fix(bigtable): Allow GC updates on emulated aggregate column family

[wesnel]

Closes #11498:

• bigtable: Emulator does not allow GC policy changes on existing aggregate column family #11498

[bhshkh]

This condition needs to be changed.

Right now, column family has only two fields that can be updated. So, it makes sense to assume that when a field is nil, it is not getting updated and the other one is. This is not future compatible in the scenario where more fields get added to column family.

google-cloud-go/bigtable/admin/apiv2/adminpb/table.pb.go

Lines 953 to 976 in a2a926d

	// A set of columns within a table which share a common configuration.
	type ColumnFamily struct {
	state protoimpl.MessageState
	sizeCache protoimpl.SizeCache
	unknownFields protoimpl.UnknownFields
	// Garbage collection rule specified as a protobuf.
	// Must serialize to at most 500 bytes.
	//
	// NOTE: Garbage collection executes opportunistically in the background, and
	// so it's possible for reads to return a cell even if it matches the active
	// GC expression for its family.
	GcRule *GcRule `protobuf:"bytes,1,opt,name=gc_rule,json=gcRule,proto3" json:"gc_rule,omitempty"`
	// The type of data stored in each of this family's cell values, including its
	// full encoding. If omitted, the family only serves raw untyped bytes.
	//
	// For now, only the `Aggregate` type is supported.
	//
	// `Aggregate` can only be set at family creation and is immutable afterwards.
	//
	// If `value_type` is `Aggregate`, written data must be compatible with:
	// - `value_type.input_type` for `AddInput` mutations
	ValueType *Type `protobuf:"bytes,3,opt,name=value_type,json=valueType,proto3" json:"value_type,omitempty"`
	}

Instead, use the UpdateMask field. As documented here, read the updatemask from the req. Then, check if it is nil. If it is nil, assume it is updating gcrule.

[wesnel]

Thank you, this was extremely helpful! I've just pushed some changes that will allow for partial updates of the column family based on the UpdateMask, as well as some additional tests to validate the expected behavior of the UpdateMask in this case. Please let me know if there's anything else I need to change.