aboutsummaryrefslogtreecommitdiff
path: root/mod/mod-ci-github.cxx
blob: 3ff2330424f55c3c925e3d9687b66c27691e8a52 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
// file      : mod/mod-ci-github.cxx -*- C++ -*-
// license   : MIT; see accompanying LICENSE file

#include <mod/mod-ci-github.hxx>

#include <libbutl/json/parser.hxx>

#include <mod/jwt.hxx>
#include <mod/hmac.hxx>
#include <mod/module-options.hxx>

#include <mod/mod-ci-github-gq.hxx>
#include <mod/mod-ci-github-post.hxx>
#include <mod/mod-ci-github-service-data.hxx>

#include <stdexcept>

// @@ TODO
//
//    Building CI checks with a GitHub App
//    https://docs.github.com/en/apps/creating-github-apps/writing-code-for-a-github-app/building-ci-checks-with-a-github-app
//

// @@ TODO Best practices
//
//    Webhooks:
//    https://docs.github.com/en/webhooks/using-webhooks/best-practices-for-using-webhooks
//    https://docs.github.com/en/webhooks/using-webhooks/validating-webhook-deliveries
//
//    REST API:
//    https://docs.github.com/en/rest/using-the-rest-api/best-practices-for-using-the-rest-api?apiVersion=2022-11-28
//
//    Creating an App:
//    https://docs.github.com/en/apps/creating-github-apps/about-creating-github-apps/best-practices-for-creating-a-github-app
//
//    Use a webhook secret to ensure request is coming from Github. HMAC:
//    https://en.wikipedia.org/wiki/HMAC#Definition. A suitable implementation
//    is provided by OpenSSL.

// @@ TODO Centralize exception/error handling around calls to
//         github_post(). Currently it's mostly duplicated and there is quite
//         a lot of it.
//
using namespace std;
using namespace butl;
using namespace web;
using namespace brep::cli;

namespace brep
{
  ci_github::
  ci_github (tenant_service_map& tsm)
      : tenant_service_map_ (tsm)
  {
  }

  ci_github::
  ci_github (const ci_github& r, tenant_service_map& tsm)
      : handler (r),
        ci_start (r),
        options_ (r.initialized_ ? r.options_ : nullptr),
        tenant_service_map_ (tsm)
  {
  }

  void ci_github::
  init (scanner& s)
  {
    {
      shared_ptr<tenant_service_base> ts (
        dynamic_pointer_cast<tenant_service_base> (shared_from_this ()));

      assert (ts != nullptr); // By definition.

      tenant_service_map_["ci-github"] = move (ts);
    }

    options_ = make_shared<options::ci_github> (
      s, unknown_mode::fail, unknown_mode::fail);

    // Prepare for the CI requests handling, if configured.
    //
    if (options_->ci_github_app_webhook_secret_specified ())
    {
      ci_start::init (make_shared<options::ci_start> (*options_));
    }
  }

  bool ci_github::
  handle (request& rq, response&)
  {
    using namespace bpkg;

    HANDLER_DIAG;

    if (!options_->ci_github_app_webhook_secret_specified ())
      throw invalid_request (404, "GitHub CI request submission disabled");

    // Process headers.
    //
    // @@ TMP Shouldn't we also error<< in some of these header problem cases?
    //
    // @@ TMP From GitHub docs: "You can create webhooks that subscribe to the
    //        events listed on this page."
    //
    //        So it seems appropriate to generally use the term "event" (which
    //        we already do for the most part), and "webhook event" only when
    //        more context would be useful?
    //
    string event; // Webhook event.
    string hmac;  // Received HMAC.
    {
      bool content_type (false);

      for (const name_value& h: rq.headers ())
      {
        // HMAC authenticating this request. Note that it won't be present
        // unless a webhook secret has been set in the GitHub app's settings.
        //
        if (icasecmp (h.name, "x-hub-signature-256") == 0)
        {
          if (!h.value)
            throw invalid_request (400, "missing x-hub-signature-256 value");

