Memory leak

[paurkedal]

There seems to be a memory leak apparent by the stress tests from the example directory. To make the issue clear, I elevated the iteration counts,

diff --git a/examples/blocking/blocking_stress_test.ml b/examples/blocking/blocking_stress_test.ml
index 022e872..0ac77d4 100644
--- a/examples/blocking/blocking_stress_test.ml
+++ b/examples/blocking/blocking_stress_test.ml
@@ -41,4 +41,4 @@ let test () =
   M.Stmt.close !stmt |> or_die "Stmt.close";
   M.close dbh
 
-let () = for _ = 1 to 500 do test () done
+let () = for _ = 1 to 20000 do test () done
diff --git a/examples/nonblocking/nonblocking_stress_test.ml b/examples/nonblocking/nonblocking_stress_test.ml
index 8ba5807..bfda03a 100644
--- a/examples/nonblocking/nonblocking_stress_test.ml
+++ b/examples/nonblocking/nonblocking_stress_test.ml
@@ -163,5 +163,5 @@ module Make (W : Mariadb.Nonblocking.Wait) = struct
       stmt_cache (return ()) >>= fun () ->
     M.close dbh
 
-  let main () = repeat 500 test
+  let main () = repeat 5000 test
 end

and monitored it with top. For all three concurrency variants, the memory usage starts around 40 MiB and climbs steadily to around 160--180 MiB during the run on Ubuntu 18.04 with OCaml 4.07.1.

[paurkedal]

I did a re-test, and the conclusion is the same as noted in the comment #41 (comment) which I should have repeated here for clarity:

• The issue is completely fixed for the nonblocking test case.
• Memory usage is improved in the blocking cases, though it still grows without limit.