fix(storage): preserve tags during pool reconciliation

Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/erasure-multipart.go

@@ -1179,7 +1179,7 @@ func (er erasureObjects) CompleteMultipartUpload(ctx context.Context, bucket str
 		switch {
 		case gerr == nil:
 			reconcileStoredObjectLock(fi.Metadata, storedObjectLockState(curr.UserDefined))
-			reconcileStoredObjectTags(fi.Metadata, curr.UserDefined)
+			reconcileStoredObjectTags(fi.Metadata, curr.UserTags, curr.UserDefined[ReservedMetadataPrefixLower+TaggingTimestamp])
 		case isErrVersionNotFound(gerr) || isErrObjectNotFound(gerr):
 			// No existing version to order against: keep the upload's own accepted
 			// lock, including a pre-upgrade upload that persisted values without

cmd/erasure-object.go

@@ -135,7 +135,7 @@ func (er erasureObjects) CopyObject(ctx context.Context, srcBucket, srcObject, d
 
 	if dstOpts.ReplicaLockReconcile {
 		reconcileStoredObjectLock(srcInfo.UserDefined, storedObjectLockState(fi.Metadata))
-		reconcileStoredObjectTags(srcInfo.UserDefined, fi.Metadata)
+		reconcileStoredObjectTags(srcInfo.UserDefined, fi.Metadata[xhttp.AmzObjectTagging], fi.Metadata[ReservedMetadataPrefixLower+TaggingTimestamp])
 	}
 
 	filterOnlineDisksInplace(fi, metaArr, onlineDisks)
@@ -1311,7 +1311,7 @@ func (er erasureObjects) putObject(ctx context.Context, bucket string, object st
 		// existing version contributes independently ordered lock and tags.
 		if opts.ReplicaLockReconcile && err == nil {
 			reconcileStoredObjectLock(opts.UserDefined, storedObjectLockState(obj.UserDefined))
-			reconcileStoredObjectTags(opts.UserDefined, obj.UserDefined)
+			reconcileStoredObjectTags(opts.UserDefined, obj.UserTags, obj.UserDefined[ReservedMetadataPrefixLower+TaggingTimestamp])
 		}
 	}
 

cmd/erasure-server-pool-consistency.go

@@ -126,10 +126,9 @@ func mergedPoolObjectInfo(copies []PoolObjInfo) ObjectInfo {
 		stamp, _ := time.Parse(time.RFC3339Nano, ts)
 		if olderThan(oi.UserDefined[ReservedMetadataPrefixLower+TaggingTimestamp], stamp) {
 			oi.UserDefined[ReservedMetadataPrefixLower+TaggingTimestamp] = ts
-			oi.UserDefined[xhttp.AmzObjectTagging] = copy.ObjInfo.UserDefined[xhttp.AmzObjectTagging]
+			oi.UserTags = copy.ObjInfo.UserTags
 		}
 	}
-	oi.UserTags = oi.UserDefined[xhttp.AmzObjectTagging]
 	return oi
 }
 
@@ -190,6 +189,12 @@ func (z *erasureServerPools) updatePoolMetadata(ctx context.Context, bucket, obj
 			changes[key] = ""
 		}
 	}
+	// Metadata callbacks may explicitly replace tags in the raw write map.
+	// Otherwise retain the merged value from the ObjectInfo read model.
+	tags, ok := updated.UserDefined[xhttp.AmzObjectTagging]
+	if !ok {
+		tags = updated.UserTags
+	}
 	state := storedObjectLockState(updated.UserDefined)
 	opts.VersionID = updated.VersionID
 	if opts.VersionID == "" {
@@ -199,11 +204,13 @@ func (z *erasureServerPools) updatePoolMetadata(ctx context.Context, bucket, obj
 	opts.EvalMetadataFn = func(oi *ObjectInfo, _ error) (ReplicateDecision, error) {
 		maps.Copy(oi.UserDefined, changes)
 		replaceObjectLockMetadata(oi.UserDefined, state)
-		for _, key := range []string{xhttp.AmzObjectTagging, ReservedMetadataPrefixLower + TaggingTimestamp} {
-			value, exists := updated.UserDefined[key]
-			if exists || oi.UserDefined[key] != "" {
-				oi.UserDefined[key] = value
-			}
+		// Reassemble the tag value and its ordering timestamp for storage.
+		if tags != "" || oi.UserTags != "" {
+			oi.UserDefined[xhttp.AmzObjectTagging] = tags
+		}
+		key := ReservedMetadataPrefixLower + TaggingTimestamp
+		if value, exists := updated.UserDefined[key]; exists || oi.UserDefined[key] != "" {
+			oi.UserDefined[key] = value
 		}
 		return ReplicateDecision{}, nil
 	}
@@ -220,16 +227,18 @@ func (z *erasureServerPools) updatePoolMetadata(ctx context.Context, bucket, obj
 	return primary, nil
 }
 
