fix: s3proxy ListBuckets 400 error with listbuckets v1-compatible backends

The AWS SDK serializes all non-nil pointer fields into query parameters.
Previously, ContinuationToken and Prefix were always passed as non-nil
pointers (even when empty), and MaxBuckets was always forwarded with its
default value of 10000. This produced a request like:

  GET /?continuation-token=&max-buckets=10000&prefix=

Some servers implement the original S3 v1 ListBuckets API which accepts
no query parameters, causing it to return 400 InvalidArgument.

Fix by only setting ContinuationToken and Prefix on the downstream SDK
input when they are non-empty, and applying MaxBuckets pagination
client-side (consistent with the posix and azure backends) rather than
forwarding it to the upstream endpoint.

backend/s3proxy/s3.go

@@ -102,20 +102,36 @@ func (s *S3Proxy) validate(ctx context.Context) error {
 }
 
 func (s *S3Proxy) ListBuckets(ctx context.Context, input s3response.ListBucketsInput) (s3response.ListAllMyBucketsResult, error) {
-	output, err := s.client.ListBuckets(ctx, &s3.ListBucketsInput{
-		ContinuationToken: &input.ContinuationToken,
-		MaxBuckets:        &input.MaxBuckets,
-		Prefix:            &input.Prefix,
-	})
+	// Only set parameters when they carry a meaningful value. Passing empty
+	// string pointers (instead of nil) causes the AWS SDK to serialize them
+	// as empty query parameters (e.g. "continuation-token=&prefix="), and
+	// always forwarding max-buckets breaks backends such as GCS that only
+	// implement the original v1 ListBuckets API with no query parameters.
+	listInput := &s3.ListBucketsInput{}
+	if input.ContinuationToken != "" {
+		listInput.ContinuationToken = &input.ContinuationToken
+	}
+	if input.Prefix != "" {
+		listInput.Prefix = &input.Prefix
+	}
+
+	output, err := s.client.ListBuckets(ctx, listInput)
 	if err != nil {
 		return s3response.ListAllMyBucketsResult{}, handleError(err)
 	}
 
 	var buckets []s3response.ListAllMyBucketsEntry
+	var cToken string
 	for _, b := range output.Buckets {
 		if *b.Name == s.metaBucket {
 			continue
 		}
+		// Apply MaxBuckets client-side so that backends which do not support
+		// the max-buckets query parameter are still correctly paginated.
+		if input.MaxBuckets > 0 && int32(len(buckets)) >= input.MaxBuckets {
+			cToken = buckets[len(buckets)-1].Name
+			break
+		}
 		if input.IsAdmin || s.metaBucket == "" {
 			buckets = append(buckets, s3response.ListAllMyBucketsEntry{
 				Name:         *b.Name,
@@ -142,6 +158,13 @@ func (s *S3Proxy) ListBuckets(ctx context.Context, input s3response.ListBucketsI
 		}
 	}
 
+	// Prefer a server-side continuation token when available; fall back to
+	// the client-side token produced by the MaxBuckets cutoff above.
+	ct := backend.GetStringFromPtr(output.ContinuationToken)
+	if ct != "" {
+		cToken = ct
+	}
+
 	return s3response.ListAllMyBucketsResult{
 		Owner: s3response.CanonicalUser{
 			ID: *output.Owner.ID,
@@ -149,7 +172,7 @@ func (s *S3Proxy) ListBuckets(ctx context.Context, input s3response.ListBucketsI
 		Buckets: s3response.ListAllMyBucketsList{
 			Bucket: buckets,
 		},
-		ContinuationToken: backend.GetStringFromPtr(output.ContinuationToken),
+		ContinuationToken: cToken,
 		Prefix:            backend.GetStringFromPtr(output.Prefix),
 	}, nil
 }