mempool: only enable mempool use when linked to libsystemd-shared.so

Mempool use is enabled or disabled based on the mempool_use_allowed symbol that
is linked in.

Should fix assert crashes in external programs caused by #9792.
Replaces #10286.

v2:
- use two different source files instead of a gcc constructor

(cherry picked from commit a5d8835c78112206bbf0812dd4cb471f803bfe88)

2 files changed, 4 insertions, 1 deletions

diff --git a/src/basic/mempool.c b/src/basic/mempool.c
index aaf46ceda..24a7d2b11 100644
--- a/src/basic/mempool.c
+++ b/src/basic/mempool.c
@@ -78,6 +78,8 @@ bool mempool_enabled(void) {
         if (!is_main_thread())
                 return false;
 
+        if (!mempool_use_allowed)
+                b = false;
         if (b < 0)
                 b = getenv_bool("SYSTEMD_MEMPOOL") != 0;
 
diff --git a/src/basic/mempool.h b/src/basic/mempool.h
index e223fc16f..a9d6b4691 100644
--- a/src/basic/mempool.h
+++ b/src/basic/mempool.h
@@ -1,7 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
-
+//#include <stdbool.h>
 #include <stddef.h>
 
 struct pool;
@@ -23,6 +23,7 @@ static struct mempool pool_name = { \
         .at_least = alloc_at_least, \
 }
 
+extern const bool mempool_use_allowed;
 bool mempool_enabled(void);
 
 #if VALGRIND