          // Parse the x-hub-signature-256 header value. For example:
          //
          // sha256=5e82258...
          //
          // Check for the presence of the "sha256=" prefix and then strip it
          // to leave only the HMAC value.
          //
          if (h.value->find ("sha256=", 0, 7) == string::npos)
            throw invalid_request (400, "invalid x-hub-signature-256 value");

          hmac = h.value->substr (7);
        }
        // This event's UUID.
        //
        else if (icasecmp (h.name, "x-github-delivery") == 0)
        {
          // @@ TODO Check that delivery UUID has not been received before
          //         (replay attack).
        }
        else if (icasecmp (h.name, "content-type") == 0)
        {
          if (!h.value)
            throw invalid_request (400, "missing content-type value");

          if (icasecmp (*h.value, "application/json") != 0)
          {
            throw invalid_request (400,
                                   "invalid content-type value: '" + *h.value +
                                   '\'');
          }

          content_type = true;
        }
        // The webhook event.
        //
        else if (icasecmp (h.name, "x-github-event") == 0)
        {
          if (!h.value)
            throw invalid_request (400, "missing x-github-event value");

          event = *h.value;
        }
      }

      if (!content_type)
        throw invalid_request (400, "missing content-type header");

      if (event.empty ())
        throw invalid_request (400, "missing x-github-event header");

      if (hmac.empty ())
        throw invalid_request (400, "missing x-hub-signature-256 header");
    }

    // Read the entire request body into a buffer because we need to compute
    // an HMAC over it and then parse it as JSON. The alternative of reading
    // from the stream twice works out to be more complicated (see also @@
    // TODO item in web/server/module.hxx).
    //
    string body;
    {
      // Note that even though we may not need caching right now, we may later
      // (e.g., to support cancel) so let's just enable it right away.
      //
      size_t limit (128 * 1024);

      istream& is (rq.content (limit, limit));

      try
      {
        getline (is, body, '\0');
      }
      catch (const io_error& e)
      {
        fail << "unable to read request body: " << e;
      }
    }

    // Verify the received HMAC.
    //
    // Compute the HMAC value over the request body using the configured
    // webhook secret as key and compare it to the received HMAC.
    //
    try
    {
      string h (
        compute_hmac (*options_,
                      body.data (), body.size (),
                      options_->ci_github_app_webhook_secret ().c_str ()));

      if (!icasecmp (h, hmac))
      {
        string m ("computed HMAC does not match received HMAC");

        error << m;

        throw invalid_request (400, move (m));
      }
    }
    catch (const system_error& e)
    {
      fail << "unable to compute request HMAC: " << e;
    }

    // There is a webhook event (specified in the x-github-event header) and
    // each event contains a bunch of actions (specified in the JSON request
    // body).
    //
    // Note: "GitHub continues to add new event types and new actions to
    // existing event types." As a result we ignore known actions that we are
    // not interested in and log and ignore unknown actions. The thinking here
    // is that we want be "notified" of new actions at which point we can decide
    // whether to ignore them or to handle.
    //
    if (event == "check_suite")
    {
      gh_check_suite_event cs;
      try
      {
        json::parser p (body.data (), body.size (), "check_suite event");

        cs = gh_check_suite_event (p);
      }
      catch (const json::invalid_json_input& e)
      {
        string m ("malformed JSON in " + e.name + " request body");

        error << m << ", line: " << e.line << ", column: " << e.column
              << ", byte offset: " << e.position << ", error: " << e;

        throw invalid_request (400, move (m));
      }

      if (cs.action == "requested")
      {
        return handle_check_suite_request (move (cs));
      }
      else if (cs.action == "rerequested")
      {
        // Someone manually requested to re-run the check runs in this check
        // suite. Treat as a new request.
        //
        return handle_check_suite_request (move (cs));
      }
      else if (cs.action == "completed")
      {
        // GitHub thinks that "all the check runs in this check suite have
        // completed and a conclusion is available". Looks like this one we
        // ignore?
        //
        // @@ TODO What if our bookkeeping says otherwise? See conclusion
        //    field which includes timedout. Need to come back to this once
        //    have the "happy path" implemented.
        //
        return true;
      }
      else
      {
        // Ignore unknown actions by sending a 200 response with empty body
        // but also log as an error since we want to notice new actions.
        //
        error << "unknown action '" << cs.action << "' in check_suite event";

        return true;
      }
    }
    else if (event == "pull_request")
    {
      // @@ TODO

      throw invalid_request (501, "pull request events not implemented yet");
    }
    else
    {
      // Log to investigate.
      //
      error << "unexpected event '" << event << "'";

      throw invalid_request (400, "unexpected event: '" + event + "'");
    }
  }

  bool ci_github::
  handle_check_suite_request (gh_check_suite_event cs)
  {
    HANDLER_DIAG;

    l3 ([&]{trace << "check_suite event { " << cs << " }";});

    optional<string> jwt (generate_jwt (trace, error));
    if (!jwt)
      throw server_error ();

    optional<gh_installation_access_token> iat (
        obtain_installation_access_token (cs.installation.id,
                                          move (*jwt),
                                          error));

    if (!iat)
      throw server_error ();

    l3 ([&]{trace << "installation_access_token { " << *iat << " }";});

