Wasm: Keep file descriptor open in fread.c until finished with mmap()

[georgestagg]

See #5969 for discussion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.49%. Comparing base (46ee525) to head (52ec7d6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5974   +/-   ##
=======================================
  Coverage   97.49%   97.49%           
=======================================
  Files          80       80           
  Lines       14861    14861           
=======================================
  Hits        14488    14488           
  Misses        373      373           

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[MichaelChirico]

Thanks for the PR!! Really appreciate it. Please write a NEWS item to cite yourself, I guess this is a bug fix? Otherwise a note.

Lastly, can you think of a way to regression test this? It's probably not worth the effort, but maybe you're familiar with an easy way to do so.

[georgestagg]

Lastly, can you think of a way to regression test this? It's probably not worth the effort, but maybe you're familiar with an easy way to do so.

Not easily, sorry. We have thought a little about what automated testing could look like inside the Wasm environment, but the infrastructure is not there yet.

[MichaelChirico]

Thanks so much for handling this!