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

meson.build

@@ -1380,6 +1380,7 @@ libjournal_core = static_library(
 libsystemd_sym_path = '@0@/@1@'.format(meson.current_source_dir(), libsystemd_sym)
 libsystemd = shared_library(
         'systemd',
+        disable_mempool_c,
         version : libsystemd_version,
         include_directories : includes,
         link_args : ['-shared',
@@ -1461,6 +1462,7 @@ subdir('test')
 test_dlopen = executable(
         'test-dlopen',
         test_dlopen_c,
+        disable_mempool_c,
         include_directories : includes,
         link_with : [libbasic],
         dependencies : [libdl],
@@ -1481,6 +1483,7 @@ foreach tuple : [['myhostname', 'ENABLE_NSS_MYHOSTNAME'],
                 nss = shared_library(
                         'nss_' + module,
                         'src/nss-@0@/nss-@0@.c'.format(module),
+                        disable_mempool_c,
                         version : '2',
                         include_directories : includes,
                         # Note that we link NSS modules with '-z nodelete' so that mempools never get orphaned

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;
 

src/basic/mempool.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
+#include <stdbool.h>
 #include <stddef.h>
 
 struct pool;
@@ -22,6 +23,7 @@ static struct mempool pool_name = { \
         .at_least = alloc_at_least, \
 }
 
+extern const bool mempool_use_allowed;
 bool mempool_enabled(void);
 
 #if VALGRIND

src/libsystemd/disable-mempool.c

@@ -0,0 +1,5 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include "mempool.h"
+
+const bool mempool_use_allowed = false;

src/libsystemd/meson.build

@@ -81,6 +81,8 @@ libsystemd_sources = files('''
         sd-utf8/sd-utf8.c
 '''.split()) + id128_sources + sd_daemon_c + sd_event_c + sd_login_c
 
+disable_mempool_c = files('disable-mempool.c')
+
 libsystemd_c_args = ['-fvisibility=default']
 
 libsystemd_static = static_library(

src/shared/enable-mempool.c

@@ -0,0 +1,5 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include "mempool.h"
+
+const bool mempool_use_allowed = true;

src/shared/meson.build

@@ -36,6 +36,7 @@ shared_sources = files('''
         dropin.h
         efivars.c
         efivars.h
+        enable-mempool.c
         fdset.c
         fdset.h
         firewall-util.h