[Impala-ASF-CR] Clarify synchronization policy for 'done ' in KuduScanNode

["Matthew Jacobs (Code Review)" <ge...@cloudera.org>]

Matthew Jacobs has posted comments on this change.

Change subject: Clarify synchronization policy for 'done_' in KuduScanNode
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Patch Set 1:

(6 comments)

http://gerrit.cloudera.org:8080/#/c/5494/1//COMMIT_MSG
Commit Message:

PS1, Line 9: calrify
sp


http://gerrit.cloudera.org:8080/#/c/5494/1/be/src/exec/kudu-scan-node.cc
File be/src/exec/kudu-scan-node.cc:

PS1, Line 202:     materialized_row_batches_->Shutdown();
not really in scope for your change but it's confusing that we call Shutdown() from 3 different places. I think it might be possible for it to be called multiple times though I don't think it's an issue, just confusing. Maybe we can clean this up when we move this to MT.


Line 217:   unique_lock<mutex> lock(lock_);
it would be handy for this to return null if done_. I don't think it's strictly necessary but it might get confusing right now where the caller in RunScannerThread ends up incrementing next_scan_token_idx_ unnecessarily, and there is some logic elsewhere which does check next_scan_token_idx_.


PS1, Line 257:     while (!done_) {
Now that I look at this more, I think this is weird that we check_ done but still have this wrapped in a while(!eos) loop , so even after we find this is done_ we keep fetching rows until eos. Also, at that point we incr scan_ranges_complete_counter which isn't really accurate.

I think it'd be better to have l253 check !eos && !done_ so that the outer while loop stops as well.


PS1, Line 281: done_
please add a comment that even if this read conflicts with a write to done_, ProcessScanToken() will return early, GetNextScanToken() will not change the state of the token_idx/it will return null (per my comment on that fn), and then this while loop will exit.


http://gerrit.cloudera.org:8080/#/c/5494/1/be/src/exec/kudu-scan-node.h
File be/src/exec/kudu-scan-node.h:

PS1, Line 96: For performance reasons, 
remove this, it makes it sound like the performance reason _is what makes it safe_.


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Gerrit-MessageType: comment
Gerrit-Change-Id: I72c5f1fecf4bcf737d71a1fa13e59361604485f0
Gerrit-PatchSet: 1
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Sailesh Mukil <sa...@cloudera.com>
Gerrit-Reviewer: Lars Volker <lv...@cloudera.com>
Gerrit-Reviewer: Matthew Jacobs <mj...@cloudera.com>
Gerrit-HasComments: Yes