westonpace commented on a change in pull request #12662:
URL: https://github.com/apache/arrow/pull/12662#discussion_r830312415
##########
File path: cpp/src/arrow/util/async_generator.h
##########
@@ -60,6 +60,9 @@ namespace arrow {
// Readahead operators, and some other operators, may introduce queueing. Any
operators
// that introduce buffering should detail the amount of buffering they
introduce in their
// MakeXYZ function comments.
+//
+// A generator should always be fully consumed before it is destroyed.
+// A generator should not emit a terminal item until it has finished all
ongoing futures.
Review comment:
Well I hope the generator can see the future because it returned it ;)
I can try and fix up the warning. I agree there is probably some ambiguity
whether `emit` means "returning the future" or "completing a future" (this
statement assumes it means the latter).
Something like:
```
A generator should ensure that all ongoing futures have completed before it
completes a future with a terminal (error or end-of-stream) result. All
futures completed after that result should be the end-of-stream result.
```
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