-func reconcileStoredObjectTags(metadata, stored map[string]string) {
+// Pass the stored tag value explicitly: ObjectInfo.UserDefined excludes it,
+// whereas FileInfo.Metadata retains the raw storage key.
+func reconcileStoredObjectTags(metadata map[string]string, storedTags, storedTimestamp string) {
 	key := ReservedMetadataPrefixLower + TaggingTimestamp
-	stamp, err := time.Parse(time.RFC3339Nano, stored[key])
+	stamp, err := time.Parse(time.RFC3339Nano, storedTimestamp)
 	if err != nil {
 		return
 	}
 	incoming, err := time.Parse(time.RFC3339Nano, metadata[key])
 	if err != nil || !stamp.Before(incoming) {
-		metadata[key] = stored[key]
-		metadata[xhttp.AmzObjectTagging] = stored[xhttp.AmzObjectTagging]
+		metadata[key] = storedTimestamp
+		metadata[xhttp.AmzObjectTagging] = storedTags
 	}
 }
 

cmd/erasure-server-pool-tags_test.go

@@ -0,0 +1,459 @@
+// Copyright (c) 2026 Feng Ruohang
+//
+// This file is part of Silo Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package cmd
+
+import (
+	"context"
+	"fmt"
+	"maps"
+	"strings"
+	"testing"
+
+	xhttp "github.com/minio/minio/internal/http"
+)
+
+// The fixture places copies directly in real erasure pools. It models a
+// duplicated version; it does not claim to exercise a rebalance workflow.
+func TestPoolsMetadataUpdatePreservesTags(t *testing.T) {
+	z, bucket := consistencyPools(t)
+	const (
+		old       = "2026-09-09T09:00:00Z"
+		recent    = "2026-09-09T10:00:00Z"
+		timestamp = ReservedMetadataPrefixLower + TaggingTimestamp
+	)
+	for _, test := range []struct {
+		name         string
+		tags, stamps [2]string
+		winner       int
+		single       bool
+	}{
+		{name: "newer-secondary", tags: [2]string{"key=old", "key=new"}, stamps: [2]string{old, recent}, winner: 1},
+		{name: "newer-primary", tags: [2]string{"key=new", "key=old"}, stamps: [2]string{recent, old}},
+		{name: "empty-secondary", tags: [2]string{"key=old", ""}, stamps: [2]string{old, recent}, winner: 1},
+		{name: "empty-primary", tags: [2]string{"", "key=old"}, stamps: [2]string{recent, old}},
+		{name: "single-copy-primary", tags: [2]string{"key=only", ""}, stamps: [2]string{recent, ""}, single: true},
+		{name: "single-copy-secondary", tags: [2]string{"", "key=only"}, stamps: [2]string{"", recent}, winner: 1, single: true},
+		{name: "legacy-single-copy", tags: [2]string{"key=legacy", ""}, single: true},
+		{name: "legacy-duplicates", tags: [2]string{"key=legacy", "key=legacy"}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			object := test.name
+			var original ObjectInfo
+			for pool := range 2 {
+				if test.single && pool != test.winner {
+					continue
+				}
+				metadata := map[string]string{
+					xhttp.AmzObjectTagging: test.tags[pool],
+					"copy-local":           fmt.Sprint(pool),
+				}
+				if test.stamps[pool] != "" {
+					metadata[timestamp] = test.stamps[pool]
+				}
+				original = putConsistencyObject(t, z, bucket, object, pool, "data", ObjectOptions{
+					Versioned: true, VersionID: original.VersionID, MTime: original.ModTime, UserDefined: metadata,
+				})
+				got, err := z.serverPools[pool].GetObjectInfo(t.Context(), bucket, object, ObjectOptions{VersionID: original.VersionID})
+				if err != nil || got.UserTags != test.tags[pool] || got.UserDefined[timestamp] != test.stamps[pool] {
