util: add set of macros for declaring _cleanup_-style destructors for static variables

src/basic/meson.build

@@ -183,6 +183,7 @@ basic_sources = files('''
         special.h
         stat-util.c
         stat-util.h
+        static-destruct.h
         stdio-util.h
         strbuf.c
         strbuf.h

src/basic/static-destruct.h

@@ -0,0 +1,51 @@
+#pragma once
+
+#include "macro.h"
+
+/* A framework for registering static variables that shall be freed on shutdown of a process. It's a bit like gcc's
+ * destructor attribute, but allows us to precisely schedule when we want to free the variables. This is supposed to
+ * feel a bit like the gcc cleanup attribute, but for static variables. Note that this does not work for static
+ * variables declared in .so's, as the list is private to the same linking unit. But maybe that's a good thing. */
+
+typedef struct StaticDestructor {
+        void *data;
+        void (*destroy)(void *p);
+} StaticDestructor;
+
+#define STATIC_DESTRUCTOR_REGISTER(variable, func) \
+        _STATIC_DESTRUCTOR_REGISTER(UNIQ, variable, func)
+
+#define _STATIC_DESTRUCTOR_REGISTER(uq, variable, func)                 \
+        /* Type-safe destructor */                                      \
+        static void UNIQ_T(static_destructor_wrapper, uq)(void *p) {    \
+                typeof(variable) *q = p;                                \
+                func(q);                                                \
+        }                                                               \
+        /* The actual destructor structure */                           \
+        __attribute__ ((__section__("SYSTEMD_STATIC_DESTRUCT")))        \
+        __attribute__ ((__aligned__(__BIGGEST_ALIGNMENT__)))            \
+        __attribute__ ((__used__))                                      \
+        static const StaticDestructor UNIQ_T(static_destructor_entry, uq) = { \
+                .data = &(variable),                                    \
+                .destroy = UNIQ_T(static_destructor_wrapper, uq),       \
+        }
+
+/* Beginning and end of our section listing the destructors. We define these as weak as we want this to work even if
+ * there's not a single destructor is defined in which case the section will be missing. */
+extern const struct StaticDestructor __attribute__((__weak__)) __start_SYSTEMD_STATIC_DESTRUCT[];
+extern const struct StaticDestructor __attribute__((__weak__)) __stop_SYSTEMD_STATIC_DESTRUCT[];
+
+/* The function to destroy everything. (Note that this must be static inline, as it's key that it remains in the same
+ * linking unit as the variables we want to destroy. */
+static inline void static_destruct(void) {
+        const StaticDestructor *d;
+
+        if (!__start_SYSTEMD_STATIC_DESTRUCT)
+                return;
+
+        d = ALIGN_TO_PTR(__start_SYSTEMD_STATIC_DESTRUCT, __BIGGEST_ALIGNMENT__);
+        while (d < __stop_SYSTEMD_STATIC_DESTRUCT) {
+                d->destroy(d->data);
+                d = ALIGN_TO_PTR(d + 1, __BIGGEST_ALIGNMENT__);
+        }
+}