s3: drop binary metadata with an ERROR message

Before this change, rclone would attempt to upload metadata with
binary contents which fail to be uploaded by net/http.

This checks the keys and values for validity as http header values
before uploading.

See: https://forum.rclone.org/t/invalid-metadata-key-names-result-in-a-failure-to-transfer-xattr-results-in-failure-to-upload-net-http-invalid-header-field-value-for-x-amz-meta-samba-pai/33406/

backend/s3/s3.go

@@ -57,6 +57,7 @@ import (
 	"github.com/rclone/rclone/lib/readers"
 	"github.com/rclone/rclone/lib/rest"
 	"github.com/rclone/rclone/lib/version"
+	"golang.org/x/net/http/httpguts"
 	"golang.org/x/sync/errgroup"
 )
 
@@ -5241,6 +5242,20 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 		}
 	}
 
+	// Check metadata keys and values are valid
+	for key, value := range req.Metadata {
+		if !httpguts.ValidHeaderFieldName(key) {
+			fs.Errorf(o, "Dropping invalid metadata key %q", key)
+			delete(req.Metadata, key)
+		} else if value == nil {
+			fs.Errorf(o, "Dropping nil metadata value for key %q", key)
+			delete(req.Metadata, key)
+		} else if !httpguts.ValidHeaderFieldValue(*value) {
+			fs.Errorf(o, "Dropping invalid metadata value %q for key %q", *value, key)
+			delete(req.Metadata, key)
+		}
+	}
+
 	var wantETag string        // Multipart upload Etag to check
 	var gotEtag string         // Etag we got from the upload
 	var lastModified time.Time // Time we got from the upload