Fix SetBucketLifecycle() for global rules

https://docs.aws.amazon.com/AmazonS3/latest/API/API_LifecycleRule.html
mentions that

       Filter is required
       if the LifecycleRule does not contain a Prefix element.

After #2177, this requirement is broken for bucket-global (=prefixless)
rules. And clients setting such may run into server side validation
issues like.

HTTP/1.1 400 Bad Request

<?xml version="1.0" encoding="UTF-8"?>
<Error>
  <Code>MalformedXML</Code>
  ...
</Error>

Author: isimluk

pkg/lifecycle/lifecycle.go

@@ -267,10 +267,6 @@ func (f Filter) MarshalJSON() ([]byte, error) {
 // MarshalXML - produces the xml representation of the Filter struct
 // only one of Prefix, And and Tag should be present in the output.
 func (f Filter) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
-	if f.IsNull() {
-		return nil
-	}
-
 	if err := e.EncodeToken(start); err != nil {
 		return err
 	}
@@ -508,6 +504,50 @@ func (r Rule) MarshalJSON() ([]byte, error) {
 	return json.Marshal(newr)
 }
 
+// MarshalXML customizes marshal XML and ensures that <Filter/> is present if Prefix is empty.
+func (r Rule) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+	// If both Filter and Prefix are empty, we need to emit an empty <Filter/> element.
+	if r.RuleFilter.IsNull() {
+		type ruleWrapper struct {
+			XMLName                        xml.Name                       `xml:"Rule,omitempty"`
+			AbortIncompleteMultipartUpload AbortIncompleteMultipartUpload `xml:"AbortIncompleteMultipartUpload,omitempty"`
+			Expiration                     Expiration                     `xml:"Expiration,omitempty"`
+			DelMarkerExpiration            DelMarkerExpiration            `xml:"DelMarkerExpiration,omitempty"`
+			AllVersionsExpiration          AllVersionsExpiration          `xml:"AllVersionsExpiration,omitempty"`
+			ID                             string                         `xml:"ID"`
+			RuleFilter                     *Filter                        `xml:"Filter,omitempty"`
+			NoncurrentVersionExpiration    NoncurrentVersionExpiration    `xml:"NoncurrentVersionExpiration,omitempty"`
+			NoncurrentVersionTransition    NoncurrentVersionTransition    `xml:"NoncurrentVersionTransition,omitempty"`
+			Prefix                         string                         `xml:"Prefix,omitempty"`
+			Status                         string                         `xml:"Status"`
+			Transition                     Transition                     `xml:"Transition,omitempty"`
+		}
+
+		w := ruleWrapper{
+			AbortIncompleteMultipartUpload: r.AbortIncompleteMultipartUpload,
+			Expiration:                     r.Expiration,
+			DelMarkerExpiration:            r.DelMarkerExpiration,
+			AllVersionsExpiration:          r.AllVersionsExpiration,
+			ID:                             r.ID,
+			NoncurrentVersionExpiration:    r.NoncurrentVersionExpiration,
+			NoncurrentVersionTransition:    r.NoncurrentVersionTransition,
+			Prefix:                         r.Prefix,
+			Status:                         r.Status,
+			Transition:                     r.Transition,
+		}
+		if r.Prefix == "" {
+			// will be explicitly marshaled as empty <Filter/>
+			w.RuleFilter = &r.RuleFilter
+		}
+
+		return e.EncodeElement(w, start)
+	}
+
+	// Default XML marshal for Rule (uses struct tags as defined).
+	type ruleAlias Rule
+	return e.EncodeElement(ruleAlias(r), start)
+}
+
 // Rule represents a single rule in lifecycle configuration
 type Rule struct {
 	XMLName                        xml.Name                       `xml:"Rule,omitempty" json:"-"`

pkg/lifecycle/lifecycle_test.go

@@ -518,7 +518,6 @@ func TestLifecycleMarshalXML(t *testing.T) {
 			input: Configuration{
 				Rules: []Rule{
 					{
-
 						ID:                             "expire-incomplete-uploads-1",
 						Status:                         "Enabled",
 						Prefix:                         "my_dir",
@@ -543,8 +542,7 @@ func TestLifecycleMarshalXML(t *testing.T) {
 					},
 				},
 			},
-			// this is wrong fixed behavior fixed in a following commit
-			expectedXMLOut: "<LifecycleConfiguration><Rule><AbortIncompleteMultipartUpload><DaysAfterInitiation>1</DaysAfterInitiation></AbortIncompleteMultipartUpload><ID>expire-incomplete-uploads-2</ID><Status>Enabled</Status></Rule></LifecycleConfiguration>",
+			expectedXMLOut: "<LifecycleConfiguration><Rule><AbortIncompleteMultipartUpload><DaysAfterInitiation>1</DaysAfterInitiation></AbortIncompleteMultipartUpload><ID>expire-incomplete-uploads-2</ID><Filter><Prefix></Prefix></Filter><Status>Enabled</Status></Rule></LifecycleConfiguration>",
 		},
 		{
 			testDescription: "Ensure we always export Filter or Prefix. Specification explicitly mentions: 'Filter is required if the LifecycleRule does not contain a Prefix element.' (https://docs.aws.amazon.com/AmazonS3/latest/API/API_LifecycleRule.html)",
@@ -559,8 +557,7 @@ func TestLifecycleMarshalXML(t *testing.T) {
 					},
 				},
 			},
-			// this is wrong fixed behavior fixed in a following commit
-			expectedXMLOut: "<LifecycleConfiguration><Rule><AbortIncompleteMultipartUpload><DaysAfterInitiation>1</DaysAfterInitiation></AbortIncompleteMultipartUpload><ID>expire-incomplete-uploads-3</ID><Status>Enabled</Status></Rule></LifecycleConfiguration>",
+			expectedXMLOut: "<LifecycleConfiguration><Rule><AbortIncompleteMultipartUpload><DaysAfterInitiation>1</DaysAfterInitiation></AbortIncompleteMultipartUpload><ID>expire-incomplete-uploads-3</ID><Filter><Prefix></Prefix></Filter><Status>Enabled</Status></Rule></LifecycleConfiguration>",
 		},
 	}