add api endpoint: /build/<id>/constituents

Returns a list of constituent builds

hydra-api.yaml

@@ -504,6 +504,32 @@ paths:
               schema:
                 $ref: '#/components/schemas/Error'
 
+  /build/{build-id}/constituents:
+    get:
+      summary: Retrieves a build's constituent jobs
+      parameters:
+      - name: build-id
+        in: path
+        description: build identifier
+        required: true
+        schema:
+            type: integer
+      responses:
+        '200':
+          description: build
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Build'
+        '404':
+          description: build couldn't be found
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
+
   /eval/{build-id}:
     get:
       summary: Retrieves evaluations identified by build id

src/lib/Hydra/Controller/Build.pm

@@ -104,6 +104,19 @@ sub build_GET {
     $c->stash->{binaryCachePublicUri} = $c->config->{binary_cache_public_uri};
 }
 
+sub constituents :Chained('buildChain') :PathPart('constituents') :Args(0) :ActionClass('REST') { }
+
+sub constituents_GET {
+    my ($self, $c) = @_;
+
+    my $build = $c->stash->{build};
+
+    $self->status_ok(
+        $c,
+        entity => [$build->constituents_->search({}, {order_by => ["job"]})]
+    );
+}
+
 
 sub view_nixlog : Chained('buildChain') PathPart('nixlog') {
     my ($self, $c, $stepnr, $mode) = @_;

t/Controller/Build/constituents.t

@@ -0,0 +1,46 @@
+use strict;
+use Setup;
+use JSON qw(decode_json encode_json);
+use Data::Dumper;
+use URI;
+my %ctx = test_init();
+
+require Hydra::Schema;
+require Hydra::Model::DB;
+require Hydra::Helper::Nix;
+
+use Test2::V0;
+require Catalyst::Test;
+Catalyst::Test->import('Hydra');
+use HTTP::Request::Common;
+
+my $db = Hydra::Model::DB->new;
+hydra_setup($db);
+
+my $project = $db->resultset('Projects')->create({name => "tests", displayname => "", owner => "root"});
+
+my $jobset = createBaseJobset("aggregate", "aggregate.nix", $ctx{jobsdir});
+
+ok(evalSucceeds($jobset),               "Evaluating jobs/aggregate.nix should exit with return code 0");
+is(nrQueuedBuildsForJobset($jobset), 3, "Evaluating jobs/aggregate.nix should result in 3 builds");
+for my $build (queuedBuildsForJobset($jobset)) {
+    ok(runBuild($build), "Build '".$build->job."' from jobs/aggregate.nix should exit with return code 0");
+}
+
+my $build_redirect = request(GET '/job/tests/aggregate/aggregate/latest-finished');
+
+my $url = URI->new($build_redirect->header('location'))->path . "/constituents";
+my $constituents = request(GET $url,
+      Accept => 'application/json',
+  );
+
+ok($constituents->is_success, "Getting the constituent builds");
+my $data = decode_json($constituents->content);
+
+my ($buildA) = grep { $_->{nixname} eq "empty-dir-a" } @$data;
+my ($buildB) = grep { $_->{nixname} eq "empty-dir-b" } @$data;
+
+is($buildA->{job}, "a");
+is($buildB->{job}, "b");
+
+done_testing;

t/jobs/aggregate.nix

@@ -0,0 +1,26 @@
+with import ./config.nix;
+{
+  a =
+    mkDerivation {
+      name = "empty-dir-a";
+      builder = ./empty-dir-builder.sh;
+    };
+
+  b =
+    mkDerivation {
+      name = "empty-dir-b";
+      builder = ./empty-dir-builder.sh;
+    };
+
+  aggregate =
+    mkDerivation {
+      name = "aggregate";
+      builder = ./empty-dir-builder.sh; # doesn't matter, just needs to pass a build
+
+      _hydraAggregate = true;
+      constituents = [
+        "a"
+        "b"
+      ];
+    };
+}