sysroot: Add _require_booted_deployment() API

This is a common pattern that is replicated both in our code
and in rpm-ostree a lot.  Let's add a canonical API.

src/libostree/libostree-devel.sym

@@ -25,6 +25,7 @@
 LIBOSTREE_2021.1 {
 global:
   ostree_commit_metadata_for_bootable;
+  ostree_sysroot_require_booted_deployment;
 } LIBOSTREE_2020.8;
 
 /* Stub section for the stable release *after* this development one; don't

src/libostree/ostree-sysroot-deploy.c

@@ -3066,9 +3066,9 @@ ostree_sysroot_stage_tree_with_options (OstreeSysroot     *self,
   if (!_ostree_sysroot_ensure_writable (self, error))
     return FALSE;
 
-  OstreeDeployment *booted_deployment = ostree_sysroot_get_booted_deployment (self);
+  OstreeDeployment *booted_deployment = ostree_sysroot_require_booted_deployment (self, error);
   if (booted_deployment == NULL)
-    return glnx_throw (error, "Cannot stage a deployment when not currently booted into an OSTree system");
+    return glnx_prefix_error (error, "Cannot stage deployment");
 
   /* This is used by the testsuite to exercise the path unit, until it becomes the default
    * (which is pending on the preset making it everywhere). */

src/libostree/ostree-sysroot.c

@@ -1246,6 +1246,27 @@ ostree_sysroot_get_booted_deployment (OstreeSysroot       *self)
   return self->booted_deployment;
 }
 
+
+/**
+ * ostree_sysroot_require_booted_deployment:
+ * @self: Sysroot
+ *
+ * Find the booted deployment, or return an error if not booted via OSTree.
+ *
+ * Returns: (transfer none) (not nullable): The currently booted deployment, or an error
+ * Since: 2021.1
+ */
+OstreeDeployment *
+ostree_sysroot_require_booted_deployment (OstreeSysroot *self, GError **error)
+{
+  g_return_val_if_fail (self->loadstate == OSTREE_SYSROOT_LOAD_STATE_LOADED, NULL);
+
+  if (!self->booted_deployment)
+    return glnx_null_throw (error, "Not currently booted into an OSTree system");
+  return self->booted_deployment;
+}
+
+
 /**
  * ostree_sysroot_get_staged_deployment:
  * @self: Sysroot

src/libostree/ostree-sysroot.h

@@ -85,6 +85,9 @@ GPtrArray *ostree_sysroot_get_deployments (OstreeSysroot  *self);
 _OSTREE_PUBLIC
 OstreeDeployment *ostree_sysroot_get_booted_deployment (OstreeSysroot *self);
 _OSTREE_PUBLIC
+OstreeDeployment *
+ostree_sysroot_require_booted_deployment (OstreeSysroot *self, GError **error);
+_OSTREE_PUBLIC
 OstreeDeployment *ostree_sysroot_get_staged_deployment (OstreeSysroot *self);
 
 _OSTREE_PUBLIC