+					t.Fatalf("pool %d fixture: tags=%q timestamp=%q err=%v", pool, got.UserTags, got.UserDefined[timestamp], err)
+				}
+				if _, exists := got.UserDefined[xhttp.AmzObjectTagging]; exists {
+					t.Fatal("fixture must use the cleaned ObjectInfo representation")
+				}
+			}
+			wantTags, wantStamp := test.tags[test.winner], test.stamps[test.winner]
+			called := 0
+			got, err := z.PutObjectMetadata(t.Context(), bucket, object, ObjectOptions{
+				VersionID: original.VersionID, MTime: original.ModTime,
+				EvalMetadataFn: func(current *ObjectInfo, _ error) (ReplicateDecision, error) {
+					called++
+					if current.UserTags != wantTags || current.UserDefined[timestamp] != wantStamp {
+						t.Errorf("callback tags=%q timestamp=%q; want %q %q", current.UserTags, current.UserDefined[timestamp], wantTags, wantStamp)
+					}
+					current.UserDefined["unrelated-update"] = "preserved"
+					return ReplicateDecision{}, nil
+				},
+			})
+			if err != nil || called != 1 {
+				t.Fatalf("metadata update: %v, callbacks=%d", err, called)
+			}
+			if got.UserTags != wantTags || got.UserDefined[timestamp] != wantStamp {
+				t.Errorf("response tags=%q timestamp=%q; want %q %q", got.UserTags, got.UserDefined[timestamp], wantTags, wantStamp)
+			}
+			for pool := range 2 {
+				got, err := z.serverPools[pool].GetObjectInfo(t.Context(), bucket, object, ObjectOptions{VersionID: original.VersionID})
+				if test.single && pool != test.winner {
+					if !isErrVersionNotFound(err) {
+						t.Errorf("metadata update created another copy: %v", err)
+					}
+					continue
+				}
+				if err != nil {
+					t.Fatal(err)
+				}
+				t.Logf("pool %d persisted tags=%q timestamp=%q", pool, got.UserTags, got.UserDefined[timestamp])
+				if got.UserTags != wantTags || got.UserDefined[timestamp] != wantStamp {
+					t.Errorf("pool %d persisted tags=%q timestamp=%q; want %q %q", pool, got.UserTags, got.UserDefined[timestamp], wantTags, wantStamp)
+				}
+				if got.UserDefined["unrelated-update"] != "preserved" || got.UserDefined["copy-local"] != fmt.Sprint(pool) {
+					t.Errorf("pool %d lost unrelated metadata: %v", pool, got.UserDefined)
+				}
+			}
+		})
+	}
+}
+
+func TestReplicaWritesPreserveTagOrdering(t *testing.T) {
+	z, bucket := consistencyPools(t)
+	// Pool allocation checks the host's used-space percentage. Present only
+	// its free space as fixture capacity; all reads and writes still use the
+	// real disks. This keeps unrelated host disk usage out of the tag test.
+	for _, pool := range z.serverPools {
+		for _, set := range pool.sets {
+			getDisks := set.getDisks
+			disks := append([]StorageAPI(nil), getDisks()...)
+			for i := range disks {
+				disks[i] = tagTestCapacityDisk{StorageAPI: disks[i]}
+			}
+			set.getDisks = func() []StorageAPI { return disks }
+			t.Cleanup(func() { set.getDisks = getDisks })
+		}
+	}
+	const (
+		old       = "2026-09-09T09:00:00Z"
+		recent    = "2026-09-09T10:00:00Z"
+		timestamp = ReservedMetadataPrefixLower + TaggingTimestamp
+	)
+	for _, path := range []struct {
+		name, operation string
+		direct          bool
+		winner          int
+	}{
+		{name: "set-put", operation: "put", direct: true},
+		{name: "set-multipart", operation: "multipart", direct: true},
