BUG/MEDIUM: proxy: fix email-alert invalid free

In fa90a7d ("BUG/MINOR: proxy: fix email-alert leak on deinit()"), I
tried to fix email-alert deinit() leak the simple way by leveraging
existing free_email_alert() helper function which was already used for
freeing email alert settings used in a default section.

However, as described in GH #2608, there is a subtelty that makes
free_email_alert() not suitable for use from free_proxy().

Indeed, proxy 'mailers.name' hint shares the same memory space than the
pointer to the corresponding mailers section (once the proxy is resolved,
name hint is replaced by the pointer to the section). However, since both
values share the same space (through union), we have to take care of not
freeing `mailers.name` once init_email_alert() was called on the proxy.

Unfortunately, free_email_alert() isn't protected against that, causing
double free() during deinit when mailers section is referenced from
multiple proxy sections. Since there is no easy fix, and that the leak in
itself isn't a big deal (fa90a7d was simply an opportunistic fix rather
than a must-have given that the leak only occurs during deinit and not
during runtime), let's actually revert the fix to restore legacy behavior
and prevent deinit errors.

Thanks to @snetAT for having reported the issue on Github as well as
providing relevant infos to pinpoint the bug.

It should be backported everywhere fa90a7d was backported.
[ada: for versions prior to 3.0, simply revert the offending commit using
'git revert' as proxy_free_common() first appears in 3.0]

(cherry picked from commit 8e22668)
Signed-off-by: Christopher Faulet <[email protected]>

Author: Darlelet

src/proxy.c

@@ -253,8 +253,6 @@ static inline void proxy_free_common(struct proxy *px)
 		LIST_DEL_INIT(&lf->list);
 
 	chunk_destroy(&px->log_tag);
-
-	free_email_alert(px);
 }
 
 void free_proxy(struct proxy *p)
@@ -1483,6 +1481,7 @@ void proxy_free_defaults(struct proxy *defproxy)
 
 	proxy_release_conf_errors(defproxy);
 	deinit_proxy_tcpcheck(defproxy);
+	free_email_alert(defproxy);
 
 	/* FIXME: we cannot free uri_auth because it might already be used by
 	 * another proxy (legacy code for stats URI ...). Refcount anyone ?