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.

meson.build

@@ -1928,7 +1928,6 @@ alias_target('devel', libsystemd_pc, libudev_pc)
 
 libsystemd = shared_library(
         'systemd',
-        disable_mempool_c,
         version : libsystemd_version,
         include_directories : libsystemd_includes,
         link_args : ['-shared',
@@ -1953,7 +1952,6 @@ install_libsystemd_static = static_library(
         basic_gcrypt_sources,
         basic_compress_sources,
         fundamental_sources,
-        disable_mempool_c,
         include_directories : libsystemd_includes,
         build_by_default : static_libsystemd != 'false',
         install : static_libsystemd != 'false',
@@ -1975,7 +1973,6 @@ install_libsystemd_static = static_library(
 
 libudev = shared_library(
         'udev',
-        disable_mempool_c,
         version : libudev_version,
         include_directories : includes,
         link_args : ['-shared',
@@ -1997,7 +1994,6 @@ install_libudev_static = static_library(
         shared_sources,
         libsystemd_sources,
         libudev_sources,
-        disable_mempool_c,
         include_directories : includes,
         build_by_default : static_libudev != 'false',
         install : static_libudev != 'false',
@@ -2118,7 +2114,6 @@ subdir('test')
 test_dlopen = executable(
         'test-dlopen',
         test_dlopen_c,
-        disable_mempool_c,
         include_directories : includes,
         link_with : [libbasic],
         dependencies : [libdl],
@@ -2148,7 +2143,6 @@ foreach tuple : [['myhostname', 'ENABLE_NSS_MYHOSTNAME'],
                 nss = shared_library(
                         'nss_' + module,
                         sources,
-                        disable_mempool_c,
                         version : '2',
                         include_directories : incs,
                         # Note that we link NSS modules with '-z nodelete' so that mempools never get orphaned

src/basic/hashmap.c

@@ -771,16 +771,15 @@ static void shared_hash_key_initialize(void) {
 static struct HashmapBase* hashmap_base_new(const struct hash_ops *hash_ops, enum HashmapType type  HASHMAP_DEBUG_PARAMS) {
         HashmapBase *h;
         const struct hashmap_type_info *hi = &hashmap_type_info[type];
-        bool up;
 
-        up = mempool_enabled();
+        bool use_pool = mempool_enabled && mempool_enabled();
 
-        h = up ? mempool_alloc0_tile(hi->mempool) : malloc0(hi->head_size);
+        h = use_pool ? mempool_alloc0_tile(hi->mempool) : malloc0(hi->head_size);
         if (!h)
                 return NULL;
 
         h->type = type;
-        h->from_pool = up;
+        h->from_pool = use_pool;
         h->hash_ops = hash_ops ?: &trivial_hash_ops;
 
         if (type == HASHMAP_TYPE_ORDERED) {

src/basic/mempool.c

@@ -3,12 +3,9 @@
 #include <stdint.h>
 #include <stdlib.h>
 
-#include "env-util.h"
 #include "macro.h"
 #include "memory-util.h"
 #include "mempool.h"
-#include "process-util.h"
-#include "util.h"
 
 struct pool {
         struct pool *next;
@@ -73,20 +70,6 @@ void mempool_free_tile(struct mempool *mp, void *p) {
         mp->freelist = p;
 }
 
-bool mempool_enabled(void) {
-        static int b = -1;
-
-        if (!is_main_thread())
-                return false;
-
-        if (!mempool_use_allowed)
-                b = false;
-        if (b < 0)
-                b = getenv_bool("SYSTEMD_MEMPOOL") != 0;
-
-        return b;
-}
-
 #if VALGRIND
 void mempool_drop(struct mempool *mp) {
         struct pool *p = mp->first_pool;

src/basic/mempool.h

@@ -23,8 +23,7 @@ static struct mempool pool_name = { \
         .at_least = alloc_at_least, \
 }
 
-extern const bool mempool_use_allowed;
-bool mempool_enabled(void);
+__attribute__((weak)) bool mempool_enabled(void);
 
 #if VALGRIND
 void mempool_drop(struct mempool *mp);

src/libsystemd/disable-mempool.c

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

src/libsystemd/meson.build

@@ -158,7 +158,6 @@ libsystemd_sources = files(
         'sd-utf8/sd-utf8.c',
 ) + sd_journal_sources + id128_sources + sd_daemon_sources + sd_event_sources + sd_login_sources
 
-disable_mempool_c = files('disable-mempool.c')
 
 libsystemd_c_args = ['-fvisibility=default']
 

src/shared/enable-mempool.c

@@ -1,5 +1,19 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
-#include "mempool.h"
+#include <stdbool.h>
 
-const bool mempool_use_allowed = true;
+#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;
+
+        return cache;
+}

src/test/test-set-disable-mempool.c

@@ -2,6 +2,7 @@
 
 #include <pthread.h>
 
+#include "mempool.h"
 #include "process-util.h"
 #include "set.h"
 #include "tests.h"
@@ -15,6 +16,9 @@ static void* thread(void *p) {
         assert_se(*s);
 
         assert_se(!is_main_thread());
+        assert_se(mempool_enabled);
+        assert_se(!mempool_enabled());
+
         assert_se(set_size(*s) == NUM);
         *s = set_free(*s);
 
@@ -29,7 +33,10 @@ static void test_one(const char *val) {
 
         log_info("Testing with SYSTEMD_MEMPOOL=%s", val);
         assert_se(setenv("SYSTEMD_MEMPOOL", val, true) == 0);
+
         assert_se(is_main_thread());
+        assert_se(mempool_enabled);    /* It is a weak symbol, but we expect it to be available */
+        assert_se(!mempool_enabled());
 
         assert_se(s = set_new(NULL));
         for (i = 0; i < NUM; i++)