switchroot: Allow letting ostree-prepare-root mount /var

In some scenarios, it might make sense to let `ostree-prepare-root` do
the `/var` mount from the state root as before. For example, one may
want to do some system configuration before the switch root. This of
course comes at the expense of supporting `/var` as a mount point in
`/etc/fstab`.

Closes: #1617
Approved by: cgwalters

src/switchroot/ostree-mount-util.h

@@ -31,6 +31,8 @@
 #include <fcntl.h>
 #include <string.h>
 
+#define INITRAMFS_MOUNT_VAR "/run/ostree/initramfs-mount-var"
+
 static inline int
 path_is_on_readonly_fs (char *path)
 {

src/switchroot/ostree-prepare-root.c

@@ -151,13 +151,18 @@ main(int argc, char *argv[])
   if (chdir (deploy_path) < 0)
     err (EXIT_FAILURE, "failed to chdir to deploy_path");
 
-  /* In the systemd case, this is handled by ostree-system-generator */
+  bool mount_var = true;
+  /* In the systemd case, this is handled by ostree-system-generator by default */
 #ifndef HAVE_SYSTEMD_AND_LIBMOUNT
-  /* Link to the deployment's /var */
-  if (mount ("../../var", "var", NULL, MS_MGC_VAL|MS_BIND, NULL) < 0)
-    err (EXIT_FAILURE, "failed to bind mount ../../var to var");
+  /* file in /run can override that behaviour */
+  if (lstat (INITRAMFS_MOUNT_VAR, &stbuf) < 0)
+    mount_var = false;
 #endif
 
+  /* Link to the deployment's /var */
+  if (mount_var && mount ("../../var", "var", NULL, MS_MGC_VAL|MS_BIND, NULL) < 0)
+    err (EXIT_FAILURE, "failed to bind mount ../../var to var");
+
   char srcpath[PATH_MAX];
   /* If /boot is on the same partition, use a bind mount to make it visible
    * at /boot inside the deployment. */

src/switchroot/ostree-system-generator.c

@@ -49,6 +49,16 @@ main(int argc, char *argv[])
       exit (EXIT_SUCCESS);
   }
 
+  /* We conflict with the magic ostree-mount-deployment-var file for ostree-prepare-root */
+  { struct stat stbuf;
+    if (fstatat (AT_FDCWD, INITRAMFS_MOUNT_VAR, &stbuf, 0) == 0)
+      {
+        if (unlinkat (AT_FDCWD, INITRAMFS_MOUNT_VAR, 0) < 0)
+          err (EXIT_FAILURE, "Can't unlink " INITRAMFS_MOUNT_VAR);
+        exit (EXIT_SUCCESS);
+      }
+  }
+
   if (argc > 1 && argc != 4)
     errx (EXIT_FAILURE, "This program takes three or no arguments");