error: Poison g_autoptr(Error) to prevent its use

The previous commit reverted support for g_autoptr(Error).  This one
should stop it from coming back.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Tested-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20251201194510.1121221-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

include/qapi/error.h

@@ -437,6 +437,26 @@ Error *error_copy(const Error *err);
  */
 void error_free(Error *err);
 
+/*
+ * Poison g_autoptr(Error) to prevent its use.
+ *
+ * Functions that report or propagate an error take ownership of the
+ * Error object.  Explicit error_free() is needed when you handle an
+ * error in some other way.  This is rare.
+ *
+ * g_autoptr(Error) would call error_free() automatically on return.
+ * To avoid a double-free, we'd have to manually clear the pointer
+ * every time we propagate or report.
+ *
+ * Thus, g_autoptr(Error) would make the rare case easier to get right
+ * (less prone to leaks), and the common case easier to get wrong
+ * (more prone to double-free).
+ */
+extern void
+__attribute__((error("Do not use g_autoptr() to declare Error * variables")))
+error_free_poisoned(Error *err);
+G_DEFINE_AUTOPTR_CLEANUP_FUNC(Error, error_free_poisoned)
+
 /*
  * Convenience function to assert that *@errp is set, then silently free it.
  */