+		{name: "set-copy", operation: "copy", direct: true},
+		{name: "pools-put", operation: "put", winner: 1},
+		{name: "pools-multipart", operation: "multipart", winner: 1},
+		{name: "pools-copy-primary", operation: "copy"},
+		{name: "pools-copy-secondary", operation: "copy", winner: 1},
+	} {
+		for _, test := range []struct {
+			name, storedTags, incomingTags, storedStamp, incomingStamp, wantTags string
+		}{
+			{"stored-newer", "key=stored", "key=incoming", recent, old, "key=stored"},
+			{"stored-deleted", "", "key=incoming", recent, old, ""},
+			{"incoming-newer", "key=stored", "key=incoming", old, recent, "key=incoming"},
+			{"incoming-deleted", "key=stored", "", old, recent, ""},
+		} {
+			t.Run(path.name+"/"+test.name, func(t *testing.T) {
+				object := path.name + "-" + test.name
+				var original ObjectInfo
+				for pool := range 2 {
+					if path.direct && pool != 0 {
+						continue
+					}
+					metadata := map[string]string{
+						xhttp.AmzObjectTagging: "key=older-copy",
+						timestamp:              "2026-09-09T08:00:00Z",
+					}
+					if pool == path.winner {
+						metadata[xhttp.AmzObjectTagging] = test.storedTags
+						metadata[timestamp] = test.storedStamp
+					}
+					original = putConsistencyObject(t, z, bucket, object, pool, "data", ObjectOptions{
+						Versioned: true, VersionID: original.VersionID, MTime: original.ModTime, UserDefined: metadata,
+					})
+				}
+				opts := ObjectOptions{
+					Versioned: true, VersionID: original.VersionID, MTime: original.ModTime, ReplicaLockReconcile: true,
+					UserDefined: map[string]string{
+						xhttp.AmzObjectTagging: test.incomingTags,
+						timestamp:              test.incomingStamp,
+					},
+				}
+				var got ObjectInfo
+				var err error
+				switch path.operation {
+				case "put":
+					put := z.PutObject
+					if path.direct {
+						put = z.serverPools[0].PutObject
+					}
+					got, err = put(t.Context(), bucket, object, mustGetPutObjReader(t, strings.NewReader("data"), 4, "", ""), opts)
+				case "multipart":
+					// Persist the incoming tags with the upload, before completion
+					// reconciles the destination version through its real resolver.
+					mp, err := z.serverPools[0].NewMultipartUpload(t.Context(), bucket, object, opts)
+					if err != nil {
+						t.Fatal(err)
+					}
+					part, err := z.serverPools[0].PutObjectPart(t.Context(), bucket, object, mp.UploadID, 1,
+						mustGetPutObjReader(t, strings.NewReader("data"), 4, "", ""), ObjectOptions{})
+					if err != nil {
+						t.Fatal(err)
+					}
+					complete := z.CompleteMultipartUpload
+					if path.direct {
+						complete = z.serverPools[0].CompleteMultipartUpload
+					}
+					got, err = complete(t.Context(), bucket, object, mp.UploadID, []CompletePart{{PartNumber: 1, ETag: part.ETag}}, ObjectOptions{
+						Versioned: true, MTime: original.ModTime, ReplicaLockReconcile: true,
+					})
+					if err != nil {
+						t.Fatal(err)
+					}
+				case "copy":
+					src := original
+					src.metadataOnly = true
+					src.UserDefined = maps.Clone(opts.UserDefined)
+					copyObject := z.CopyObject
+					if path.direct {
+						copyObject = z.serverPools[0].CopyObject
+					}
+					got, err = copyObject(t.Context(), bucket, object, bucket, object, src, ObjectOptions{VersionID: original.VersionID}, opts)
