fix when the vcl_synth {} Process timestamp gets emitted

Found by @Dridi

Conflicts:
	doc/changes.rst

bin/varnishd/cache/cache_req_fsm.c

@@ -301,8 +301,6 @@ cnt_synth(struct worker *wrk, struct req *req)
 
 	wrk->stats->s_synth++;
 
-	VSLb_ts_req(req, "Process", W_TIM_real(wrk));
-
 	if (req->err_code < 100)
 		req->err_code = 501;
 
@@ -315,6 +313,8 @@ cnt_synth(struct worker *wrk, struct req *req)
 
 	AZ(VSB_finish(synth_body));
 
+	VSLb_ts_req(req, "Process", W_TIM_real(wrk));
+
 	if (wrk->handling == VCL_RET_FAIL) {
 		VSB_destroy(&synth_body);
 		req->doclose = SC_VCL_FAILURE;

bin/varnishtest/tests/c00018.vtc

@@ -122,7 +122,6 @@ logexpect l1 -v v1 -g raw {
 	expect 0 1011	VCL_return      {^synth$}
 	expect 0 1011	VCL_call        {^HASH$}
 	expect 0 1011	VCL_return      {^lookup$}
-	expect 0 1011	Timestamp       {^Process:}
 	expect 0 1011	RespProtocol    {^HTTP/1.1$}
 	expect 0 1011	RespStatus      {^405$}
 	expect 0 1011	RespReason      {^Method Not Allowed$}
@@ -130,6 +129,7 @@ logexpect l1 -v v1 -g raw {
 	expect 1 1011	RespHeader      {^Date:}
 	expect 0 1011	RespHeader      {^Server: Varnish$}
 	expect 0 1011	RespHeader      {^X-Varnish: 1011$}
+	expect * 1011	Timestamp       {^Process:}
 
 } -start
 

doc/changes.rst

@@ -26,6 +26,14 @@ http://varnish-cache.org/docs/trunk/whats-new/index.html and via
 individual releases. These documents are updated as part of the
 release process.
 
+=================================
+Varnish Cache 6.0.13 (unreleased)
+=================================
+
+* The ``Process`` timestamp for ``vcl_synth {}`` was wrongly issued
+  before the VCL callback, now it gets emitted after VCL returns for
+  consistency with ``vcl_deliver {}``
+
 =================================
 Varnish Cache 6.0.12 (2023-11-13)
 =================================