Partial responses to range requests (beresp.status == 206) are ignored.

As do the g(un)zip VFPs for Varnish.

src/tests/range.vtc

+# looks like -*- vcl -*-
+
+varnishtest "the filters ignore backend responses with status 206"
+
+server s1 {
+	loop 2 {
+		rxreq
+		expect req.http.Range == "bytes=0-15"
+		txresp -status 206 -body "foo bar baz quux"
+	}
+} -start
+
+varnish v1 -arg "-p vsl_mask=+VfpAcct" -vcl+backend {
+	import ${vmod_brotli};
+
+	sub vcl_backend_fetch {
+		set bereq.http.Range = "bytes=0-15";
+	}
+
+	sub vcl_backend_response {
+		set beresp.filters = "unbr";
+		set beresp.uncacheable = true;
+	}
+} -start
+
+client c1 {
+	txreq -hdr "Range: bytes=0-15"
+	rxresp
+	expect resp.status == 206
+	expect resp.http.Content-Encoding == <undef>
+	expect resp.body == "foo bar baz quux"
+} -run
+
+varnish v1 -vcl+backend {
+	import ${vmod_brotli};
+
+	sub vcl_backend_fetch {
+		set bereq.http.Range = "bytes=0-15";
+	}
+
+	sub vcl_backend_response {
+		set beresp.filters = "br";
+		set beresp.uncacheable = true;
+	}
+}
+
+client c1 -run
+
+logexpect l1 -v v1 -d 1 -g vxid -q "VfpAcct" {
+	expect 0 * Begin	bereq
+ 	expect * = VfpAcct	"unbr 0 0"
+ 	expect * = End
+
+	expect 0 * Begin	bereq
+ 	expect * = VfpAcct	"br 0 0"
+ 	expect * = End
+} -run

src/vfp_brotli.c

@@ -222,11 +222,12 @@ vfp_br_init(struct vfp_ctx *ctx, struct vfp_entry *ent)
 	CHECK_OBJ_NOTNULL(ctx, VFP_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(ent, VFP_ENTRY_MAGIC);
 
-	/* XXX: cache_gzip.c has:
-	 if (http_GetStatus(vc->resp) == 206)
+	/*
+	 * Ignore partial responses to range requests (as Varnish does for
+	 * gzip and gunzip).
+	 */
+	if (http_GetStatus(ctx->resp) == 206)
 	 	return (VFP_NULL);
-	 What do we want to do about partial responses?
-	*/
 
 	if (ent->vfp == &vfp_br) {
 		if (http_GetHdr(ctx->resp, H_Content_Encoding, NULL))