fileio: add new helper fdopen_independent()

This is a combination of fdopen() and fd_reopen(). i.e. it first reopens
the fd, and then converts that into a FILE*.

We do this at various places already manually. let's move this into a
helper call of its own.

src/basic/env-file.c

@@ -381,22 +381,15 @@ int parse_env_file_fd_sentinel(
                 const char *fname, /* only used for logging */
                 ...) {
 
-        _cleanup_close_ int fd_ro = -EBADF;
         _cleanup_fclose_ FILE *f = NULL;
         va_list ap;
         int r;
 
         assert(fd >= 0);
 
-        fd_ro = fd_reopen(fd, O_CLOEXEC | O_RDONLY);
-        if (fd_ro < 0)
-                return fd_ro;
-
-        f = fdopen(fd_ro, "re");
-        if (!f)
-                return -errno;
-
-        TAKE_FD(fd_ro);
+        r = fdopen_independent(fd, "re", &f);
+        if (r < 0)
+                return r;
 
         va_start(ap, fname);
         r = parse_env_filev(f, fname, ap);

src/basic/fileio.c

@@ -1020,6 +1020,35 @@ int xfopenat(int dir_fd, const char *path, const char *mode, int flags, FILE **r
         return 0;
 }
 
+int fdopen_independent(int fd, const char *mode, FILE **ret) {
+        _cleanup_close_ int copy_fd = -EBADF;
+        _cleanup_fclose_ FILE *f = NULL;
+        int mode_flags;
+
+        assert(fd >= 0);
+        assert(mode);
+        assert(ret);
+
+        /* A combination of fdopen() + fd_reopen(). i.e. reopens the inode the specified fd points to and
+         * returns a FILE* for it */
+
+        mode_flags = fopen_mode_to_flags(mode);
+        if (mode_flags < 0)
+                return mode_flags;
+
+        copy_fd = fd_reopen(fd, mode_flags);
+        if (copy_fd < 0)
+                return copy_fd;
+
+        f = fdopen(copy_fd, mode);
+        if (!f)
+                return -errno;
+
+        TAKE_FD(copy_fd);
+        *ret = TAKE_PTR(f);
+        return 0;
+}
+
 static int search_and_fopen_internal(
                 const char *path,
                 const char *mode,

src/basic/fileio.h

@@ -106,6 +106,8 @@ int get_proc_field(const char *filename, const char *pattern, const char *termin
 DIR *xopendirat(int dirfd, const char *name, int flags);
 int xfopenat(int dir_fd, const char *path, const char *mode, int flags, FILE **ret);
 
+int fdopen_independent(int fd, const char *mode, FILE **ret);
+
 int search_and_fopen(const char *path, const char *mode, const char *root, const char **search, FILE **ret, char **ret_path);
 int search_and_fopen_nulstr(const char *path, const char *mode, const char *root, const char *search, FILE **ret, char **ret_path);