s3api: always write XML timestamps with three fractional digits (#11520)

CopyObject responses carried LastModified values such as
"2026-09-29T20:30:04.56Z": trailing zeros of the fractional seconds were
trimmed, and a whole-second value had no fraction at all. AWS S3 always
writes exactly three digits ("...04.560Z"), and clients that parse with a
fixed-width pattern reject anything else. minio-java 8.6.0
(yyyy-MM-dd'T'HH:mm:ss.SSS'Z') throws DateTimeParseException, so roughly
one CopyObject in ten fails on the client even though the copy succeeded.

Two causes:

- xsdDateTime marshalled with "2006-01-02T15:04:05.999999999", which
  drops trailing zeros. It now writes UTC with ".000Z".
- CopyObjectResult.MarshalXML had a pointer receiver, but the handlers
  pass the result by value, so encoding/xml never called it and fell back
  to time.Time's RFC 3339 encoding. It now has a value receiver.
  CopyPartResult had no custom marshaller at all; it now uses xsdDateTime.

Follow-up to #8394 / #8398, which truncated these timestamps to
milliseconds but kept the trimmed format.

weed/s3api/s3api_object_handlers_copy.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"crypto/rand"
 	"encoding/base64"
+	"encoding/xml"
 	"errors"
 	"fmt"
 	"io"
@@ -792,6 +793,19 @@ type CopyPartResult struct {
 	ChecksumResult
 }
 
+// MarshalXML writes LastModified in the S3 timestamp format (see xsdDateTime)
+// instead of encoding/xml's RFC 3339 with trimmed fractional seconds.
+func (r CopyPartResult) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+	type T CopyPartResult
+	var layout struct {
+		*T
+		LastModified xsdDateTime `xml:"LastModified"`
+	}
+	layout.T = (*T)(&r)
+	layout.LastModified = xsdDateTime(r.LastModified)
+	return e.EncodeElement(layout, start)
+}
+
 func buildCopyPartResult(etag string, lastModified time.Time, metadata SSEResponseMetadata) CopyPartResult {
 	result := CopyPartResult{
 		ETag:         etag,

weed/s3api/s3api_object_handlers_list_test.go

@@ -119,7 +119,7 @@ func TestListObjectsHandler(t *testing.T) {
 	// https://docs.aws.amazon.com/AmazonS3/latest/API/v2-RESTBucketGET.html
 
 	expected := `<?xml version="1.0" encoding="UTF-8"?>
-<ListBucketResult><Name>test_container</Name><Prefix></Prefix><Marker></Marker><MaxKeys>1000</MaxKeys><IsTruncated>false</IsTruncated><Contents><Key>1.zip</Key><ETag>&#34;4397da7a7649e8085de9916c240e8166&#34;</ETag><Size>1234567</Size><Owner><ID>65a011niqo39cdf8ec533ec3d1ccaafsa932</ID></Owner><StorageClass>STANDARD</StorageClass><LastModified>2011-04-09T12:34:49Z</LastModified></Contents><EncodingType></EncodingType></ListBucketResult>`
+<ListBucketResult><Name>test_container</Name><Prefix></Prefix><Marker></Marker><MaxKeys>1000</MaxKeys><IsTruncated>false</IsTruncated><Contents><Key>1.zip</Key><ETag>&#34;4397da7a7649e8085de9916c240e8166&#34;</ETag><Size>1234567</Size><Owner><ID>65a011niqo39cdf8ec533ec3d1ccaafsa932</ID></Owner><StorageClass>STANDARD</StorageClass><LastModified>2011-04-09T12:34:49.000Z</LastModified></Contents><EncodingType></EncodingType></ListBucketResult>`
 