+				}
+				if err != nil {
+					t.Fatal(err)
+				}
+				if got.UserTags != test.wantTags || got.UserDefined[timestamp] != recent {
+					t.Errorf("response tags=%q timestamp=%q; want %q %q", got.UserTags, got.UserDefined[timestamp], test.wantTags, recent)
+				}
+				copies := 0
+				for pool := range 2 {
+					got, err := z.serverPools[pool].GetObjectInfo(t.Context(), bucket, object, ObjectOptions{VersionID: original.VersionID})
+					if isErrVersionNotFound(err) {
+						continue
+					}
+					if err != nil {
+						t.Fatal(err)
+					}
+					copies++
+					if got.UserTags != test.wantTags || got.UserDefined[timestamp] != recent {
+						t.Errorf("pool %d persisted tags=%q timestamp=%q; want %q %q", pool, got.UserTags, got.UserDefined[timestamp], test.wantTags, recent)
+					}
+				}
+				if copies != 1 {
+					t.Errorf("replacement left %d copies; want 1", copies)
+				}
+			})
+		}
+	}
+}
+
+type tagTestCapacityDisk struct{ StorageAPI }
+
+func (d tagTestCapacityDisk) DiskInfo(ctx context.Context, opts DiskInfoOptions) (DiskInfo, error) {
+	info, err := d.StorageAPI.DiskInfo(ctx, opts)
+	info.Total, info.Used = info.Free, 0
+	return info, err
+}
+
+func TestMergedPoolObjectInfoTagOrdering(t *testing.T) {
+	const (
+		old       = "2026-09-09T09:00:00Z"
+		recent    = "2026-09-09T10:00:00Z"
+		timestamp = ReservedMetadataPrefixLower + TaggingTimestamp
+	)
+	for _, test := range []struct {
+		name, firstStamp, secondStamp, secondTags string
+		winner                                    int
+	}{
+		{"newer", old, recent, "key=second", 1},
+		{"newer-removal", old, recent, "", 1},
+		{"equal", recent, recent, "key=second", 0},
+		{"unordered", "", "", "key=second", 0},
+		{"missing-first", "", recent, "key=second", 1},
+		{"missing-second", recent, "", "key=second", 0},
+		{"invalid-first", "invalid", recent, "key=second", 1},
+		{"invalid-second", recent, "invalid", "key=second", 0},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			tagValues := []string{"key=first", test.secondTags}
+			stamps := []string{test.firstStamp, test.secondStamp}
+			copies := make([]PoolObjInfo, 2)
+			before := make([]map[string]string, 2)
+			for i := range copies {
+				fi := FileInfo{Metadata: map[string]string{xhttp.AmzObjectTagging: tagValues[i]}}
+				if stamps[i] != "" {
+					fi.Metadata[timestamp] = stamps[i]
+				}
+				copies[i] = PoolObjInfo{Index: i, ObjInfo: fi.ToObjectInfo("bucket", "object", true)}
+				before[i] = maps.Clone(copies[i].ObjInfo.UserDefined)
+			}
+			got := mergedPoolObjectInfo(copies)
+			if got.UserTags != tagValues[test.winner] || got.UserDefined[timestamp] != stamps[test.winner] {
+				t.Errorf("merged tags=%q timestamp=%q; want %q %q", got.UserTags, got.UserDefined[timestamp], tagValues[test.winner], stamps[test.winner])
+			}
+			if _, exists := got.UserDefined[xhttp.AmzObjectTagging]; exists {
+				t.Error("merged ObjectInfo leaked the raw tagging key into UserDefined")
+			}
+			for i := range copies {
+				if !maps.Equal(copies[i].ObjInfo.UserDefined, before[i]) || copies[i].ObjInfo.UserTags != tagValues[i] {
+					t.Errorf("merge mutated input copy %d", i)
+				}
+			}
+		})
+	}
+}
+
+func TestPoolsMetadataCallbackReplacesTags(t *testing.T) {
+	z, bucket := consistencyPools(t)
