Bugfix polling for stderr.

src/tests/error.vtc

+# looks like -*- vcl -*-
+
+varnishtest "vdp error handling"
+
+shell "rm -f ${tmpdir}/foo"
+
+server s1 {
+	rxreq
+	txresp -body {foo bar baz quux}
+} -start
+
+varnish v1 -vcl+backend {
+	import ${vmod_pipe};
+
+	sub vcl_init {
+		new cat = pipe.vdp(path="${cat}");
+		cat.arg("${tmpdir}/foo");
+	}
+
+	sub vcl_deliver {
+		set resp.filters = "cat";
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp -no_obj
+} -run
+
+logexpect l1 -v v1 -g vxid -d 1 -q Error {
+	expect 0 * Begin  {^req \d+ rxreq$}
+	expect * = Error  {^vdfp_pipe: vdp cat: ${cat} stderr}
+	expect * = End
+} -run

src/vdfp_pipe.c

@@ -198,7 +198,7 @@ vdp_init(struct req *req, void **priv)
 	state->chldpid = -1;
 	state->fds[STDIN_FILENO].events = POLLOUT;
 	state->fds[STDOUT_FILENO].events = POLLIN;
-	state->fds[STDERR_FILENO].events = POLLOUT;
+	state->fds[STDERR_FILENO].events = POLLIN;
 
 	argv = obj->argv;
 	/* Hackishly using non-API knowledge of VRT_priv_task() */