shell: delete an empty volume only when every copy is empty

volume.deleteEmpty judged each copy on its own, so it removed an empty
copy while another copy still held data or was offline, leaving that
one as the volume's only copy. Low-disk copies now report
readOnlyCanDelete and passed the check as well. Apply the master
sweep's rule: every copy is listed and every copy is empty.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

weed/shell/command_volume_delete_empty_test.go

@@ -39,3 +39,46 @@ func TestIsEmptyVolumeDeleteCandidate(t *testing.T) {
 		})
 	}
 }
+
+func TestIsEmptyVolumeCopiesDeleteCandidate(t *testing.T) {
+	matcher, err := wildcard.CompileCollectionMatcher("")
+	assert.NoError(t, err)
+
+	const quietSeconds = int64(60)
+	const now = int64(1_000_000)
+	old := now - quietSeconds - 1
+	copyOf := func(rp uint32, v *master_pb.VolumeInformationMessage) *VolumeReplica {
+		v.ReplicaPlacement = rp
+		return &VolumeReplica{location: &location{dataNode: &master_pb.DataNodeInfo{Id: "dn"}}, info: v}
+	}
+	empty := func() *master_pb.VolumeInformationMessage {
+		return &master_pb.VolumeInformationMessage{Size: super_block.SuperBlockSize, ModifiedAtSecond: old}
+	}
+	lowDisk := func() *master_pb.VolumeInformationMessage {
+		return &master_pb.VolumeInformationMessage{Size: super_block.SuperBlockSize, ReadOnly: true, ReadOnlyCanDelete: true, ModifiedAtSecond: old}
+	}
+	withData := func() *master_pb.VolumeInformationMessage {
+		return &master_pb.VolumeInformationMessage{Size: 1 << 20, FileCount: 3, ModifiedAtSecond: old}
+	}
+	protected := func() *master_pb.VolumeInformationMessage {
+		return &master_pb.VolumeInformationMessage{Size: super_block.SuperBlockSize, ReadOnly: true, ModifiedAtSecond: old}
+	}
+
+	tests := []struct {
+		name     string
+		replicas []*VolumeReplica
+		want     bool
+	}{
+		{"single copy volume", []*VolumeReplica{copyOf(0, empty())}, true},
+		{"every copy empty", []*VolumeReplica{copyOf(1, empty()), copyOf(1, lowDisk())}, true},
+		{"other copy holds data", []*VolumeReplica{copyOf(1, lowDisk()), copyOf(1, withData())}, false},
+		{"other copy is missing", []*VolumeReplica{copyOf(1, lowDisk())}, false},
+		{"other copy is protected read-only", []*VolumeReplica{copyOf(1, empty()), copyOf(1, protected())}, false},
+		{"no copies", nil, false},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.want, isEmptyVolumeCopiesDeleteCandidate(tc.replicas, quietSeconds, now, matcher))
+		})
+	}
+}