Turn mempool_enabled() into a weak symbol

Before we had the following scheme:
mempool_enabled() would check mempool_use_allowed, and
libsystemd-shared would be linked with a .c file that provides mempool_use_allowed=true,
while other things would linked with a different .c file with mempool_use_allowed=false.

In the new scheme, mempool_enabled() itself is a weak symbol. If it's
not found, we assume false. So it only needs to be provided for libsystemd-shared,
where it can return false or true.

test-set-disable-mempool is libshared, so it gets the symbol. But then we
actually disable the mempool via envvar. mempool_enable() is called to check
its return value directly.

1 files changed, 15 insertions, 1 deletions

diff --git a/src/shared/enable-mempool.c b/src/shared/enable-mempool.c
index 1abfccbd81..fd582c0e78 100644
--- a/src/shared/enable-mempool.c
+++ b/src/shared/enable-mempool.c
@@ -1,5 +1,19 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <stdbool.h>
+
+#include "env-util.h"
 #include "mempool.h"
+#include "process-util.h"
+
+bool mempool_enabled(void) {
+        static int cache = -1;
+
+        if (!is_main_thread())
+                return false;
+
+        if (cache < 0)
+                cache = getenv_bool("SYSTEMD_MEMPOOL") != 0;
 
-const bool mempool_use_allowed = true;
+        return cache;
+}