repo/commit: Add support for --selinux-policy-from-base

The [dev-overlay](https://github.com/coreos/coreos-assembler/blob/332c6ab3b91778d904224c3c960d9cc4739d60bd/src/cmd-dev-overlay)
script shipped in coreos-assembler mostly exists to deal
with the nontrivial logic around SELinux policy.  Let's make
the use case of "commit some binaries overlaying a base tree, using
the base's selinux policy" just require a magical
`--selinux-policy-from-base` argument to `ostree commit`.

A new C API was added to implement this in the case of `--tree=ref`;
when the base directory is already checked out, we can just reuse
the existing logic that `--selinux-policy` was using.

Requires: https://github.com/ostreedev/ostree/pull/2039

src/libostree/ostree-repo-commit.c

@@ -4227,9 +4227,11 @@ ostree_repo_commit_modifier_unref (OstreeRepoCommitModifier *modifier)
   if (modifier->xattr_destroy)
     modifier->xattr_destroy (modifier->xattr_user_data);
 
-  g_clear_object (&modifier->sepolicy);
   g_clear_pointer (&modifier->devino_cache, (GDestroyNotify)g_hash_table_unref);
 
+  g_clear_object (&modifier->sepolicy);
+  (void) glnx_tmpdir_delete (&modifier->sepolicy_tmpdir, NULL, NULL);
+
   g_free (modifier);
   return;
 }
@@ -4279,6 +4281,60 @@ ostree_repo_commit_modifier_set_sepolicy (OstreeRepoCommitModifier
   modifier->sepolicy = sepolicy ? g_object_ref (sepolicy) : NULL;
 }
 
+/**
+ * ostree_repo_commit_modifier_set_sepolicy_from_commit:
+ * @modifier: Commit modifier
+ * @repo: OSTree repo containing @rev
+ * @rev: Find SELinux policy from this base commit
+ * @cancellable:
+ * @error:
+ *
+ * In many cases, one wants to create a "derived" commit from base commit.
+ * SELinux policy labels are part of that base commit.  This API allows
+ * one to easily set up SELinux labeling from a base commit.
+ */
+gboolean 
+ostree_repo_commit_modifier_set_sepolicy_from_commit (OstreeRepoCommitModifier              *modifier,
+                                                      OstreeRepo                            *repo,
+                                                      const char                            *rev,
+                                                      GCancellable                          *cancellable,
+                                                      GError                               **error)
+{
+  GLNX_AUTO_PREFIX_ERROR ("setting sepolicy from commit", error);
+  g_autofree char *commit = NULL;
+  g_autoptr(GFile) root = NULL;
+  if (!ostree_repo_read_commit (repo, rev, &root, &commit, cancellable, error))
+    return FALSE;
+  const char policypath[] = "usr/etc/selinux";
+  g_autoptr(GFile) policyroot = g_file_get_child (root, policypath);
+  if (!g_file_query_exists (policyroot, NULL))
+    return TRUE;  /* No policy, nothing to do */
+
+  GLnxTmpDir tmpdir = {0,};
+  if (!glnx_mkdtemp ("ostree-commit-sepolicy-XXXXXX", 0700, &tmpdir, error))
+    return FALSE;
+  if (!glnx_shutil_mkdir_p_at (tmpdir.fd, "usr/etc", 0755, cancellable, error))
+    return FALSE;
+
+  OstreeRepoCheckoutAtOptions coopts = {0,};
+  coopts.mode = OSTREE_REPO_CHECKOUT_MODE_USER;
+  coopts.subpath = glnx_strjoina ("/", policypath);
+
+  if (!ostree_repo_checkout_at (repo, &coopts, tmpdir.fd, policypath, commit, cancellable, error))
+    return glnx_prefix_error (error, "policy checkout");
+
+  g_autoptr(OstreeSePolicy) policy = ostree_sepolicy_new_at (tmpdir.fd, cancellable, error);
+  if (!policy)
+    return glnx_prefix_error (error, "reading policy");
+
+  ostree_repo_commit_modifier_set_sepolicy (modifier, policy);
+  /* Transfer ownership */
+  modifier->sepolicy_tmpdir = tmpdir;
+  tmpdir.initialized = FALSE;
+
+  return TRUE;
+}
+
 /**
  * ostree_repo_commit_modifier_set_devino_cache:
  * @modifier: Modifier