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fix: allow event loop to process during wait queue processing#2537

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mbroadst merged 4 commits into3.6from
NODE-2803/process-wait-queue-nextTick
Sep 9, 2020
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fix: allow event loop to process during wait queue processing#2537
mbroadst merged 4 commits into3.6from
NODE-2803/process-wait-queue-nextTick

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@mbroadst mbroadst commented Sep 8, 2020

Running processWaitQueue on the next tick allows the event loop to process while the connection pool is processing large numbers of wait queue members.

NODE-2803

@mbroadst
fix: allow event loop to process during wait queue processing
7b1d9bd 
Running `processWaitQueue` on the next tick allows the event loop
to process while the connection pool is processing large numbers of
wait queue members.

NODE-2803
emadum
emadum approved these changes Sep 9, 2020
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LGTM, but it looks like this made a test start failing in CI (unless that's unrelated)

@mbroadst mbroadst force-pushed the NODE-2803/process-wait-queue-nextTick branch from 0c92d66 to 71038b7 Compare September 9, 2020 14:01
@mbroadst
correct timing issues around transactions
e340b8f 
`endSession` must wait for all the machinery behind the scenes to
check out a connection and write a message before considering its
job finished.
@mbroadst mbroadst force-pushed the NODE-2803/process-wait-queue-nextTick branch from 71038b7 to e340b8f Compare September 9, 2020 14:16
@mbroadst mbroadst requested review from emadum and reggi September 9, 2020 20:59
nbbeeken
nbbeeken approved these changes Sep 9, 2020
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LGTM

@mbroadst mbroadst merged commit 4e03dfa into 3.6 Sep 9, 2020
@mbroadst mbroadst deleted the NODE-2803/process-wait-queue-nextTick branch September 9, 2020 21:19
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vkarpov15 commented Sep 17, 2020

Unfortunately this is a breaking change for anyone that uses Sinon's useFakeTimers() for testing, see Automattic/mongoose#9417. Mongoose strongly advises against using timer mocks, but people use them anyway, so I think it's worth thinking about: do we need to use setImmediate() here, or can we get away with using process.nextTick(), which sinon doesn't stub?

@vkarpov15 vkarpov15 mentioned this pull request Sep 17, 2020
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mbroadst commented Sep 18, 2020

@vkarpov15 hmm, the original implementation used process.nextTick but after discussion around the guidance in this document (specifically "We recommend developers use setImmediate() in all cases because it's easier to reason about.") we opted to use setImmediate.

There's a possibility when using process.nextTick that we will still be too greedy and won't ever allow the event loop to process critical events. I think your guidance to those users is correct, they should not be mocking timers outside of unit tests. Perhaps your page on using mock timers could be updated to use the workarounds you provided them?

@vkarpov15 vkarpov15 mentioned this pull request Sep 28, 2020
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@nbbeeken nbbeeken nbbeeken approved these changes

@reggi reggi Awaiting requested review from reggi

@emadum emadum Awaiting requested review from emadum

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@mbroadst @vkarpov15 @reggi @emadum @nbbeeken