Write stderr messages one line at a time to the Varnish log.

src/tests/error.vtc

@@ -2,8 +2,6 @@
 
 varnishtest "vdp error handling"
 
-shell "rm -f ${tmpdir}/foo"
-
 server s1 {
 	rxreq
 	txresp -body {foo bar baz quux}
@@ -14,7 +12,7 @@ varnish v1 -vcl+backend {
 
 	sub vcl_init {
 		new cat = pipe.vdp(path="${cat}");
-		cat.arg("${tmpdir}/foo");
+		cat.arg("-notalegaloption");
 	}
 
 	sub vcl_deliver {
@@ -29,6 +27,6 @@ client c1 {
 
 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 * = Error  {^vdfp_pipe: vdp cat: ${cat} stderr: }
 	expect * = End
 } -run

src/vdfp_pipe.c

@@ -437,10 +437,28 @@ vdp_bytes(struct req *req, enum vdp_action act, void **priv, const void *ptr,
 				}
 				continue;
 			}
-			// XXX write one line at a time
-			VSLb(req->vsl, SLT_Error, "vdfp_pipe: vdp %s: %s "
-			     "stderr ...", obj->name, obj->path);
-			VSLb_bin(req->vsl, SLT_Error, nbytes, state->buf);
+
+			/* Log the stderr message one line at a time. */
+			for (char *errmsg = state->buf; nbytes > 0;) {
+				size_t linelen;
+				char *newline =	memchr(errmsg, '\n', nbytes);
+				if (newline == NULL)
+					linelen = nbytes;
+				else
+					linelen = newline - errmsg;
+				assert(linelen <= (unsigned)nbytes);
+				if (linelen == 0) {
+					nbytes--;
+					errmsg++;
+					continue;
+				}
+				VSLb(req->vsl, SLT_Error,
+				     "vdfp_pipe: vdp %s: %s stderr: %.*s",
+				     obj->name, obj->path, (int)linelen,
+				     errmsg);
+				nbytes -= (linelen + 1);
+				errmsg += (linelen + 1);
+			}
 		}
 		if (act == VDP_END) {
 			if (fds[STDIN_FILENO].fd != -1)