    // Submit the CI request.
    //
    repository_location rl (cs.repository.clone_url + '#' +
                                cs.check_suite.head_branch,
                            repository_type::git);

    string sd (service_data (move (iat->token),
                             iat->expires_at,
                             cs.installation.id,
                             move (cs.repository.node_id),
                             move (cs.check_suite.head_sha))
                   .json ());

    optional<start_result> r (
        start (error,
               warn,
               verb_ ? &trace : nullptr,
               tenant_service (move (cs.check_suite.node_id),
                               "ci-github",
                               move (sd)),
               move (rl),
               vector<package> {},
               nullopt, // client_ip
               nullopt  // user_agent
               ));

    if (!r)
      fail << "unable to submit CI request";

    return true;
  }

  // Build state change notifications (see tenant-services.hxx for
  // background). Mapping our state transitions to GitHub pose multiple
  // problems:
  //
  // 1. In our model we have the building->queued (interrupted) and
  //    built->queued (rebuild) transitions. We are going to ignore both of
  //    them when notifying GitHub. The first is not important (we expect the
  //    state to go back to building shortly). The second should normally not
  //    happen and would mean that a completed check suite may go back on its
  //    conclusion (which would be pretty confusing for the user).
  //
  //    So, for GitHub notifications, we only have the following linear
  //    transition sequence:
  //
  //    -> queued -> building -> built
  //
  //    Note, however, that because we ignore certain transitions, we can now
  //    observe "degenerate" state changes that we need to ignore:
  //
  //    building -> [queued] -> building
  //    built -> [queued] -> ...
  //
  // 2. As mentioned in tenant-services.hxx, we may observe the notifications
  //    as arriving in the wrong order. Unfortunately, GitHub provides no
  //    mechanisms to help with that. In fact, GitHub does not even prevent
  //    the creation of multiple check runs with the same name (it will always
  //    use the last created instance, regardless of the status, timestamps,
  //    etc). As a result, we cannot, for example, rely on the failure to
  //    create a new check run in response to the queued notification as an
  //    indication of a subsequent notification (e.g., building) having
  //    already occurred.
  //
  //    The only aid in this area that GitHub provides is that it prevents
  //    updating a check run in the built state to a former state (queued or
  //    building). But one can still create a new check run with the same name
  //    and a former state.
  //
  //    (Note that we should also be careful if trying to take advantage of
  //    this "check run override" semantics: each created check run gets a new
  //    URL and while the GitHub UI will always point to the last created when
  //    showing the list of check runs, if the user is already on the previous
  //    check run's URL, nothing will automatically cause them to be
  //    redirected to the new URL. And so the user may sit on the abandoned
  //    check run waiting forever for it to completed.)
  //
  //    As a result, we will deal with the out of order problem differently
  //    depending on the notification:
  //
  //    queued    Skip if there is already a check run in service data,
  //              otherwise create new.
  //
  //    building  Skip if there is no check run in service data or it's
  //              not in the queued state, otherwise update.
  //
  //    built     Update if there is check run in service data and its
  //              state is not built, otherwise create new.
  //
  //    The rationale for this semantics is as follows: the building
  //    notification is a "nice to have" and can be skipped if things are not
  //    going normally. In contrast, the built notification cannot be skipped
  //    and we must either update the existing check run or create a new one
  //    (hopefully overriding the one created previously, if any). Note that
  //    the likelihood of the built notification being performed at the same
  //    time as queued/building is quite low (unlike queued and building).
  //
  //    Note also that with this semantics it's unlikely but possible that we
  //    attempt to updat the service data in the wrong order. Specifically, it
  //    feels like this should not be possible in the ->building transition
  //    since we skip the building notification unless the check run in the
  //    service data is already in the queued state. But it is theoretically
  //    possible in the ->built transition. For example, we may be updating
  //    the service data for the queued notification after it has already been
  //    updated by the built notification. In such cases we should not be
  //    overriding the latter state (built) with the former (queued).
  //
  // 3. We may not be able to "conclusively" notify GitHub, for example, due
  //    to a transient network error. The "conclusively" part means that the
  //    notification may or may not have gone through (though it feels the
  //    common case will be the inability to send the request rather than
  //    receive the reply).
  //
  //    In such cases, we record in the service data that the notification was
  //    no synchronized and in subsequent notifications we do the best we can:
  //    if we have node_id, then we update, otherwise, we create (potentially
  //    overriding the check run created previously).
  //

