Currently a module from concurrent-output is inlined into propellor. This should be converted to a dependency.

Waiting on concurrent-output reaching Debian stable.

Well, it's in stable now. Not in oldstable yet, and propellor is still supporting the current oldstable, I believe.. --Joey

This was attempted again in 2018 and had to be reverted in b6ac64737b59e74d4aa2d889690e8fab3772d2c6.

The strange output I was seeing is the first line of "apt-cache policy apache2" (but not subsequent lines) and the ssh-keygen command run by genSSHFP'

Propellor also misbehaved in some other ways likely due to not seeing the command output it expected. In particular Git.cloned must have failed to see an origin url in git config output, because it nuked and re-cloned a git repo (losing data).

So, it seems that readProcess was somehow leaking output to the console and also likely not providing it to the caller.

The affected system had libghc-concurrent-output-dev 1.10.5-1 installed from debian. That is a somewhat old version and perhaps it was buggy? However, I have not had any luck reproducing the problem there running readProcess in ghci. --Joey