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Vite 3 Flaky Dev Server

Reproduction showcasing that the dev server of vite@3.0.0-alpha.9 (purposley using an old alpha version) is flaky.

The bug also exists in vite@3.0.0-alpha.13. (Why the reproduction uses vite@3.0.0-alpha.9 is explained later.)

Reproduction

Observe the CI:

Observe how the only difference between the two branches is the Vite version used.

The "interesting" part here is that all vite@3.0.0-alpha.9 tests are sometimes failing and sometimes not. In other words: if one were to run the CI over and over again, it would be green once in a while.

The flakiness only happens with tests that use Vite's dev server. All $ vite build tests are always green. So something must have changed in Vite's dev server that makes these Vite dev server tests randomly fail. The tests fail for a range of reasons:

  • Jest test timeout
  • 504 (Gateway Timeout)
  • Vue hydration mismatches
  • Page navigation not working

My guess is that the root cause is Vite's dev server to randomly stop working, which would explain all these errors.

Note that the commits re-run CI are empty commits to trigger the CI. This is to showcase that the vite@2.9.10 branch is always green whereas vite@3.0.0-alpha.9 has randomly failing tests.

This repository is a fork of the vite-plugin-ssr repository. (The reproduction is purposley not minimal to better showcase the flakiness.)

Double Source

One tricky thing about this issue is that it has two sources: the problem is caused by both a Vite change as well as a vite-plugin-ssr change.

I know this because there is an old vite-plugin-ssr version that works with vite@3.0.0-alpha.9 (as showcased by the branch vite@3.0.0-alpha.9_old-vite-pugin-ssr-version which is always green).

I plan to fix the situation by searching, and once found, reverting the vite-plugin-ssr problematic change. But fixing this on vite-plugin-ssr's side doesn't really help us: the Vite problematic change would still be around, looming around the corner to haunt us back (at some point in the future).

That said, even if we want to take a faith of leap (I think we should) by only fixing the problem on vite-plugin-ssr's side, someone familiar with the Vite 3 alphas should be aware about this issue and point us to Vite 3 changes that have potential to be the cause.

Anyways, it's good to have a reproduction for this. Especially since flakiness is hard to reproduce.

Note that the flakiness also happens with vite@3.0.0-alpha.13 but this reproduction showcases the flakiness with vite@3.0.0-alpha.9 in order to reduce the Vite search space: for example, since the 127.0.0.1 => localhost change was released later, we can safely assume that it's not the culprit.

On Vite's side, the problem is somewhere between v2.9.10 and v3.0.0-alpha.9.

On vite-plugin-ssr's side, the problem is somewhere between a3a71f and 408159.

Hit me up on discord if you have questions.

This reproduction lives in its own GitHub organization to keep the repository name vite-plugin-ssr which is needed for vite-plugin-ssr's CI.