tests for Unix.select and introduce select-as-epoll

[edwintorok]

First write tests for the semantics of Unix.select.
Then introduce a Unixext.select implemented using epoll, and switch the tests to test this function: now we know that Unixext.select semantics matches Unix.select.
(we don't test error semantics, because XAPI only calls Unix.select with an empty list as the error list).

This doesn't yet switch the code to use Unixext.select, that will be done on feature/perf as it needs more testing.

Draft, needs testing in koji and some ticket numbers.
Also I don't know why the BLK tests haven't failed before on master, maybe they are not being run correctly, they were supposed to run as part of the Quicktests...

[edwintorok]

Ah Unixext_test is not part of quicktest.ml, let me fix that.

[edwintorok]

Added, and it passes now:

ulimit -n 2048
./quicktest.exe -skip-xapi -- test unixext
qcheck random seed: 724901538
Testing `Quicktests'.
This run has ID `8CKKL4GO'.

  [SKIP]        Quicktest_example                         0   example test.
  [SKIP]        xenstore                                  0   basic.
  [SKIP]        xenstore                                  1   watch_within_timeout.
  [SKIP]        xenstore                                  2   watch_exceed_timeout.
  [SKIP]        event                                     0   event_next_unblocking_test.
  [SKIP]        event                                     1   event_next_test.
  [SKIP]        event                                     2   event_from_test.
  [SKIP]        event                                     3   event_from_parallel_test.
  [SKIP]        event                                     4   object_level_event_test.
  [SKIP]        event                                     5   event_message_test.
  [SKIP]        event                                     6   event_inject_test.
  [SKIP]        event                                     7   event_from_number_test.
  [SKIP]        import_raw_vdi                            0   import_raw_vdi.
  [SKIP]        Quicktest_date                            0   host.get_server_localtime.
  [SKIP]        Quicktest_date                            1   message.get_since.
  [OK]          unixext                                   0   Unixext_test.test_select.
  [OK]          unixext                                   1   Unixext_test.test_proxy.
  [OK]          unixext                                   2   Unixext_test.test_time_limited_write.
  [OK]          unixext                                   3   Unixext_test.test_time_limited_read.
  [SKIP]        bufio                                     0   Bufio_test.test_buf_io.
  [SKIP]        Timer                                     0   Timer.remaining.

[edwintorok]

The tests are now run by 'quicktest' in Dom0 too:

[...] 
  [OK]          unixext                                   0   Unixext_test.te...
  [OK]          unixext                                   1   Unixext_test.te...
  [OK]          unixext                                   2   Unixext_test.te...
  [OK]          unixext                                   3   Unixext_test.te...
  [OK]          bufio                                     0   Bufio_test.test...
  [OK]          Timer                                     0   Timer.remaining.

Also included #5912 to fix the CI

[Vincent-lau]

Looks good to me. Most of these are unit tests so should be relatively low risk

[edwintorok]

I've added 2 comments:

@@ -684,6 +684,8 @@ let pollerr_set = Polly.Events.pri

 let to_milliseconds ms = ms *. 1e3 |> ceil |> int_of_float

+(* we could change lists to proper Sets once the Unix.select to Unixext.select conversion is done *)
+
 let readable fd (rd, wr, ex) = (fd :: rd, wr, ex)

 let writable fd (rd, wr, ex) = (rd, fd :: wr, ex)
@@ -723,6 +723,7 @@ let select ins outs errs timeout =
       Unix.sleepf timeout ; no_events
   | _ -> (
       with_polly @@ fun polly ->
+      (* file descriptors that cannot be watched by epoll *)
       let immediate =
         no_events
         |> polly_fold polly pollin_set ins readable