commit a41ac2d710711f3ab91d92415278a73c358aedca
Author: Willy Tarreau <w@1wt.eu>
Date:   Tue Jun 4 16:43:29 2019 +0200

    BUG/MEDIUM: vars: make the tcp/http unset-var() action support conditions
    
    Patrick Hemmer reported that http-request unset-var(foo) if ... fails to
    parse. The reason is that it reuses the same parser as "set-var(foo)" which
    makes a special case of the arguments, supposed to be a sample expression
    for set-var, but which must not exist for unset-var. Unfortunately the
    parser finds "if" or "unless" and believes it's an expression. Let's simply
    drop the test so that the outer rule parser deals with potential extraneous
    keywords.
    
    This should be backported to all versions supporting unset-var().
    
    (cherry picked from commit 4b7531f48b5aa66d11fcee2836c201644bfb6a71)
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    (cherry picked from commit a47e745276662db361637914b8558984f091306b)
    Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>

diff --git a/src/vars.c b/src/vars.c
index c86f612f..d69fb838 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -684,6 +684,7 @@ static int conv_check_var(struct arg *args, struct sample_conv *conv,
  * the format:
  *
  *   set-var(<variable-name>) <expression>
+ *   unset-var(<variable-name>)
  *
  * It returns ACT_RET_PRS_ERR if fails and <err> is filled with an error
  * message. Otherwise, it returns ACT_RET_PRS_OK and the variable <expr>
@@ -727,10 +728,6 @@ static enum act_parse_ret parse_store(const char **args, int *arg, struct proxy
 	/* There is no fetch method when variable is unset. Just set the right
 	 * action and return. */
 	if (!set_var) {
-		if (*args[*arg]) {
-			memprintf(err, "fetch method not supported");
-			return ACT_RET_PRS_ERR;
-		}
 		rule->action     = ACT_CUSTOM;
 		rule->action_ptr = action_clear;
 		return ACT_RET_PRS_OK;