  function<optional<string> (const tenant_service&)> ci_github::
  build_queued (const tenant_service& ts,
                const vector<build>& builds,
                optional<build_state> istate,
                const build_hints& hs,
                const diag_epilogue& log_writer) const noexcept
  {
    NOTIFICATION_DIAG (log_writer);

    service_data sd;
    try
    {
      sd = service_data (*ts.data);
    }
    catch (const invalid_argument& e)
    {
      error << "failed to parse service data: " << e;
      return nullptr;
    }

    // All builds except those for which this notification is out of order and
    // thus would cause a spurious backwards state transition.
    //
    vector<reference_wrapper<const build>> bs;
    vector<check_run> crs; // Parallel to bs.

    // Exclude builds for which this is an out of order notification.
    //
    for (const build& b: builds)
    {
      // To keep things simple we are going to queue/create a new check run
      // only if we have no corresponding state (which means we haven't yet
      // done anything about this check run).
      //
      // In particular, this will ignore the building->queued (interrupted)
      // transition so on GitHub the check run will continue showing as
      // building, which is probably not a big deal. Also, this sidesteps
      // various "absent state" corner.
      //
      // Note: never go back on the built state.
      //
      string bid (gh_check_run_name (b)); // Full Build ID.

      const check_run* scr (sd.find_check_run (bid));

      if (scr == nullptr)
      {
        crs.emplace_back (move (bid), nullopt, nullopt);
        bs.push_back (b);
      }
      else if (!scr->state)
        ; // Ignore network issue.
      else if (istate && *istate == build_state::building)
        ; // Ignore interrupted.
      else
      {
        // Out of order queued notification or a rebuild (not allowed).
        //
        warn << *scr << ": "
             << "unexpected transition from "
             << (istate ? to_string (*istate) : "null") << " to "
             << to_string (build_state::queued)
             << "; previously recorded check_run state: "
             << scr->state_string ();
      }
    }

    if (bs.empty ()) // Notification is out of order for all builds.
      return nullptr;

    // What if we could not notify GitHub about some check runs due to, say, a
    // transient network? In this case we save them with the absent state
    // hoping for things to improve when we try to issue building or built
    // notifications.

    // Get a new installation access token if the current one has expired.
    //
    const gh_installation_access_token* iat (nullptr);
    optional<gh_installation_access_token> new_iat;

    if (system_clock::now () > sd.installation_access.expires_at)
    {
      if (optional<string> jwt = generate_jwt (trace, error))
      {
        new_iat = obtain_installation_access_token (sd.installation_id,
                                                    move (*jwt),
                                                    error);
        if (new_iat)
          iat = &*new_iat;
      }
    }
    else
      iat = &sd.installation_access;

    if (iat != nullptr)
    {
      // @@ TODO Check whether any of these check runs exist on GH before
      //         creating them.
      //
      // Queue a check_run for each build.
      //
      if (gq_create_check_runs (crs,
                                iat->token,
                                sd.repository_id, sd.head_sha,
                                bs,
                                build_state::queued,
                                hs,
                                error))
      {
        for (check_run& cr: crs)
          l3 ([&] { trace << "created check_run { " << cr << " }"; });
      }
    }

    return [bs = move (bs),
            iat = move (new_iat),
            crs = move (crs),
            error = move (error),
            warn = move (warn)] (const tenant_service& ts) -> optional<string>
    {
      // NOTE: this lambda may be called repeatedly (e.g., due to transaction
      // being aborted) and so should not move out of its captures.

      service_data sd;
      try
      {
        sd = service_data (*ts.data);
      }
      catch (const invalid_argument& e)
      {
        error << "failed to parse service data: " << e;
        return nullopt;
      }

      if (iat)
        sd.installation_access = *iat;

