Rename the obj flag OF_CHGGZIP to OF_CHGCE, for "changed Content-Encoding".

And update the comment explaining what happens. This is to make it
apparent that a VFP has changed C-E in the stored object, so C-E
should not be changed by an IMS fetch, and a new ETag from IMS has
to be weakened. The flag is appropriate for use by any VFP that
needs to manipulate C-E this way (not just gzip).

Closes #2910

bin/varnishd/cache/cache_esi_fetch.c

@@ -162,7 +162,7 @@ vfp_esi_gzip_init(struct vfp_ctx *vc, struct vfp_entry *vfe)
 	ALLOC_OBJ(vef, VEF_MAGIC);
 	if (vef == NULL)
 		return (VFP_ERROR);
-	vc->obj_flags |= OF_GZIPED | OF_CHGGZIP | OF_ESIPROC;
+	vc->obj_flags |= OF_GZIPED | OF_CHGCE | OF_ESIPROC;
 	vef->vgz = VGZ_NewGzip(vc->wrk->vsl, "G F E");
 	vef->vep = VEP_Init(vc, vc->req, vfp_vep_callback, vef);
 	vef->ibuf_sz = cache_param->gzip_buffer;

bin/varnishd/cache/cache_fetch.c

@@ -356,11 +356,12 @@ vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 		if (bo->stale_oc != NULL &&
 		    ObjCheckFlag(bo->wrk, bo->stale_oc, OF_IMSCAND)) {
 			AZ(bo->stale_oc->flags & (OC_F_HFM|OC_F_PRIVATE));
-			if (ObjCheckFlag(bo->wrk, bo->stale_oc, OF_CHGGZIP)) {
+			if (ObjCheckFlag(bo->wrk, bo->stale_oc, OF_CHGCE)) {
 				/*
-				 * If we changed the gzip status of the object
-				 * the stored Content_Encoding controls we
-				 * must weaken any new ETag we get.
+				 * If a VFP changed C-E in the stored
+				 * object, then don't overwrite C-E from
+				 * the IMS fetch, and we must weaken any
+				 * new ETag we get.
 				 */
 				http_Unset(bo->beresp, H_Content_Encoding);
 				RFC2616_Weaken_Etag(bo->beresp);

bin/varnishd/cache/cache_gzip.c

@@ -465,14 +465,14 @@ vfp_gzip_init(struct vfp_ctx *vc, struct vfp_entry *vfe)
 		if (http_GetHdr(vc->resp, H_Content_Encoding, NULL))
 			return (VFP_NULL);
 		vg = VGZ_NewGzip(vc->wrk->vsl, vfe->vfp->priv1);
-		vc->obj_flags |= OF_GZIPED | OF_CHGGZIP;
+		vc->obj_flags |= OF_GZIPED | OF_CHGCE;
 	} else {
 		if (!http_HdrIs(vc->resp, H_Content_Encoding, "gzip"))
 			return (VFP_NULL);
 		if (vfe->vfp == &VFP_gunzip) {
 			vg = VGZ_NewGunzip(vc->wrk->vsl, vfe->vfp->priv1);
 			vc->obj_flags &= ~OF_GZIPED;
-			vc->obj_flags |= OF_CHGGZIP;
+			vc->obj_flags |= OF_CHGCE;
 		} else {
 			vg = VGZ_NewTestGunzip(vc->wrk->vsl, vfe->vfp->priv1);
 			vc->obj_flags |= OF_GZIPED;

include/tbl/obj_attr.h

@@ -55,7 +55,7 @@
 #ifdef OBJ_FLAG
 /* upper, lower, val */
   OBJ_FLAG(GZIPED,	gziped,		(1<<1))
-  OBJ_FLAG(CHGGZIP,	chggzip,	(1<<2))
+  OBJ_FLAG(CHGCE,	chgce,		(1<<2))
   OBJ_FLAG(IMSCAND,	imscand,	(1<<3))
   OBJ_FLAG(ESIPROC,	esiproc,	(1<<4))
   #undef OBJ_FLAG