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https://issues.apache.org/jira/browse/THRIFT-3752?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18115797#comment-18115797
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Sylwester Lachiewicz commented on THRIFT-3752:
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Status on master: the case this ticket reports, an optional collection, is
fixed. The generator guards an optional collection with an {{IsSet}} helper
that tests for nil, so a nil optional set or list is omitted from the wire.
Verified with a compiler built from master. What still writes an empty
collection is a nil collection with default requiredness, which has no guard.
That is the same "write if set" question as [PR
#3827|https://github.com/apache/thrift/pull/3827] for THRIFT-6204, which
currently guards pointer fields only, so the decision there should settle this
half as well rather than a separate change.
_This comment was created with AI assistance._
> nil collections are serialized as empty collections
> ---------------------------------------------------
>
> Key: THRIFT-3752
> URL: https://issues.apache.org/jira/browse/THRIFT-3752
> Project: Thrift
> Issue Type: Bug
> Components: Go - Compiler
> Reporter: John Sirois
> Assignee: John Sirois
> Priority: Major
>
> See discussion here: https://reviews.apache.org/r/45193/
> This is likely related to THRIFT-3700.
> In short, for this struct:
> {noformat}
> struct TaskQuery {
> 4: optional set<string> taskIds
> }
> {noformat}
> The following go struct is generated:
> {noformat}
> type TaskQuery struct {
> TaskIds map[string]bool `thrift:"taskIds,4" json:"taskIds"`
> }
> {noformat}
> This is all well and good, since {{TaskQuery{}.TaskIds == nil}}; ie the
> {{TaskIds}} collection field is - by default - unset.
> The problem is in the serialization for the field, which wipes out the unset
> ({{nil}}) vs empty collection distinction at the wire level:
> {noformat}
> func (p *TaskQuery) writeField4(oprot thrift.TProtocol) (err error) {
> if err := oprot.WriteFieldBegin("taskIds", thrift.SET, 4); err != nil {
> return thrift.PrependError(fmt.Sprintf("%T write field begin
> error 4:taskIds: ", p), err)
> }
> if err := oprot.WriteSetBegin(thrift.STRING, len(p.TaskIds)); err !=
> nil {
> return thrift.PrependError("error writing set begin: ", err)
> }
> for v, _ := range p.TaskIds {
> if err := oprot.WriteString(string(v)); err != nil {
> return thrift.PrependError(fmt.Sprintf("%T. (0) field
> write error: ", p), err)
> }
> }
> if err := oprot.WriteSetEnd(); err != nil {
> return thrift.PrependError("error writing set end: ", err)
> }
> if err := oprot.WriteFieldEnd(); err != nil {
> return thrift.PrependError(fmt.Sprintf("%T write field end
> error 4:taskIds: ", p), err)
> }
> return err
> }
> {noformat}
> So on the receiving end of the wire, a {{nil}} collection is turned into an
> empty collection and so unset-ness cannot be distinguished from set but empty.
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