MINOR: proxy: prevent backend removal when unsupported

Prevent removal of a backend which relies on features not compatible
with dynamic backends. This is the case if either dispatch or
transparent option is used, or if a stick-table is declared.

These limitations are similar to the "add backend" ones.

Author: a-denoyelle

doc/management.txt

@@ -2130,6 +2130,11 @@ del backend <name>
   This operation is only possible for TCP or HTTP proxies. To succeed, the
   backend instance must have been first unpublished.
 
+  There is also additional restrictions which prevent backend removal.
+  Currently, this is the case when deprecated dispatch or option transparent
+  are used. Also, a backend cannot be removed if there is a stick-table
+  declared in it.
+
   This command is restricted and can only be issued on sockets configured for
   level "admin". Moreover, this feature is still considered in development so it
   also requires experimental mode (see "experimental-mode on").

src/proxy.c

@@ -5014,6 +5014,16 @@ int be_check_for_deletion(const char *bename, struct proxy **pb, const char **pm
 		goto out;
 	}
 
+	if (be->options & (PR_O_DISPATCH|PR_O_TRANSP)) {
+		msg = "Deletion of backend with deprecated dispatch/transparent options is not supported.";
+		goto out;
+	}
+
+	if (be->table) {
+		msg = "Cannot remove a backend with stick-table.";
+		goto out;
+	}
+
 	if (be->cap & PR_CAP_FE) {
 		msg = "Cannot delete a listen section.";
 		goto out;