ostree-remount: Don't skip remount if root is composefs

When using composefs the root fs will always be read-only, but in this
case we should still continue remounting /sysroot. So, we record a
/run/ostree-composefs-root.stamp file in ostree-prepare-root if composefs
is used, and then react to it in ostree-remount.

src/switchroot/ostree-mount-util.h

@@ -32,6 +32,7 @@
 
 #define INITRAMFS_MOUNT_VAR "/run/ostree/initramfs-mount-var"
 #define _OSTREE_SYSROOT_READONLY_STAMP "/run/ostree-sysroot-ro.stamp"
+#define _OSTREE_COMPOSEFS_ROOT_STAMP "/run/ostree-composefs-root.stamp"
 
 static inline int
 path_is_on_readonly_fs (const char *path)

src/switchroot/ostree-prepare-root.c

@@ -330,7 +330,14 @@ main (int argc, char *argv[])
             err (EXIT_FAILURE, "Failed to mount composefs");
         }
       else
-        using_composefs = 1;
+        {
+          int fd = open (_OSTREE_COMPOSEFS_ROOT_STAMP, O_WRONLY | O_CREAT | O_CLOEXEC, 0644);
+          if (fd < 0)
+            err (EXIT_FAILURE, "failed to create %s", _OSTREE_COMPOSEFS_ROOT_STAMP);
+          (void)close (fd);
+
+          using_composefs = 1;
+        }
 #else
       err (EXIT_FAILURE, "Composefs not supported");
 #endif

src/switchroot/ostree-remount.c

@@ -95,7 +95,12 @@ main (int argc, char *argv[])
   if (mount ("none", "/sysroot", NULL, MS_REC | MS_PRIVATE, NULL) < 0)
     perror ("warning: While remounting /sysroot MS_PRIVATE");
 
-  if (path_is_on_readonly_fs ("/"))
+  bool root_is_composefs = false;
+  struct stat stbuf;
+  if (fstatat (AT_FDCWD, _OSTREE_COMPOSEFS_ROOT_STAMP, &stbuf, 0) == 0)
+    root_is_composefs = true;
+
+  if (path_is_on_readonly_fs ("/") && !root_is_composefs)
     {
       /* If / isn't writable, don't do any remounts; we don't want
        * to clear the readonly flag in that case.