os-util: add helpers for finding /etc/os-release

Place this new helpers in a new source file os-util.[ch], and move the
existing and related call path_is_os_tree() to it as well.

src/basic/meson.build

@@ -131,6 +131,8 @@ basic_sources = files('''
         ordered-set.h
         pager.c
         pager.h
+        os-util.c
+        os-util.h
         parse-util.c
         parse-util.h
         path-util.c

src/basic/os-util.c

@@ -0,0 +1,117 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include "alloc-util.h"
+#include "fd-util.h"
+#include "fs-util.h"
+#include "macro.h"
+#include "os-util.h"
+#include "strv.h"
+#include "fileio.h"
+#include "string-util.h"
+
+int path_is_os_tree(const char *path) {
+        int r;
+
+        assert(path);
+
+        /* Does the path exist at all? If not, generate an error immediately. This is useful so that a missing root dir
+         * always results in -ENOENT, and we can properly distuingish the case where the whole root doesn't exist from
+         * the case where just the os-release file is missing. */
+        if (laccess(path, F_OK) < 0)
+                return -errno;
+
+        /* We use {/etc|/usr/lib}/os-release as flag file if something is an OS */
+        r = open_os_release(path, NULL, NULL);
+        if (r == -ENOENT) /* We got nothing */
+                return 0;
+        if (r < 0)
+                return r;
+
+        return 1;
+}
+
+int open_os_release(const char *root, char **ret_path, int *ret_fd) {
+        _cleanup_free_ char *q = NULL;
+        const char *p;
+        int k;
+
+        FOREACH_STRING(p, "/etc/os-release", "/usr/lib/os-release") {
+                k = chase_symlinks(p, root, CHASE_PREFIX_ROOT|(ret_fd ? CHASE_OPEN : 0), (ret_path ? &q : NULL));
+                if (k != -ENOENT)
+                        break;
+        }
+        if (k < 0)
+                return k;
+
+        if (ret_fd) {
+                int real_fd;
+
+                /* Convert the O_PATH fd into a proper, readable one */
+                real_fd = fd_reopen(k, O_RDONLY|O_CLOEXEC|O_NOCTTY);
+                safe_close(k);
+                if (real_fd < 0)
+                        return real_fd;
+
+                *ret_fd = real_fd;
+        }
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(q);
+
+        return 0;
+}
+
+int fopen_os_release(const char *root, char **ret_path, FILE **ret_file) {
+        _cleanup_free_ char *p = NULL;
+        _cleanup_close_ int fd = -1;
+        FILE *f;
+        int r;
+
+        if (!ret_file)
+                return open_os_release(root, ret_path, NULL);
+
+        r = open_os_release(root, ret_path ? &p : NULL, &fd);
+        if (r < 0)
+                return r;
+
+        f = fdopen(fd, "re");
+        if (!f)
+                return -errno;
+        fd = -1;
+
+        *ret_file = f;
+
+        if (ret_path)
+                *ret_path = TAKE_PTR(p);
+
+        return 0;
+}
+
+int parse_os_release(const char *root, ...) {
+        _cleanup_fclose_ FILE *f = NULL;
+        _cleanup_free_ char *p = NULL;
+        va_list ap;
+        int r;
+
+        r = fopen_os_release(root, &p, &f);
+        if (r < 0)
+                return r;
+
+        va_start(ap, root);
+        r = parse_env_filev(f, p, NEWLINE, ap);
+        va_end(ap);
+
+        return r;
+}
+
+int load_os_release_pairs(const char *root, char ***ret) {
+        _cleanup_fclose_ FILE *f = NULL;
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        r = fopen_os_release(root, &p, &f);
+        if (r < 0)
+                return r;
+
+        return load_env_file_pairs(f, p, NEWLINE, ret);
+}

src/basic/os-util.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <stdio.h>
+
+int path_is_os_tree(const char *path);
+
+int open_os_release(const char *root, char **ret_path, int *ret_fd);
+int fopen_os_release(const char *root, char **ret_path, FILE **ret_file);
+
+int parse_os_release(const char *root, ...);
+int load_os_release_pairs(const char *root, char ***ret);

src/basic/stat-util.c

@@ -132,32 +132,6 @@ int path_is_read_only_fs(const char *path) {
         return false;
 }
 
-int path_is_os_tree(const char *path) {
-        int r;
-
-        assert(path);
-
-        /* Does the path exist at all? If not, generate an error immediately. This is useful so that a missing root dir
-         * always results in -ENOENT, and we can properly distuingish the case where the whole root doesn't exist from
-         * the case where just the os-release file is missing. */
-        if (laccess(path, F_OK) < 0)
-                return -errno;
-
-        /* We use /usr/lib/os-release as flag file if something is an OS */
-        r = chase_symlinks("/usr/lib/os-release", path, CHASE_PREFIX_ROOT, NULL);
-        if (r == -ENOENT) {
-
-                /* Also check for the old location in /etc, just in case. */
-                r = chase_symlinks("/etc/os-release", path, CHASE_PREFIX_ROOT, NULL);
-                if (r == -ENOENT)
-                        return 0; /* We got nothing */
-        }
-        if (r < 0)
-                return r;
-
-        return 1;
-}
-
 int files_same(const char *filea, const char *fileb, int flags) {
         struct stat a, b;
 

src/basic/stat-util.h

@@ -35,7 +35,6 @@ int null_or_empty_path(const char *fn);
 int null_or_empty_fd(int fd);
 
 int path_is_read_only_fs(const char *path);
-int path_is_os_tree(const char *path);
 
 int files_same(const char *filea, const char *fileb, int flags);