commit 54948f3fd310ffc74a6c252dc11046a8a18ab230
Author: Jerome Magnin <jmagnin@haproxy.com>
Date:   Sat Nov 9 18:00:47 2019 +0100

    BUG/MINOR: stream: init variables when the list is empty
    
    We need to call vars_init() when the list is empty otherwise we
    can't use variables in the response scope. This regression was
    introduced by cda7f3f5 (MINOR: stream: don't prune variables if
    the list is empty).
    
    The following config reproduces the issue:
    
     defaults
       mode http
    
     frontend in
       bind *:11223
       http-request set-var(req.foo) str("foo")  if { path /bar }
       http-request set-header bar %[var(req.foo)]  if { var(req.foo) -m found }
       http-response set-var(res.bar) str("bar")
       http-response set-header foo %[var(res.bar)] if { var(res.bar) -m found }
       use_backend out
    
     backend out
       server s1 127.0.0.1:11224
    
     listen back
       bind *:11224
       http-request deny deny_status 200
    
     > GET /ba HTTP/1.1
     > Host: localhost:11223
     > User-Agent: curl/7.66.0
     > Accept: */*
     >
     < HTTP/1.0 200 OK
     < Cache-Control: no-cache
     < Content-Type: text/html
    
     > GET /bar HTTP/1.1
     > Host: localhost:11223
     > User-Agent: curl/7.66.0
     > Accept: */*
     >
     < HTTP/1.0 200 OK
     < Cache-Control: no-cache
     < Content-Type: text/html
     < foo: bar
    
    This must be backported as far as 1.9.
    
    (cherry picked from commit 2f44e8843a553ef0f9c53c9b27899727de097777)
    Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/src/stream.c b/src/stream.c
index 4a7ced22..9fe0efaf 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -2399,10 +2399,9 @@ struct task *process_stream(struct task *t, void *context, unsigned short state)
 	if (si_state_in(si_b->state, SI_SB_REQ|SI_SB_QUE|SI_SB_TAR|SI_SB_ASS)) {
 		/* prune the request variables and swap to the response variables. */
 		if (s->vars_reqres.scope != SCOPE_RES) {
-			if (!LIST_ISEMPTY(&s->vars_reqres.head)) {
+			if (!LIST_ISEMPTY(&s->vars_reqres.head))
 				vars_prune(&s->vars_reqres, s->sess, s);
-				vars_init(&s->vars_reqres, SCOPE_RES);
-			}
+			vars_init(&s->vars_reqres, SCOPE_RES);
 		}
 
 		do {