Simplify the collection of task values and errors #5
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danderson
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Roughly compare the performance of accumulating values with a separate goroutine via a channel, vs. accumulating them directly under a lock.
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Instead of maintaining a separate goroutine to synchronize delivery of values, rework the collector to use a plain sync.Mutex. This: - Greatly simplifies the code (with one exception, noted below). - Eliminates the need for a separate goroutine to service values. Each task now handles its own service, mediated by the collector. That, in turn: - Eliminates the need to Wait for the Collector: Once all the goroutines running tasks in the collector have exited, the state is fully settled. The Wait method is now a no-op, and is marked as deprecated. In addition, add a new Report method, replacing Stream. Instead of a channel, tasks using this method accepts a report function that sends values to the collector. The report function ensures control does not return to the task until the reported value has been serviced, which allows tasks to ensure they do not exit until all their values have been addressed. The Stream method still works, but is deprecated. To preserve its interface, each Stream call now spins up a new goroutine to service the values from its task. This is wasteful, but easily replaced by switching to Report. Co-Authored-By: David Anderson <dave@natulte.net>
Remove the separate goroutine collecting errors, and deliver them directly to the error filter and the output field. Moreover simplify the setup and teardown so that there is not so much coordinated state. Although performance was not a primary consideration, benchmarking suggests this is actually faster than the previous implementation, and uses less memory. Also expand and clarify the documentation of the Wait method. Co-Authored-By: David Anderson <dave@natulte.net>
danderson
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| // progress. Once all Wait calls have returned, the group is ready for reuse. | ||
| func (g *Group) Wait() error { g.cleanup(); return g.err } | ||
| // As with sync.WaitGroup, new tasks can be added to g during a call to Wait | ||
| // only if there was already at least one task active when Wait was called. |
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Does this need to be stronger? Any new Go must occur while at least one task is continuously active until after Go has returned.
As stated in the comment, I believe it permits the sequence:
- G1 calls g.Go, spawns task G2
- G1 calls g.Wait (G2 is alive, contract is satisfied)
- G3 calls g.Go, gets through the activation check, is about to call wg.Add
- G2 completes and calls wg.Done concurrently with the prior Add
- wg.Add observes a zero bounce and panics
The required ordering is quite annoying to describe cleanly :/
"new tasks can be added to g during a call to Wait only if the group continuously contains at least one active task at all times, starting before the call to Wait and until after the final concurrent call to Go returns" ? :/
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You're right, I was focusing on the Wait'er too much.
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How about:
// As with sync.WaitGroup, new tasks can be added to g during a call to Wait
// only if the group contains at least one active task when Wait is called and
// continuously thereafter until the last concurrent call to g.Go returns.
?
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Simplify the implementations of Group and Collector.
Remove the separate goroutine collecting errors, and deliver them directly to
the error filter and the output field. Moreover simplify the setup and teardown
so that there is not so much coordinated state. Although performance was not a
primary consideration, benchmarking suggests this is actually faster than the
previous implementation, and uses less memory.
Also expand and clarify the documentation of the Wait method.
Instead of maintaining a separate goroutine to synchronize delivery of values,
rework the collector to use a plain sync.Mutex.
In addition, add a new Report method, replacing Stream. Instead of a channel,
tasks using this method accepts a report function that sends values to the
collector. The report function ensures control does not return to the task
until the reported value has been serviced, which allows tasks to ensure they
do not exit until all their values have been addressed.
The Stream method still works, but is deprecated. To preserve its interface,
each Stream call now spins up a new goroutine to service the values from its
task. This is wasteful, but easily replaced by switching to Report.
Co-Authored-By: David Anderson