      // Note that we've already ignored all the builds for which this
      // notification was out of order.
      //
      for (size_t i (0); i != bs.size (); ++i)
      {
        const check_run& cr (crs[i]);

        // Note that this service data may not be the same as what we observed
        // in the build_queued() function above. For example, some check runs
        // that we have queued may have already transitioned to building. So
        // we skip any check runs that are already present.
        //
        if (check_run* scr = sd.find_check_run (cr.build_id))
        {
          warn << cr << " state " << scr->state_string ()
               << " was stored before notified state " << cr.state_string ()
               << " could be stored";
        }
        else
          sd.check_runs.push_back (cr);
      }

      return sd.json ();
    };
  }

  function<optional<string> (const tenant_service&)> ci_github::
  build_building (const tenant_service& ts, const build& b,
                  const build_hints& hs,
                  const diag_epilogue& log_writer) const noexcept
  {
    // Note that we may receive this notification before the corresponding
    // check run object has been persisted in the service data (see the
    // returned by build_queued() lambda for details). Thus we wouldn't know
    // whether the check run has been created on GitHub yet or not. And given
    // that, on GitHub, creating a check run with an existent name does not
    // fail but instead replaces the existing check run (same name, different
    // node ID), we have to check whether a check run already exists on GitHub
    // before creating it.
    //
    // @@ TMP Will have to do this in build_queued() as well.
    //
    NOTIFICATION_DIAG (log_writer);

    service_data sd;
    try
    {
      sd = service_data (*ts.data);
    }
    catch (const invalid_argument& e)
    {
      error << "failed to parse service data: " << e;
      return nullptr;
    }

    check_run cr; // Updated check run.

    // Get a new installation access token if the current one has expired.
    //
    const gh_installation_access_token* iat (nullptr);
    optional<gh_installation_access_token> new_iat;

    if (system_clock::now () > sd.installation_access.expires_at)
    {
      if (optional<string> jwt = generate_jwt (trace, error))
      {
        new_iat = obtain_installation_access_token (sd.installation_id,
                                                    move (*jwt),
                                                    error);
        if (new_iat)
          iat = &*new_iat;
      }
    }
    else
      iat = &sd.installation_access;

    if (iat != nullptr)
    {
      string bid (gh_check_run_name (b)); // Full Build ID.

      // Stored check run.
      //
      const check_run* scr (sd.find_check_run (bid));

      if (scr != nullptr && scr->node_id)
      {
        // The check run exists on GitHub and in the persisted service data.
        //
        if (!scr->state || *scr->state == build_state::queued)
        {
          // If the stored state is queued (most likely), at least
          // build_queued() and its lambda has run. If this is all then GitHub
          // has the queued status. But it's also possible that build_built()
          // has also run and updated GitHub to built but its lambda has not
          // yet run.
          //
          // On the other hand, if there is no stored state, both of the other
          // notifications must have run: the first created the check run on
          // GitHub and stored the node ID; the second failed to update GitHub
          // and stored the nullopt state. Either way around, build_built()
          // has run so built is the correct state.
          //
          //   @@ TMP Maybe we should always store the state and add a flag
          //          like "gh_updated", then we would know for sure whether
          //          we need to update to built or building.
          //
          // So GitHub currently either has queued or built but we can't be
          // sure which.
          //
          // If it has queued, updating to building is correct.
          //
          // If it has built then it would be a logical mistake to update to
          // building. However, GitHub ignores updates from built to any other
          // state (the REST API responds with HTTP 200 and the full check run
          // JSON body but with the "completed" status; presumably the GraphQL
          // API has the same semantics) so we can just try to update to
          // building and see what the actual status it returns is.
          //
          // If scr->state is nullopt then GitHub has either queued or built
          // but we know build_built() has run so we need to update to built
          // instead of building.
          //
          cr = move (*scr);
          cr.state = nullopt;

          build_state st (scr->state ? build_state::building
                                     : build_state::built);

          if (gq_update_check_run (cr,
                                   iat->token,
                                   sd.repository_id,
                                   *cr.node_id,
                                   st,
                                   error))
          {
            // @@ TODO If !scr->state and GH had built then we probably don't
            //         want to run the lambda either but currently it will run
            //         and this update message is not accurate. Is stored
            //         failed state the only way?
            //
            if (cr.state == st)
              l3 ([&]{trace << "updated check_run { " << cr << " }";});
            else
            {
              // Do not persist anything if state was already built on
              // GitHub.
              //
              assert (cr.state == build_state::built);

