fd-util: introduce fd_reopen() helper for reopening an fd

We have the same code for this in place at various locations, let's
unify that. Also, let's repurpose test-fs-util.c as a test for this new
helper cal..

src/basic/fd-util.c

@@ -427,7 +427,6 @@ int move_fd(int from, int to, int cloexec) {
 
 int acquire_data_fd(const void *data, size_t size, unsigned flags) {
 
-        char procfs_path[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
         _cleanup_close_pair_ int pipefds[2] = { -1, -1 };
         char pattern[] = "/dev/shm/data-fd-XXXXXX";
         _cleanup_close_ int fd = -1;
@@ -537,12 +536,7 @@ try_dev_shm:
                         return -EIO;
 
                 /* Let's reopen the thing, in order to get an O_RDONLY fd for the original O_RDWR one */
-                xsprintf(procfs_path, "/proc/self/fd/%i", fd);
-                r = open(procfs_path, O_RDONLY|O_CLOEXEC);
-                if (r < 0)
-                        return -errno;
-
-                return r;
+                return fd_reopen(fd, O_RDONLY|O_CLOEXEC);
         }
 
 try_dev_shm_without_o_tmpfile:
@@ -725,3 +719,22 @@ finish:
 
         return r;
 }
+
+int fd_reopen(int fd, int flags) {
+        char procfs_path[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int)];
+        int new_fd;
+
+        /* Reopens the specified fd with new flags. This is useful for convert an O_PATH fd into a regular one, or to
+         * turn O_RDWR fds into O_RDONLY fds.
+         *
+         * This doesn't work on sockets (since they cannot be open()ed, ever).
+         *
+         * This implicitly resets the file read index to 0. */
+
+        xsprintf(procfs_path, "/proc/self/fd/%i", fd);
+        new_fd = open(procfs_path, flags);
+        if (new_fd < 0)
+                return -errno;
+
+        return new_fd;
+}