Commits on Apr 13, 2023

1. Fix spurious failure in api_server::tests::test_serve_vmm_action_request …

   The test calls the `serve_vmm_action_request` function and tries to
   check whether it correctly updates specific metrices related to
   measuring how long a request took. However, since the VMM side is mocked
   out in this test (the receiver end of the channel through which the
   API server communicates is simply pre-filled with VmmData::Empty, and
   not passed to any implementation that handles the requests), the
   `serve_vmm_action_request` function only goes through the following
   steps:
   
   1. Put something into a channel
   2. Read a pre-submitted value from another channel
   3. Update the metrics
   
   This can happen very fast (below 1us, which is the resolution of our
   metrics), in which cases the unittests fails at the assertion that tries
   to check whether the metric of "how long did handling this request take"
   was correctly set (as this assertion only checks that the metric is not
   0).
   
   I suspect that the reason we have been encountering this more often
   recently is due to the Rust 1.67.0 update (firecracker-microvm#3576), which replaced the
   old stdlib implementation of mpsc channels with that from the crossbeam
   crate. Since channel operations are most of what the test does, this
   could have a performance impact if the new rust version has a more
   performant implementation (which the changelog implies [1]).
   
   We fix this by subtracting 1 from the start time inside the test, so
   make sure that the difference "now - start time" which
   serve_vmm_action_request uses to compute the metric value is always at
   least 1.
   
   [1]: https://blog.rust-lang.org/2023/01/26/Rust-1.67.0.html
   
   Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

   

   roypat committed Apr 13, 2023
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