prototype for setting options using As() in gocloud.dev blob/s3

s3blob.go.patch

diff --git a/blob/s3blob/s3blob.go b/blob/s3blob/s3blob.go
index b697f237..b03ce956 100644
--- a/blob/s3blob/s3blob.go
+++ b/blob/s3blob/s3blob.go
@@ -44,9 +44,9 @@
 // s3blob exposes the following types for As:
 //   - Bucket: (V1) *s3.S3; (V2) *s3v2.Client
 //   - Error: (V1) awserr.Error; (V2) any error type returned by the service, notably smithy.APIError
-//   - ListObject: (V1) s3.Object for objects, s3.CommonPrefix for "directories"; (V2) typesv2.Object for objects, typesv2.CommonPrefix fo
r "directories
+//   - ListObject: (V1) s3.Object for objects, s3.CommonPrefix for "directories"; (V2) typesv2.Object for objects, typesv2.CommonPrefix fo
r "directories"
 //   - ListOptions.BeforeList: (V1) *s3.ListObjectsV2Input, or *s3.ListObjectsInput
-//     when Options.UseLegacyList == true; (V2) *s3v2.ListObjectsV2Input, or *s3v2.ListObjectsInput
+//     when Options.UseLegacyList == true; (V2) *s3v2.ListObjectsV2Input or *[]func(*s3v2.Options), or *s3v2.ListObjectsInput
 //     when Options.UseLegacyList == true
 //   - Reader: (V1) s3.GetObjectOutput; (V2) s3v2.GetObjectInput
 //   - ReaderOptions.BeforeRead: (V1) *s3.GetObjectInput; (V2) *s3v2.GetObjectInput
@@ -274,6 +274,7 @@ type writer struct {
        // v2
        uploaderV2 *s3managerv2.Uploader
        reqV2      *s3v2.PutObjectInput
+       optsV2     []func(*s3v2.Options)
 
        donec chan struct{} // closed when done writing
        // The following fields will be written before donec closes:
@@ -528,20 +529,24 @@ func (b *bucket) ListPaged(ctx context.Context, opts *driver.ListOptions) (*driv
 
 func (b *bucket) listObjectsV2(ctx context.Context, in *s3v2.ListObjectsV2Input, opts *driver.ListOptions) (*s3v2.ListObjectsV2Output, err
or) {
        if !b.useLegacyList {
+               var varopt []func(*s3v2.Options)
                if opts.BeforeList != nil {
                        asFunc := func(i interface{}) bool {
-                               p, ok := i.(**s3v2.ListObjectsV2Input)
-                               if !ok {
-                                       return false
+                               if p, ok := i.(**s3v2.ListObjectsV2Input); ok {
+                                       *p = in
+                                       return true
                                }
-                               *p = in
-                               return true
+                               if p, ok := i.(**[]func(*s3v2.Options)); ok {
+                                       *p = &varopt
+                                       return true
+                               }
+                               return false
                        }
                        if err := opts.BeforeList(asFunc); err != nil {
                                return nil, err
                        }
                }
-               return b.clientV2.ListObjectsV2(ctx, in)
+               return b.clientV2.ListObjectsV2(ctx, in, varopt...)
        }
 
        // Use the legacy ListObjects request.
@@ -776,19 +781,24 @@ func (b *bucket) NewRangeReader(ctx context.Context, key string, offset, length
                        Key:    aws.String(key),
                        Range:  byteRange,
                }
+               var varopt []func(*s3v2.Options)
                if opts.BeforeRead != nil {
                        asFunc := func(i interface{}) bool {
                                if p, ok := i.(**s3v2.GetObjectInput); ok {
                                        *p = in
                                        return true
                                }
+                               if p, ok := i.(**[]func(*s3v2.Options)); ok {
+                                       *p = &varopt
+                                       return true
+                               }
                                return false
                        }
                        if err := opts.BeforeRead(asFunc); err != nil {
                                return nil, err
                        }
                }
-               resp, err := b.clientV2.GetObject(ctx, in)
+               resp, err := b.clientV2.GetObject(ctx, in, varopt...)
                if err != nil {
                        return nil, err
                }
@@ -958,16 +968,19 @@ func (b *bucket) NewTypedWriter(ctx context.Context, key string, contentType str
                if len(opts.ContentMD5) > 0 {
                        reqV2.ContentMD5 = aws.String(base64.StdEncoding.EncodeToString(opts.ContentMD5))
                }
+               var varopt []func(*s3v2.Options)
                if opts.BeforeWrite != nil {
                        asFunc := func(i interface{}) bool {
-                               pu, ok := i.(**s3managerv2.Uploader)
-                               if ok {
-                                       *pu = uploaderV2
+                               if p, ok := i.(**s3managerv2.Uploader); ok {
+                                       *p = uploaderV2
                                        return true
                                }
-                               pui, ok := i.(**s3v2.PutObjectInput)
-                               if ok {
-                                       *pui = reqV2
+                               if p, ok := i.(**s3v2.PutObjectInput); ok {
+                                       *p = reqV2
+                                       return true
+                               }
+                               if p, ok := i.(**[]func(*s3v2.Options)); ok {
+                                       *p = &varopt
                                        return true
                                }
                                return false
@@ -981,6 +994,7 @@ func (b *bucket) NewTypedWriter(ctx context.Context, key string, contentType str
                        useV2:      true,
                        uploaderV2: uploaderV2,
                        reqV2:      reqV2,
+                       optsV2:     varopt,
                        donec:      make(chan struct{}),
                }, nil
        } else {