+	const timestamp = ReservedMetadataPrefixLower + TaggingTimestamp
+	for _, test := range []struct{ name, tags string }{
+		{"replace", "key=callback"},
+		{"remove", ""},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			var original ObjectInfo
+			for pool := range 2 {
+				original = putConsistencyObject(t, z, bucket, test.name, pool, "data", ObjectOptions{
+					Versioned: true, VersionID: original.VersionID, MTime: original.ModTime,
+					UserDefined: map[string]string{
+						xhttp.AmzObjectTagging: []string{"key=old", "key=new"}[pool],
+						timestamp:              []string{"2026-09-09T09:00:00Z", "2026-09-09T10:00:00Z"}[pool],
+					},
+				})
+			}
+			const updatedStamp = "2026-09-09T11:00:00Z"
+			got, err := z.PutObjectMetadata(t.Context(), bucket, test.name, ObjectOptions{
+				VersionID: original.VersionID, MTime: original.ModTime,
+				EvalMetadataFn: func(current *ObjectInfo, _ error) (ReplicateDecision, error) {
+					if current.UserTags != "key=new" {
+						t.Errorf("callback read tags=%q; want key=new", current.UserTags)
+					}
+					if _, exists := current.UserDefined[xhttp.AmzObjectTagging]; exists {
+						t.Error("callback received the raw tagging key")
+					}
+					current.UserDefined[xhttp.AmzObjectTagging] = test.tags
+					current.UserDefined[timestamp] = updatedStamp
+					return ReplicateDecision{}, nil
+				},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got.UserTags != test.tags || got.UserDefined[timestamp] != updatedStamp {
+				t.Errorf("callback update response tags=%q timestamp=%q", got.UserTags, got.UserDefined[timestamp])
+			}
+			for pool := range 2 {
+				got, err := z.serverPools[pool].GetObjectInfo(t.Context(), bucket, test.name, ObjectOptions{VersionID: original.VersionID})
+				if err != nil || got.UserTags != test.tags || got.UserDefined[timestamp] != updatedStamp {
+					t.Errorf("pool %d did not persist callback tags: tags=%q timestamp=%q err=%v", pool, got.UserTags, got.UserDefined[timestamp], err)
+				}
+			}
+		})
+	}
+}
+
+func TestReconcileStoredObjectTagOrdering(t *testing.T) {
+	const (
+		old       = "2026-09-09T09:00:00Z"
+		recent    = "2026-09-09T10:00:00Z"
+		timestamp = ReservedMetadataPrefixLower + TaggingTimestamp
+	)
+	for _, test := range []struct {
+		name, storedStamp, incomingStamp, storedTags string
+		wantStored                                   bool
+	}{
+		{"stored-newer", recent, old, "key=stored", true},
+		{"incoming-newer", old, recent, "key=stored", false},
+		{"equal", recent, recent, "key=stored", true},
+		{"equal-removal", recent, recent, "", true},
+		{"missing-stored", "", recent, "key=stored", false},
+		{"missing-incoming", recent, "", "key=stored", true},
+		{"invalid-stored", "invalid", recent, "key=stored", false},
+		{"invalid-incoming", recent, "invalid", "key=stored", true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			metadata := map[string]string{
+				xhttp.AmzObjectTagging: "key=incoming",
+				timestamp:              test.incomingStamp,
+				"unrelated":            "preserved",
+			}
+			reconcileStoredObjectTags(metadata, test.storedTags, test.storedStamp)
+			wantTags, wantStamp := "key=incoming", test.incomingStamp
+			if test.wantStored {
+				wantTags, wantStamp = test.storedTags, test.storedStamp
+			}
