) (bool, err
 		Bucket: aws.String(s.bucket),
 		Prefix: aws.String(fmt.Sprintf("%s/@v", module)),
 	}
-
-	loo, err := s.s3API.ListObjectsWithContext(ctx, lsParams)
-	if err != nil {
-		return false, errors.E(op, err, errors.M(module), errors.V(version))
-	}
 	var count int
+	err := s.s3API.ListObjectsPagesWithContext(ctx, lsParams, func(loo *s3.ListObjectsOutput, lastPage bool) bool {
 		for _, o := range loo.Contents {
 			// sane assumption: no duplicate keys.
 			switch *o.Key {
@@ -43,5 +35,12 @@ func (s *Storage) Exists(ctx context.Context, module, version string) (bool, err
 				count++
 			}
 		}
+		return count != 3
+	})
+
+	if err != nil {
+		return false, errors.E(op, err, errors.M(module), errors.V(version))
+	}
+
 	return count == 3, nil
 }