core: Add knob to make it easier to test out state overlay model

I want people to be able to test out the state overlay approach without
having to switch over the base compose in e.g. FCOS to use it.

Hackily add an env var that will forcibly fix the symlinks in the OSTree
commit when doing a local assembly. This is enough to allow properly
checking out RPMs that install into `/opt` and `/usr/local`. Have it also
enable the systemd services for the overlays. (In `/usr`, so that if the
knob is removed, the services are no longer enabled.)

src/libpriv/rpmostree-core.cxx

@@ -4045,6 +4045,55 @@ rpmostree_context_assemble (RpmOstreeContext *self, GCancellable *cancellable, G
   if (!build_rootfs_usrlinks (self, error))
     return FALSE;
 
+  /* This is purely for making it easier for people to test out the
+   * state-overlay stuff until it's stabilized and part of base composes. */
+  if (g_getenv ("RPMOSTREE_EXPERIMENTAL_FORCE_OPT_USRLOCAL_OVERLAY"))
+    {
+      rpmostree_output_message (
+          "Enabling experimental state overlay support for /opt and /usr/local");
+
+      struct stat stbuf;
+
+      if (!glnx_fstatat_allow_noent (tmprootfs_dfd, "opt", &stbuf, AT_SYMLINK_NOFOLLOW, error))
+        return FALSE;
+      if (errno == ENOENT || (errno == 0 && S_ISLNK (stbuf.st_mode)))
+        {
+          if (errno == 0 && !glnx_unlinkat (tmprootfs_dfd, "opt", 0, error))
+            return FALSE;
+          if (symlinkat ("usr/lib/opt", tmprootfs_dfd, "opt") < 0)
+            return glnx_throw_errno_prefix (error, "symlinkat(/opt)");
+        }
+
+      if (!glnx_fstatat_allow_noent (tmprootfs_dfd, "usr/local", &stbuf, AT_SYMLINK_NOFOLLOW,
+                                     error))
+        return FALSE;
+      if (errno == ENOENT || (errno == 0 && S_ISLNK (stbuf.st_mode)))
+        {
+          if (errno == 0 && !glnx_unlinkat (tmprootfs_dfd, "usr/local", 0, error))
+            return FALSE;
+          if (mkdirat (tmprootfs_dfd, "usr/local", 0755) < 0)
+            return glnx_throw_errno_prefix (error, "mkdirat(/usr/local)");
+        }
+
+      if (!glnx_shutil_mkdir_p_at (tmprootfs_dfd, "usr/lib/systemd/system/local-fs.target.wants",
+                                   0755, cancellable, error))
+        return FALSE;
+
+      if (symlinkat ("[email protected]", tmprootfs_dfd,
+                     "usr/lib/systemd/system/local-fs.target.wants/"
+                     "[email protected]")
+              < 0
+          && errno != EEXIST)
+        return glnx_throw_errno_prefix (error, "enabling ostree-state-overlay for /usr/lib/opt");
+
+      if (symlinkat (
+              "[email protected]", tmprootfs_dfd,
+              "usr/lib/systemd/system/local-fs.target.wants/[email protected]")
+              < 0
+          && errno != EEXIST)
+        return glnx_throw_errno_prefix (error, "enabling ostree-state-overlay for /usr/local");
+    }
+
   /* We need up to date labels; the set of things needing relabeling
    * will have been calculated in sort_packages()
    */