              return nullptr;
            }
          }
        }
        else if (*scr->state == build_state::built)
        {
          // Ignore out of order built notification.
          //
          assert (*scr->state == build_state::built);

          warn << *scr << ": "
               << "out of order transition from "
               << to_string (build_state::queued) << " to "
               << to_string (build_state::building) <<
            " (stored state is " << to_string (build_state::built) << ")";

          return nullptr;
        }
      }
      else // (src == nullptr || !scr->node_id)
      {
        // No state has been persisted, or one or both of the other
        // notifications were unable to create the check run on GitHub.
        //
        // It's also possible that another notification has since created the
        // check run on GitHub but its lambda just hasn't run yet.
        //
        // Thus the check run may or may not exist on GitHub.
        //
        // Because creation destroys check runs with the same name (see
        // comments at top of function) we have to check whether the check run
        // exists on GitHub before we can do anything.
        //
        // Destructive creation would be catastrophic if, for example, our
        // "new" building check run replaced the existing built one because it
        // would look exactly like a transition from built back to building in
        // the GitHub UI. And then the built lambda could run after our
        // building lambda, creating an opportunity for the real node ID to be
        // overwritten with the old one.
        //
        cr.build_id = move (bid);

        // Fetch the check run by name from GitHub.
        //
        pair<optional<gh_check_run>, bool> pr (
            gq_fetch_check_run (iat->token,
                                ts.id,
                                gh_check_run_name (b, &hs),
                                error));

        if (pr.second) // No errors.
        {
          if (!pr.first) // Check run does not exist on GitHub.
          {
            // Assume the most probable cases: build_queued() failed to create
            // the check run or build_building() is running before
            // build_queued(), so creating with building state is
            // correct. (The least likely being that build_built() ran before
            // this, in which case we should create with the built state.)
            //
            // @@ TODO Create with whatever the failed state was if we decide
            //         to store it.
            //
            if (gq_create_check_run (cr,
                                     iat->token,
                                     sd.repository_id, sd.head_sha,
                                     b,
                                     build_state::queued,
                                     hs,
                                     error))
            {
              l3 ([&]{trace << "created check_run { " << cr << " }";});
            }
          }
          else // Check run exists on GitHub.
          {
            if (pr.first->status == gh_to_status (build_state::queued))
            {
              if (scr != nullptr)
              {
                cr = move (*scr);
                cr.state = nullopt;
              }

              if (gq_update_check_run (cr,
                                       iat->token,
                                       sd.repository_id,
                                       pr.first->node_id,
                                       build_state::building,
                                       error))
              {
                l3 ([&]{trace << "updated check_run { " << cr << " }";});
              }
            }
            else
            {
              // Do nothing because the GitHub state is already built so the
              // lambda returned by build_built() will update the database to
              // built.
              //
              return nullptr;
            }
          }
        }
        else // Error communicating with GitHub.
        {
          // Can't tell whether the check run exists GitHub. Make the final
          // decision on whether or not to store nullopt in node_id and state
          // based on what's in the database when the lambda runs
          // (build_built() and its lambda could run in the meantime).
          //
          // @@ TODO Store build_state::building if we start storing failed
          //         state.
        }
      }
    }

    return [iat = move (new_iat),
            cr = move (cr),
            error = move (error),
            warn = move (warn)] (const tenant_service& ts) -> optional<string>
    {
      // NOTE: this lambda may be called repeatedly (e.g., due to transaction
      // being aborted) and so should not move out of its captures.

