basically working

src/vfp_bodyhash.c

@@ -43,7 +43,6 @@ struct bodyhash {
 #define BODYHASH_MAGIC		0xb0d16a56
 
 	struct VSHA256Context	sha256ctx;
-	char			*hdr;
 	// unsigned char	hash[VSHA256_LEN];
 };
 
@@ -59,7 +58,6 @@ vfp_bodyhash_init(struct vfp_ctx *vc, struct vfp_entry *vfe)
 	CHECK_OBJ_NOTNULL(vc, VFP_CTX_MAGIC);
 	CHECK_OBJ_NOTNULL(vfe, VFP_ENTRY_MAGIC);
 
-	AN(vfe->priv1);
 	assert(vfe->vfp == &VFP_bodyhash);
 	assert(placeholder_l == VSHA256_LEN * 2 + 2);
 
@@ -68,7 +66,6 @@ vfp_bodyhash_init(struct vfp_ctx *vc, struct vfp_entry *vfe)
 	if (bh == NULL)
 		return (VFP_ERROR);
 	VSHA256_Init(&bh->sha256ctx);
-	bh->hdr = vfe->priv1;
 
 	vfe->priv1 = bh;
 	http_ForceHeader(vc->resp, H_ETag, placeholder);
@@ -132,11 +129,6 @@ vfp_bodyhash_fini(struct vfp_ctx *vc, struct vfp_entry *vfe)
 	*p++ = '\0';
 	assert(pdiff(hex, p) == hexl);
 
-	if (vc->resp) {
-		http_ForceHeader(vc->resp, H_ETag, hex);
-		goto out;
-	}
-
 	// HACKY
 	p = TRUST_ME(ObjGetAttr(vc->wrk, vc->oc, OA_HEADERS, &l));
 	if (p == NULL) {
@@ -178,15 +170,3 @@ const struct vfp VFP_bodyhash = {
 	.pull = vfp_bodyhash_pull,
 	.fini = vfp_bodyhash_fini
 };
-
-// XXX in cache_varnishd.h
-struct vfp_entry *VFP_Push(struct vfp_ctx *, const struct vfp *);
-
-void *
-Bodyhash_push(struct vfp_ctx *vc, const char *hdr)
-{
-	struct vfp_entry *vfe = VFP_Push(vc, &VFP_bodyhash);
-	if (vfe)
-		vfe->priv1 = TRUST_ME(hdr);
-	return (vfe);
-}

src/vfp_bodyhash.h

@@ -25,4 +25,4 @@
  *
  */
 
-void *Bodyhash_push(struct vfp_ctx *vc, const char *hdr);
+extern const struct vfp VFP_bodyhash;

src/vmod_etag.c

 #include "config.h"
 
 #include <cache/cache.h>
+#include <cache/cache_filter.h>
 
 #include "vcc_etag_if.h"
+#include "vfp_bodyhash.h"
 
-VCL_STRING
-vmod_hello(VRT_CTX)
+int
+vmod_event_function(VRT_CTX, struct vmod_priv *priv, enum vcl_event_e e)
 {
+	if (e != VCL_EVENT_LOAD)
+		return (0);
 
-	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
-	return ("vmod-etag");
+	VRT_AddVFP(ctx, &VFP_bodyhash);
+	return (0);
 }

src/vmod_etag.vcc

@@ -19,10 +19,7 @@ Example
 
 XXX: define vmod-etag interface
 
-$Function STRING hello()
-
-Description
-    Hello world for vmod-etag
+$Event event_function
 
 SEE ALSO
 ========vcl\(7),varnishd\(1)

src/vtc/vmod_etag.vtc

@@ -2,20 +2,29 @@ varnishtest "test vmod-etag"
 
 server s1 {
 	rxreq
-	txresp
+	txresp -bodylen 1048576
 } -start
 
 varnish v1 -vcl+backend {
-        import etag;
+	import etag;
 
-        sub vcl_deliver {
-	    set resp.http.Hello = etag.hello();
-        }
+	sub vcl_backend_response {
+		set beresp.filters = beresp.filters + " bodyhash";
+	}
+
+	sub vcl_deliver {
+		# XXX racy
+		if (resp.http.Etag ~ "placeholder") {
+			unset resp.http.Etag;
+		}
+	}
 } -start
 
 client c1 {
 	txreq
 	rxresp
 	expect resp.status == 200
-	expect resp.http.Hello == "vmod-etag"
+	txreq
+	rxresp
+	expect resp.status == 200
 } -run