Broken Pipe: ef_unified_filter() failed

I was getting this error message in my Apache error logs while trying to setup an Apache based output filter using ExtFilterDefine and AddOutputFilter.

The error that accompanied it was
Broken pipe: apr_file_write(child input), len 0.

Not sure what either of those meant, but guessing the ef_unified_filter() failed was just a notice that the filter was broken, I tried a bunch of things and only found maybe 50 results while searching around, none of which helped.

It turns out that the problem was in the way I was defining the cmd="" argument for ExtFilterDefine. I, and the people who were having the same problem but with different commands, weren't using the full path to our command applications.

My original ExtFilterDefine directive looked like this.

ExtFilterDefine AutoWrap \
   cmd="cat -"

However I needed to specify the full path to cat, which in my case looked like this.

ExtFilterDefine AutoWrap \
   cmd="/bin/cat -"