Fix cache_req_body handling for H/2 requests

The h/2 request body VFP would drop data when the input buffer was too
small to fit the data in the received frame.

With this fix we have the VFP code call us again with a fresh buffer
when we run out.

Fixes: #2679

bin/varnishd/http2/cache_http2_proto.c

@@ -770,6 +770,12 @@ h2_vfp_body(struct vfp_ctx *vc, struct vfp_entry *vfe, void *ptr, ssize_t *lp)
 			h2->rxf_len -= l;
 		}
 		*lp = l;
+		if (h2->rxf_len > 0) {
+			/* We ran out of storage: Have VFP call us
+			 * again with a fresh buffer */
+			Lck_Unlock(&h2->sess->mtx);
+			return (VFP_OK);
+		}
 		if (h2->rxf_len == 0) {
 			if (h2->rxf_flags & H2FF_DATA_END_STREAM)
 				retval = VFP_END;

bin/varnishtest/tests/r02679.vtc

+varnishtest "#2679: H/2 rxbody vfp drops data"
+
+server s1 {
+	rxreq
+	expect req.http.content-length == "31469"
+	expect req.bodylen == 31469
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+	import std;
+	sub vcl_recv {
+#		std.cache_req_body(100KB);
+	}
+} -start
+
+varnish v1 -cliok "param.set feature +http2"
+
+client c1 {
+	stream 1 {
+		txreq -req POST -hdr "content-length" "31469" -nostrend
+		txdata -datalen 1550 -nostrend
+		rxwinup
+		txdata -datalen 16000 -nostrend
+		rxwinup
+		txdata -datalen 13919
+		rxwinup
+		rxresp
+		expect resp.status == 200
+	} -run
+} -run