[master] 780eace Add a flag to mark 304 backend response processing (aka Backend IMS/INM)

[Nils Goroll]

commit 780eace6120c4a94a0620f70c8ed4e1386d7bf1f
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Tue Mar 10 23:55:10 2015 +0100

    Add a flag to mark 304 backend response processing (aka Backend IMS/INM)
    
    This is a compromise discussed at VDD15Q1 after a previous suggestion
    to expose the 304 response status directly to VCL.
    
    This way, VCL can differenciate between 200 and 304 responses, but simpler
    processing logic just checking for status 200 can be preserved where
    differenciating is not required.

diff --git a/bin/varnishd/cache/cache_fetch.c b/bin/varnishd/cache/cache_fetch.c
index 664bf3d..1c59ab5 100644
--- a/bin/varnishd/cache/cache_fetch.c
+++ b/bin/varnishd/cache/cache_fetch.c
@@ -254,7 +254,7 @@ vbf_stp_retry(struct worker *wrk, struct busyobj *bo)
 static enum fetch_step
 vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 {
-	int i, do_ims = 0;
+	int i;
 	double now;
 
 	CHECK_OBJ_NOTNULL(wrk, WORKER_MAGIC);
@@ -385,6 +385,7 @@ vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 		bo->fetch_objcore->exp.ttl = -1.;
 
 	AZ(bo->do_esi);
+	AZ(bo->was_304);
 
 	if (http_IsStatus(bo->beresp, 304)) {
 		if (bo->stale_oc != NULL &&
@@ -401,7 +402,7 @@ vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 			http_Unset(bo->beresp, H_Content_Length);
 			HTTP_Merge(bo->wrk, bo->stale_oc, bo->beresp);
 			assert(http_IsStatus(bo->beresp, 200));
-			do_ims = 1;
+			bo->was_304 = 1;
 		} else if (!bo->do_pass) {
 			/*
 			 * Backend sent unallowed 304
@@ -451,7 +452,7 @@ vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 
 	assert(wrk->handling == VCL_RET_DELIVER);
 
-	return (do_ims ? F_STP_CONDFETCH : F_STP_FETCH);
+	return (bo->was_304 ? F_STP_CONDFETCH : F_STP_FETCH);
 }
 
 /*--------------------------------------------------------------------
diff --git a/bin/varnishtest/tests/b00039.vtc b/bin/varnishtest/tests/b00039.vtc
index d076290..397006d 100644
--- a/bin/varnishtest/tests/b00039.vtc
+++ b/bin/varnishtest/tests/b00039.vtc
@@ -13,6 +13,7 @@ varnish v1 -vcl+backend {
 		set beresp.ttl = 2s;
 		set beresp.grace = 20s;
 		set beresp.keep = 1m;
+		set beresp.http.was-304 = beresp.was_304;
 	}
 } -start
 
@@ -21,6 +22,7 @@ client c1 {
 	rxresp
 	expect resp.status == 200
 	expect resp.body == "Geoff Rules"
+	expect resp.http.was-304 == "false"
 } -run
 
 delay 3
@@ -30,6 +32,7 @@ client c1 {
 	rxresp
 	expect resp.status == 200
 	expect resp.body == "Geoff Rules"
+	expect resp.http.was-304 == "false"
 } -run
 
 client c1 {
@@ -37,4 +40,5 @@ client c1 {
 	rxresp
 	expect resp.status == 200
 	expect resp.body == "Geoff Rules"
+	expect resp.http.was-304 == "true"
 } -run
diff --git a/doc/sphinx/users-guide/vcl-built-in-subs.rst b/doc/sphinx/users-guide/vcl-built-in-subs.rst
index cd78355..8b00232 100644
--- a/doc/sphinx/users-guide/vcl-built-in-subs.rst
+++ b/doc/sphinx/users-guide/vcl-built-in-subs.rst
@@ -277,6 +277,25 @@ vcl_backend_response
 Called after the response headers have been successfully retrieved from
 the backend.
 
+For a 304 response, varnish core code amends ``beresp`` before calling
+`vcl_backend_response`:
+
+* If the gzip status changed, ``Content-Encoding`` is unset and any
+  ``Etag`` is weakened
+
+* Any headers not present in the 304 response are copied from the
+  existing cache object. ``Content-Length`` is copied if present in
+  the existing cache object and discarded otherwise.
+
+* The status gets set to 200.
+
+`beresp.was_304` marks that this conditional response processing has
+happened.
+
+Note: Backend conditional requests are independend of client
+conditional requests, so clients may receive 304 responses no matter
+if a backend request was conditional.
+
 The `vcl_backend_response` subroutine may terminate with calling
 ``return()`` with one of the following keywords:
 
diff --git a/include/tbl/bo_flags.h b/include/tbl/bo_flags.h
index bc01dbe..dac83b2 100644
--- a/include/tbl/bo_flags.h
+++ b/include/tbl/bo_flags.h
@@ -39,5 +39,6 @@ BO_FLAG(uncacheable,	0, 0, "")
 BO_FLAG(abandon,	0, 0, "")
 BO_FLAG(is_gzip,	0, 0, "")
 BO_FLAG(is_gunzip,	0, 0, "")
+BO_FLAG(was_304,	1, 0, "")
 
 /*lint -restore */
diff --git a/lib/libvcc/generate.py b/lib/libvcc/generate.py
index 448cec8..871b8bc 100755
--- a/lib/libvcc/generate.py
+++ b/lib/libvcc/generate.py
@@ -461,6 +461,15 @@ sp_variables = [
 		cache.  Defaults to false.
 		"""
 	),
+	('beresp.was_304',
+		'BOOL',
+		( 'backend_response', 'backend_error'),
+		( ), """
+		Boolean. If this is a successful 304 response to a
+		backend conditional request refreshing an existing
+		cache object.
+		"""
+	),
 	('beresp.uncacheable',
 		'BOOL',
 		( 'backend_response', 'backend_error'),