[Libguestfs] libnbd: When are callbacks freed

[Tage Johansson]

Apologize if resending, but I'm not sure my previous email was actually 
delivered.


On 7/12/2023 10:33 PM, Eric Blake wrote:


> On Wed, Jul 12, 2023 at 03:19:59PM +0000, Tage Johansson wrote:
>> Hello,
>>
>>
>> While writing some tests for the Rust bindings, I discovered a memory 
>> leak
>> with Valgrind due to a completion callback not being freed. More 
>> specificly,
>> the completion callback of `aio_opt_info()` doesn't seem to be if
>> `aio_opt_info()` failes. In this case, `aio_opt_info()` was called in the
>> connected state, so it should indeed fail, but it should perhaps also 
>> call
>> the free function associated with the callback.
> Can you paste the valgrind output showing the leak?


The Valgrind output is very Rust specific and only shows the Rust 
allocations which goes into the completion callback and are lost.


But if you apply the following patch (which modifies tests/opt-info.c) 
you shall see that the completion callback is not called.

I have replaced a call to `nbd_opt_info()` with a call to 
`nbd_aio_opt_info()` and passed in a completion callback which just 
calls `exit(EXIT_FAILURE)`. So if the completion callback is called the 
test should fail, which it doesn't, at least not on my machine.


diff --git a/tests/opt-info.c b/tests/opt-info.c
index 86a0608..ba5c72b 100644
--- a/tests/opt-info.c
+++ b/tests/opt-info.c
@@ -30,6 +30,12 @@

  #include <libnbd.h>

+int
+my_completion_cb(void *_user_data, int *_err)
+{
+  exit(EXIT_FAILURE);
+}
+
  int
  main (int argc, char *argv[])
  {
@@ -201,7 +207,10 @@ main (int argc, char *argv[])
      fprintf (stderr, "expecting export name 'good', got '%s'\n", s);
      exit (EXIT_FAILURE);
    }
-  if (nbd_opt_info (nbd) != -1) {
+  if (nbd_aio_opt_info (
+        nbd,
+        (nbd_completion_callback) { .callback = my_completion_cb }
+      ) != -1) {
      fprintf (stderr, "expecting error for opt_info\n");
      exit (EXIT_FAILURE);
    }

Best regards,

Tage