Allow struct_magic to be disabled.

When a subsystem is disabled, there will be no corresponding object
allocated, and no magic numbers on it.

1 files changed, 20 insertions, 2 deletions

diff --git a/src/lib/confmgt/structvar.c b/src/lib/confmgt/structvar.c
index de678d18c8..a2411477d6 100644
--- a/src/lib/confmgt/structvar.c
+++ b/src/lib/confmgt/structvar.c
@@ -30,13 +30,28 @@
 #include <stddef.h>
 
 /**
+ * Return true iff all fields on <b>decl</b> are NULL or 0, indicating that
+ * there is no object or no magic number to check.
+ **/
+static inline bool
+magic_is_null(const struct_magic_decl_t *decl)
+{
+  return decl->typename == NULL &&
+    decl->magic_offset == 0 &&
+    decl->magic_val == 0;
+}
+
+/**
  * Set the 'magic number' on <b>object</b> to correspond to decl.
  **/
 void
 struct_set_magic(void *object, const struct_magic_decl_t *decl)
 {
-  tor_assert(object);
   tor_assert(decl);
+  if (magic_is_null(decl))
+    return;
+
+  tor_assert(object);
   uint32_t *ptr = STRUCT_VAR_P(object, decl->magic_offset);
   *ptr = decl->magic_val;
 }
@@ -47,8 +62,11 @@ struct_set_magic(void *object, const struct_magic_decl_t *decl)
 void
 struct_check_magic(const void *object, const struct_magic_decl_t *decl)
 {
-  tor_assert(object);
   tor_assert(decl);
+  if (magic_is_null(decl))
+    return;
+
+  tor_assert(object);
 
   const uint32_t *ptr = STRUCT_VAR_P(object, decl->magic_offset);
   tor_assertf(*ptr == decl->magic_val,