move dlfcn-util into basic

I'm going to dlopen_many_sym_or_warn() in src/basic/compress.c, this
will introduce a circular dependency because libshared already depends
from libbasic.
To avoid this, move dlfcn-util.c from libshared to libbasic.

src/basic/dlfcn-util.c

@@ -0,0 +1,64 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "dlfcn-util.h"
+
+static int dlsym_many_or_warnv(void *dl, int log_level, va_list ap) {
+        void (**fn)(void);
+
+        /* Tries to resolve a bunch of function symbols, and logs an error about if it cannot resolve one of
+         * them. Note that this function possibly modifies the supplied function pointers if the whole
+         * operation fails. */
+
+        while ((fn = va_arg(ap, typeof(fn)))) {
+                void (*tfn)(void);
+                const char *symbol;
+
+                symbol = va_arg(ap, typeof(symbol));
+
+                tfn = (typeof(tfn)) dlsym(dl, symbol);
+                if (!tfn)
+                        return log_full_errno(log_level,
+                                              SYNTHETIC_ERRNO(ELIBBAD),
+                                              "Can't find symbol %s: %s", symbol, dlerror());
+                *fn = tfn;
+        }
+
+        return 0;
+}
+
+int dlsym_many_or_warn_sentinel(void *dl, int log_level, ...) {
+        va_list ap;
+        int r;
+
+        va_start(ap, log_level);
+        r = dlsym_many_or_warnv(dl, log_level, ap);
+        va_end(ap);
+
+        return r;
+}
+
+int dlopen_many_sym_or_warn_sentinel(void **dlp, const char *filename, int log_level, ...) {
+        _cleanup_(dlclosep) void *dl = NULL;
+        int r;
+
+        if (*dlp)
+                return 0; /* Already loaded */
+
+        dl = dlopen(filename, RTLD_LAZY);
+        if (!dl)
+                return log_debug_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
+                                       "%s is not installed: %s", filename, dlerror());
+
+        va_list ap;
+        va_start(ap, log_level);
+        r = dlsym_many_or_warnv(dl, log_level, ap);
+        va_end(ap);
+
+        if (r < 0)
+                return r;
+
+        /* Note that we never release the reference here, because there's no real reason to. After all this
+         * was traditionally a regular shared library dependency which lives forever too. */
+        *dlp = TAKE_PTR(dl);
+        return 1;
+}

src/basic/dlfcn-util.h

@@ -0,0 +1,41 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <dlfcn.h>
+
+#include "macro.h"
+
+static inline void* safe_dlclose(void *dl) {
+        if (!dl)
+                return NULL;
+
+        assert_se(dlclose(dl) == 0);
+        return NULL;
+}
+
+static inline void dlclosep(void **dlp) {
+        safe_dlclose(*dlp);
+}
+
+int dlsym_many_or_warn_sentinel(void *dl, int log_level, ...) _sentinel_;
+int dlopen_many_sym_or_warn_sentinel(void **dlp, const char *filename, int log_level, ...) _sentinel_;
+
+#define dlsym_many_or_warn(dl, log_level, ...) \
+        dlsym_many_or_warn_sentinel(dl, log_level, __VA_ARGS__, NULL)
+#define dlopen_many_sym_or_warn(dlp, filename, log_level, ...) \
+        dlopen_many_sym_or_warn_sentinel(dlp, filename, log_level, __VA_ARGS__, NULL)
+
+#define DLSYM_PROTOTYPE(symbol)                 \
+        extern typeof(symbol)* sym_##symbol
+#define DLSYM_FUNCTION(symbol)                  \
+        typeof(symbol)* sym_##symbol = NULL
+
+/* Macro useful for putting together variable/symbol name pairs when calling dlsym_many_or_warn(). Assumes
+ * that each library symbol to resolve will be placed in a variable with the "sym_" prefix, i.e. a symbol
+ * "foobar" is loaded into a variable "sym_foobar". */
+#define DLSYM_ARG(arg) \
+        ({ assert_cc(__builtin_types_compatible_p(typeof(sym_##arg), typeof(&arg))); &sym_##arg; }), STRINGIFY(arg)
+
+/* libbpf is a bit confused about type-safety and API compatibility. Provide a macro that can tape over that mess. Sad. */
+#define DLSYM_ARG_FORCE(arg) \
+        &sym_##arg, STRINGIFY(arg)

src/basic/meson.build

@@ -21,6 +21,7 @@ basic_sources = files(
         'confidential-virt.c',
         'devnum-util.c',
         'dirent-util.c',
+        'dlfcn-util.c',
         'efivars.c',
         'env-file.c',
         'env-util.c',