Fix handling of 304 backend responses.

Set the OF_CHGGZIP flag to indicate that:
- The ETag in a client response must be weakened if the response
  body differs from the fetched response.
- Any Content-Encoding header in the fetched 304 response should
  be removed.

src/tests/etag.vtc

+# looks like -*- vcl -*-
+
+varnishtest "Backend IMS and brotli de-/compression"
+
+# cf. Varnish g00006.vtc
+
+server s1 {
+	rxreq
+	expect req.url == /1
+	txresp -hdr  "Last-Modified: Wed, 11 Sep 2013 13:36:55 GMT" \
+		-hdr {ETag: "foozle"} \
+		-body {The quick brown fox jumps over the lazy dog}
+
+	rxreq
+	expect req.url == /1
+	expect req.http.if-modified-since == "Wed, 11 Sep 2013 13:36:55 GMT"
+	txresp -status 304 \
+		-hdr {ETag: "fizle"} \
+		-nolen
+
+	rxreq
+	expect req.url == /2
+	txresp -hdr "Last-Modified: Wed, 11 Sep 2013 13:36:55 GMT" \
+		-hdr {ETag: "foobar"} -hdr "Content-Encoding: br" \
+		-body {The quick brown fox jumps over the lazy dog}
+
+	rxreq
+	expect req.url == /2
+	expect req.http.if-modified-since == "Wed, 11 Sep 2013 13:36:55 GMT"
+	txresp -status 304 -hdr "Content-Encoding: br" \
+		-hdr {ETag: "snafu"} \
+		-nolen
+
+} -start
+
+varnish v1 -vcl+backend {
+	import ${vmod_brotli};
+
+	sub vcl_backend_response {
+		set beresp.http.foobar = beresp.http.content-encoding;
+		if (bereq.url == "/1") {
+			set beresp.filters = "br";
+		} else {
+			set beresp.filters = "unbr";
+		}
+		set beresp.ttl = 1s;
+		set beresp.grace = 0s;
+		set beresp.keep = 60s;
+	}
+} -start
+
+client c1 {
+	txreq -url /1 -hdr "Accept-Encoding: br"
+	rxresp
+	expect resp.http.content-encoding == "br"
+	expect resp.http.foobar == ""
+	expect resp.http.etag == {W/"foozle"}
+	expect resp.bodylen == 47
+	expect resp.body == "The quick brown fox jumps over the lazy dog"
+
+	delay 1
+
+	txreq -url /1 -hdr "Accept-Encoding: br"
+	rxresp
+	expect resp.http.content-encoding == "br"
+	expect resp.http.foobar == "br"
+	expect resp.http.etag == {W/"fizle"}
+	expect resp.bodylen == 47
+	expect resp.body == "The quick brown fox jumps over the lazy dog"
+
+	delay .2
+
+	txreq -url /2
+	rxresp
+	expect resp.http.content-encoding == <undef>
+	expect resp.http.foobar == "br"
+	expect resp.http.etag == {W/"foobar"}
+	expect resp.bodylen == 43
+	expect resp.body == "The quick brown fox jumps over the lazy dog"
+
+	delay 1
+
+	txreq -url /2
+	rxresp
+	expect resp.http.content-encoding == <undef>
+	# Here we see the C-E of the IMS OBJ
+	expect resp.http.foobar == ""
+	expect resp.http.etag == {W/"snafu"}
+	expect resp.bodylen == 43
+	expect resp.body == "The quick brown fox jumps over the lazy dog"
+} -run

src/vfp_brotli.c

@@ -349,6 +349,7 @@ vfp_br_init(struct vfp_ctx *ctx, struct vfp_entry *ent)
 	http_Unset(ctx->resp, H_Content_Encoding);
 	http_Unset(ctx->resp, H_Content_Length);
 	RFC2616_Weaken_Etag(ctx->resp);
+	ctx->obj_flags |= OF_CHGGZIP;
 
 	if (settings->which == ENC) {
 		http_SetHeader(ctx->resp, "Content-Encoding: br");