libcrun: document vfork shared error

There are examples in the source code where the vfork parent
process reuses an error created by the vfork child process.
Document that there is no need to create an
error in the parent process in such cases.

Related issue:
containers#2015

Amended the suggestion from gemini-code-assist in
containers#2018
with some modifications.

Signed-off-by: Erik Sjölund <erik.sjolund@gmail.com>

Author: eriksjolund

src/libcrun/linux.c

@@ -6216,6 +6216,8 @@ run_in_container_namespace (libcrun_container_status_t *status, int (*callback)
   if (UNLIKELY (ret < 0))
     return crun_make_error (err, errno, "waitpid for exec child pid");
 
+  /* Since vfork() is used, the child process shares the parent's memory.
+   * This allows the parent to reuse the error created by the child.  */
   return get_process_exit_status (wait_status);
 }
 

src/libcrun/net_device.c

@@ -484,7 +484,11 @@ move_network_device (const char *ifname, const char *newifname, int netns_fd, li
     return crun_make_error (err, errno, "waitpid for exec child pid");
 
   if (wait_status != 0)
-    return -get_process_exit_status (wait_status);
+    {
+      /* Since vfork() is used, the child process shares the parent's memory.
+       * This allows the parent to reuse the error created by the child.  */
+      return -get_process_exit_status (wait_status);
+    }
 
   return 0;
 }