      service_data sd;
      try
      {
        sd = service_data (*ts.data);
      }
      catch (const invalid_argument& e)
      {
        error << "failed to parse service data: " << e;
        return nullopt;
      }

      if (iat)
        sd.installation_access = *iat;

      if (check_run* scr = sd.find_check_run (cr.build_id))
      {
        // Update existing check run.
        //
        // @@ TODO What if the failed GH update was for built? May end up with
        //         permanently building check run.
        //
        if (!scr->state || scr->state == build_state::queued)
        {
          scr->state = cr.state;
          if (!scr->node_id)
            scr->node_id = move (cr.node_id);
        }
      }
      else
      {
        // Store new check run.
        //
        sd.check_runs.push_back (cr);

        warn << "check run { " << cr << " }: "
             << to_string (build_state::building)
             << " state being persisted before "
             << to_string (build_state::queued);
      }

      return sd.json ();
    };
  }

  function<optional<string> (const tenant_service&)> ci_github::
  build_built (const tenant_service&, const build&,
               const build_hints&,
               const diag_epilogue& /* log_writer */) const noexcept
  {
    return nullptr;
  }

  optional<string> ci_github::
  generate_jwt (const basic_mark& trace,
                const basic_mark& error) const
  {
    string jwt;
    try
    {
      // Set token's "issued at" time 60 seconds in the past to combat clock
      // drift (as recommended by GitHub).
      //
      jwt = brep::generate_jwt (
          *options_,
          options_->ci_github_app_private_key (),
          to_string (options_->ci_github_app_id ()),
          chrono::seconds (options_->ci_github_jwt_validity_period ()),
          chrono::seconds (60));

      l3 ([&]{trace << "JWT: " << jwt;});
    }
    catch (const system_error& e)
    {
      error << "unable to generate JWT (errno=" << e.code () << "): " << e;
      return nullopt;
    }

    return jwt;
  }

  // There are three types of GitHub API authentication:
  //
  //   1) Authenticating as an app. Used to access parts of the API concerning
  //      the app itself such as getting the list of installations. (Need to
  //      authenticate as an app as part of authenticating as an app
  //      installation.)
  //
  //   2) Authenticating as an app installation (on a user or organisation
  //      account). Used to access resources belonging to the user/repository
  //      or organisation the app is installed in.
  //
  //   3) Authenticating as a user. Used to perform actions as the user.
  //
  // We need to authenticate as an app installation (2).
  //
  // How to authenticate as an app installation
  //
  // Reference:
  // https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/authenticating-as-a-github-app-installation
  //
  // The final authentication token we need is an installation access token
  // (IAT), valid for one hour, which we will pass in the `Authentication`
  // header of our Github API requests:
  //
  //   Authorization: Bearer <INSTALLATION_ACCESS_TOKEN>
  //
  // To generate an IAT:
  //
  // - Generate a JSON Web Token (JWT)
  //
  // - Get the installation ID. This will be included in the webhook request
  //   in our case
  //
  // - Send a POST to /app/installations/<INSTALLATION_ID>/access_tokens which
  //   includes the JWT (`Authorization: Bearer <JWT>`). The response will
  //   include the IAT. Can pass the name of the repository included in the
  //   webhook request to restrict access, otherwise we get access to all
  //   repos covered by the installation if installed on an organisation for
  //   example.
  //
  optional<gh_installation_access_token> ci_github::
  obtain_installation_access_token (uint64_t iid,
                                    string jwt,
                                    const basic_mark& error) const
  {
    gh_installation_access_token iat;
    try
    {
      // API endpoint.
      //
      string ep ("app/installations/" + to_string (iid) + "/access_tokens");

      uint16_t sc (
          github_post (iat, ep, strings {"Authorization: Bearer " + jwt}));

      // Possible response status codes from the access_tokens endpoint:
      //
      // 201 Created
      // 401 Requires authentication
      // 403 Forbidden
      // 404 Resource not found
      // 422 Validation failed, or the endpoint has been spammed.
      //
      // Note that the payloads of non-201 status codes are undocumented.
      //
      if (sc != 201)
      {
        error << "unable to get installation access token: error HTTP "
              << "response status " << sc;
        return nullopt;
      }

      // Create a clock drift safety window.
      //
      iat.expires_at -= chrono::minutes (5);
    }
    catch (const json::invalid_json_input& e)
    {
      // Note: e.name is the GitHub API endpoint.
      //
      error << "malformed JSON in response from " << e.name << ", line: "
            << e.line << ", column: " << e.column << ", byte offset: "
            << e.position << ", error: " << e;
      return nullopt;
    }
    catch (const invalid_argument& e)
    {
      error << "malformed header(s) in response: " << e;
      return nullopt;
    }
    catch (const system_error& e)
    {
      error << "unable to get installation access token (errno=" << e.code ()
            << "): " << e.what ();
      return nullopt;
    }

    return iat;
  }
}