 	response := ListBucketResult{
 		Name:        "test_container",

weed/s3api/s3api_xml_timestamp_test.go

@@ -0,0 +1,56 @@
+package s3api
+
+import (
+	"encoding/xml"
+	"strings"
+	"testing"
+	"time"
+)
+
+// S3 clients such as minio-java parse LastModified with a fixed-width
+// "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'" pattern, so trailing zeros in the
+// fractional seconds must not be trimmed.
+func TestXMLTimestampsHaveFixedMilliseconds(t *testing.T) {
+	cases := []struct {
+		in   time.Time
+		want string
+	}{
+		{time.Date(2026, 9, 29, 20, 30, 4, 560_000_000, time.UTC), "2026-09-29T20:30:04.560Z"},
+		{time.Date(2026, 9, 29, 20, 30, 4, 500_000_000, time.UTC), "2026-09-29T20:30:04.500Z"},
+		{time.Date(2026, 9, 29, 20, 30, 4, 0, time.UTC), "2026-09-29T20:30:04.000Z"},
+		{time.Date(2026, 9, 29, 20, 30, 4, 123_456_789, time.UTC), "2026-09-29T20:30:04.123Z"},
+		{time.Date(2026, 9, 29, 14, 30, 4, 560_000_000, time.FixedZone("MDT", -6*3600)), "2026-09-29T20:30:04.560Z"},
+	}
+	for _, c := range cases {
+		for name, v := range map[string]any{
+			// by value, the way the handlers pass them to writeSuccessResponseXML
+			"CopyObjectResult":       CopyObjectResult{ETag: "e", LastModified: c.in},
+			"CopyPartResult":         CopyPartResult{ETag: "e", LastModified: c.in},
+			"CopyObjectResult (ptr)": &CopyObjectResult{ETag: "e", LastModified: c.in},
+			"CopyPartResult (ptr)":   &CopyPartResult{ETag: "e", LastModified: c.in},
+		} {
+			out, err := xml.Marshal(v)
+			if err != nil {
+				t.Fatalf("%s: %v", name, err)
+			}
+			if !strings.Contains(string(out), "<LastModified>"+c.want+"</LastModified>") {
+				t.Errorf("%s(%v): got %s, want LastModified %s", name, c.in, out, c.want)
+			}
+		}
+	}
+}
+
+func TestXMLTimestampRoundTrip(t *testing.T) {
+	in := CopyObjectResult{ETag: "e", LastModified: time.Date(2026, 9, 29, 20, 30, 4, 560_000_000, time.UTC)}
+	out, err := xml.Marshal(&in)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var back CopyObjectResult
+	if err := xml.Unmarshal(out, &back); err != nil {
+		t.Fatal(err)
+	}
+	if !back.LastModified.Equal(in.LastModified) {
+		t.Fatalf("round trip: got %v, want %v", back.LastModified, in.LastModified)
+	}
+}

weed/s3api/s3api_xsd_generated.go

@@ -623,13 +623,16 @@ type CopyObjectResult struct {
 	ETag         string    `xml:"ETag"`
 }
 
-func (t *CopyObjectResult) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+// MarshalXML has a value receiver: handlers pass CopyObjectResult by value, and
+// encoding/xml does not call a pointer-receiver MarshalXML on a non-addressable
+// value, which silently fell back to time.Time's RFC 3339 encoding.
+func (t CopyObjectResult) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
 	type T CopyObjectResult
 	var layout struct {
 		*T
 		LastModified *xsdDateTime `xml:"LastModified"`
 	}
-	layout.T = (*T)(t)
+	layout.T = (*T)(&t)
 	layout.LastModified = (*xsdDateTime)(&layout.T.LastModified)
 	return e.EncodeElement(layout, start)
 }
@@ -1512,8 +1515,15 @@ type xsdDateTime time.Time
 func (t *xsdDateTime) UnmarshalText(text []byte) error {
 	return _unmarshalTime(text, (*time.Time)(t), "2006-01-02T15:04:05.999999999")
 }
+
+// s3TimestampFormat is the timestamp layout AWS S3 uses in XML responses:
+// UTC with exactly three fractional digits. Trimming trailing zeros (".56Z",
+// or no fraction at all) breaks clients that parse with a fixed-width pattern,
+// e.g. minio-java's "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'".
+const s3TimestampFormat = "2006-01-02T15:04:05.000Z"
+
 func (t xsdDateTime) MarshalText() ([]byte, error) {
-	return _marshalTime((time.Time)(t), "2006-01-02T15:04:05.999999999")
+	return []byte((time.Time)(t).UTC().Format(s3TimestampFormat)), nil
 }
 func (t xsdDateTime) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
 	if (time.Time)(t).IsZero() {