binary-env intermittently fails to read from stdout/stderr pipe of spawned subprocess

[jaynis]

I experience the situation where a simple binary-env function which serves static content from a shell script sometimes fails with the following error:

Error: failed to get stderr pipe: |0: file already closed

Even worse, this sometimes results in an empty HTTP 200 response.

I am not entirely sure how to reliably reproduce this but it seems to often happen when multiple requests are sent simultaneously to a binary-env function backed by a poolmgr.