Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.17%. Comparing base (29c7824) to head (8726804).

❗ Current head 8726804 differs from pull request most recent head 4a50a84

Please upload reports for the commit 4a50a84 to get more accurate results.

@@            Coverage Diff             @@
##             main     #481      +/-   ##
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- Coverage   69.18%   69.17%   -0.01%     
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  Files         195      195              
  Lines       26144    26109      -35     
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- Hits        18087    18061      -26     
+ Misses       8057     8048       -9     

I haven't written that code, but the primary issue is that, if a write couldn't be sent through (if buffer full), we'll have to retry the write. The rust async writes machinery will retry the write with a given buf then. But it doesn't have any idea of the adjacent file descriptions sent. So we'll have to store these separately and try sending them again when possible.
As Linux doesn't allow sending just file descriptors, it requires a non-empty accompanying message, we have to mix in the fd sending with the writes.

Also, we rely on in-order message delivery. So, we'll just buffer file descriptions until we know they were sent. (and may send file descriptions before the message they're actually attached to) The receiver side will then just pop handles as soon as a message expecting a file description is received.