add failing test for edge case max_batch_size=max_entries #11378
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I think I found a reasonable bugfix (already pushed, tests are fine again): potential fix for race conditionThis commit might fix #11377 by removing currently processed elements Two tests needed changes:
Why do I think this works?
These assumptions strongly need verification by some lua experts! |
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hanshuebner
suggested changes
Aug 17, 2023
kong/tools/queue.lua
Outdated
| @@ -251,11 +251,21 @@ function Queue:process_once() | |||
| end | |||
| end | |||
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| local batch = {unpack(self.entries, self.front, self.front + entry_count - 1)} | |||
| -- Guard against queue shrinkage during handler invocation by using math.min below. | |||
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This is no longer needed. The comment kind of pointed at the bug that you've discovered without me actually realizing that it was a bug. With your change, we'll actually always be able to remove entry_count entries.
When defining a very special edge case configuration having max_batch_size=max_entries, the queue can fail with an assertion error when removing the frontmost element. This happens especially when the callback repeatedly fails (eg. an unavailable backend system receiving data). What happens: 1. we add max_batch_size elements, all of which "post" resources 2. the batch queue consumes all of those resources in `process_once` by `wait()`ing for them, but gets stuck processing/sending the batch 3. as `process_once` is stuck until `max_retry_time` passed, the function does not run `delete_frontmost_entry()` and thus actually moves the `front` reference 4. when enqueuing the next item, it tries to drop the oldest entry, but triggers the assertion in queue.lua as no resources are left
This commit might fix Kong#11377 by removing currently processed elements out of the race condition window. Two tests needed changes: 1. "giving up sending after retrying" needed another (otherwise) ignored value, such that we can wait long enough in `wait_until_queue_done` (there might be a more elegant solution here) 2. the new test required reactivating the handler to succeed to finally clear the queue Why do I think this works? - immediately after the last call on `semaphore:wait()`, we'll start actually removing items from `entries` - the code cannot be interrupted by other light threads before we actually start the handler These assumptions strongly need verification by some lua experts!
JensErat
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Removed the min statement and added the changelog entry. I bundled both queue changes in subsequent lines, I guess the changelog order is not relevant otherwise. |
hanshuebner
approved these changes
Aug 21, 2023
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I've moved these changes to #11431 so that I can deal with the CI issues that we currently have. |
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When defining a very special edge case configuration having
max_batch_size=max_entries, the queue can fail with an assertion error when removing the frontmost element. This happens especially when the callback repeatedly fails (eg. an unavailable backend system receiving data).What happens:
process_oncebywait()ing for them, but gets stuck processing/sending the batchprocess_onceis stuck untilmax_retry_timepassed, the function does not rundelete_frontmost_entry()and thus actually moves thefrontreferenceKudos to @27ascii for discovering the edge case configuration.
Summary
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Full changelog
Issue reference
FixTest case for #11377Jens Erat <jens.erat@mercedes-benz.com>, Mercedes-Benz Tech Innovation GmbH, imprint