+			if metadata[xhttp.AmzObjectTagging] != wantTags || metadata[timestamp] != wantStamp || metadata["unrelated"] != "preserved" {
+				t.Errorf("reconciled metadata=%v; want tags=%q timestamp=%q and unrelated field preserved", metadata, wantTags, wantStamp)
+			}
+		})
+	}
+}
+
+func TestPoolsMetadataUpdatePreservesAbsentTags(t *testing.T) {
+	z, bucket := consistencyPools(t)
+	const (
+		old       = "2026-09-09T09:00:00Z"
+		recent    = "2026-09-09T10:00:00Z"
+		timestamp = ReservedMetadataPrefixLower + TaggingTimestamp
+	)
+	for _, removal := range []bool{false, true} {
+		t.Run(fmt.Sprintf("timestamp-only-removal=%t", removal), func(t *testing.T) {
+			object := fmt.Sprintf("absent-tags-%t", removal)
+			var original ObjectInfo
+			checkStored := func(pool int, wantKey bool, wantTags, wantStamp string) {
+				t.Helper()
+				infos, errs := readAllFileInfo(t.Context(), z.serverPools[pool].getHashedSet(object).getDisks(), "", bucket, object, original.VersionID, false, false)
+				for disk, info := range infos {
+					if errs[disk] != nil {
+						t.Fatal(errs[disk])
+					}
+					tags, exists := info.Metadata[xhttp.AmzObjectTagging]
+					if exists != wantKey || tags != wantTags || info.Metadata[timestamp] != wantStamp {
+						t.Errorf("pool %d disk %d raw tagging key=%t value=%q stamp=%q; want %t %q %q", pool, disk, exists, tags, info.Metadata[timestamp], wantKey, wantTags, wantStamp)
+					}
+				}
+			}
+			for pool := range 2 {
+				metadata := map[string]string{}
+				if removal {
+					metadata[timestamp] = recent
+					if pool == 1 {
+						metadata[xhttp.AmzObjectTagging] = "key=old"
+						metadata[timestamp] = old
+					}
+				}
+				original = putConsistencyObject(t, z, bucket, object, pool, "data", ObjectOptions{
+					Versioned: true, VersionID: original.VersionID, MTime: original.ModTime, UserDefined: metadata,
+				})
+				checkStored(pool, removal && pool == 1, metadata[xhttp.AmzObjectTagging], metadata[timestamp])
+			}
+			_, err := z.PutObjectMetadata(t.Context(), bucket, object, ObjectOptions{
+				VersionID: original.VersionID, MTime: original.ModTime,
+				EvalMetadataFn: func(current *ObjectInfo, _ error) (ReplicateDecision, error) {
+					current.UserDefined["unrelated-update"] = "preserved"
+					return ReplicateDecision{}, nil
+				},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			wantStamp := ""
+			if removal {
+				wantStamp = recent
+			}
+			for pool := range 2 {
+				// A previously non-empty key needs an explicit empty value to
+				// propagate deletion; an absent key should remain absent.
+				checkStored(pool, removal && pool == 1, "", wantStamp)
+			}
+		})
+	}
+}

cmd/erasure-server-pool.go

@@ -1447,7 +1447,7 @@ func (z *erasureServerPools) CopyObject(ctx context.Context, srcBucket, srcObjec
 		}
 		stored := mergedPoolObjectInfo(copies)
 		reconcileStoredObjectLock(srcInfo.UserDefined, storedObjectLockState(stored.UserDefined))
-		reconcileStoredObjectTags(srcInfo.UserDefined, stored.UserDefined)
+		reconcileStoredObjectTags(srcInfo.UserDefined, stored.UserTags, stored.UserDefined[ReservedMetadataPrefixLower+TaggingTimestamp])
 		idx := copies[0].Index
 		oi, err := z.serverPools[idx].CopyObject(ctx, srcBucket, srcObject, dstBucket, dstObject, srcInfo, srcOpts, dstOpts)
 		if err == nil {