+# Used for streaming the stdout of one process as a PSGI response.
+#
+# $env is the PSGI env.
+# optional keys in $env:
+# $env->{'qspawn.wcb'} - the write callback from the PSGI server
+# optional, use this if you've already
+# captured it elsewhere. If not given,
+# psgi_return will return an anonymous
+# sub for the PSGI server to call
+#
+# $env->{'qspawn.filter'} - filter callback, receives a string as input,
+# undef on EOF
+#
+# $limiter - the Limiter object to use (uses the def_limiter if not given)
+#
+# $parse_hdr - Initial read function; often for parsing CGI header output.
+# It will be given the return value of sysread from the pipe
+# and a string ref of the current buffer. Returns an arrayref
+# for PSGI responses. 2-element arrays in PSGI mean the
+# body will be streamed, later, via writes (push-based) to
+# psgix.io. 3-element arrays means the body is available
+# immediately (or streamed via ->getline (pull-based)).