diff --git a/lib/tests/fixtures/task_fixtures.php b/lib/tests/fixtures/task_fixtures.php
index 9ece85de6811b..eb910106e9288 100644
--- a/lib/tests/fixtures/task_fixtures.php
+++ b/lib/tests/fixtures/task_fixtures.php
@@ -26,16 +26,78 @@
 namespace core\task;
 defined('MOODLE_INTERNAL') || die();
 
+/**
+ * Test class.
+ *
+ * @copyright 2022 Catalyst IT Australia Pty Ltd
+ * @author Cameron Ball <cameron@cameron1729.xyz>
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
 class adhoc_test_task extends \core\task\adhoc_task {
-    public function execute() {
+
+    /**
+     * Constructor.
+     *
+     * @param int|null $nextruntime Next run time
+     * @param int|null $timestarted Time started
+     */
+    public function __construct(?int $nextruntime = null, ?int $timestarted = null) {
+        if ($nextruntime) {
+            $this->set_next_run_time($nextruntime);
+        }
+
+        if ($timestarted) {
+            $this->set_timestarted($timestarted);
+        }
     }
-}
 
-class adhoc_test2_task extends \core\task\adhoc_task {
+    /**
+     * Execute.
+     */
     public function execute() {
     }
 }
 
+/**
+ * Test class.
+ *
+ * @copyright 2022 Catalyst IT Australia Pty Ltd
+ * @author Cameron Ball <cameron@cameron1729.xyz>
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class adhoc_test2_task extends adhoc_test_task {
+}
+
+/**
+ * Test class.
+ *
+ * @copyright 2022 Catalyst IT Australia Pty Ltd
+ * @author Cameron Ball <cameron@cameron1729.xyz>
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class adhoc_test3_task extends adhoc_test_task {
+}
+
+/**
+ * Test class.
+ *
+ * @copyright 2022 Catalyst IT Australia Pty Ltd
+ * @author Cameron Ball <cameron@cameron1729.xyz>
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class adhoc_test4_task extends adhoc_test_task {
+}
+
+/**
+ * Test class.
+ *
+ * @copyright 2022 Catalyst IT Australia Pty Ltd
+ * @author Cameron Ball <cameron@cameron1729.xyz>
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class adhoc_test5_task extends adhoc_test_task {
+}
+
 class scheduled_test_task extends \core\task\scheduled_task {
     public function get_name() {
         return "Test task";
diff --git a/lib/tests/task_manager_test.php b/lib/tests/task_manager_test.php
new file mode 100644
index 0000000000000..59eb8f38393eb
--- /dev/null
+++ b/lib/tests/task_manager_test.php
@@ -0,0 +1,248 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * This file contains the unit tests for the task manager.
+ *
+ * @package   core
+ * @copyright 2019 Brendan Heywood <brendan@catalyst-au.net>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace core\task;
+
+defined('MOODLE_INTERNAL') || die();
+require_once(__DIR__ . '/fixtures/task_fixtures.php');
+
+/**
+ * This file contains the unit tests for the task manager.
+ *
+ * @copyright 2019 Brendan Heywood <brendan@catalyst-au.net>
+ * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class task_manager_test extends \advanced_testcase {
+
+    /**
+     * Data provider for test_get_candidate_adhoc_tasks.
+     *
+     * @return array
+     */
+    public function test_get_candidate_adhoc_tasks_provider(): array {
+        return [
+            [
+                'concurrencylimit' => 5,
+                'limit' => 100,
+                'pertasklimits' => [],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null)
+                ],
+                'expected' => [
+                    adhoc_test_task::class,
+                    adhoc_test_task::class,
+                    adhoc_test_task::class,
+                    adhoc_test_task::class,
+                    adhoc_test_task::class
+                ]
+            ],
+            [
+                'concurrencylimit' => 5,
+                'limit' => 100,
+                'pertasklimits' => [],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null)
+                ],
+                'expected' => [
+                    adhoc_test_task::class,
+                    adhoc_test_task::class,
+                    adhoc_test_task::class,
+                    adhoc_test_task::class
+                ]
+            ],
+            [
+                'concurrencylimit' => 1,
+                'limit' => 100,
+                'pertasklimits' => [],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null)
+                ],
+                'expected' => []
+            ],
+            [
+                'concurrencylimit' => 2,
+                'limit' => 100,
+                'pertasklimits' => [],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null)
+                ],
+                'expected' => []
+            ],
+            [
+                'concurrencylimit' => 2,
+                'limit' => 100,
+                'pertasklimits' => [],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test2_task(time() - 20, time()),
+                    new adhoc_test2_task(time() - 20, time()),
+                    new adhoc_test3_task(time() - 20, null)
+                ],
+                'expected' => [adhoc_test3_task::class]
+            ],
+            [
+                'concurrencylimit' => 2,
+                'limit' => 2,
+                'pertasklimits' => [],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test2_task(time() - 20, null),
+                ],
+                'expected' => [
+                    adhoc_test_task::class,
+                    adhoc_test_task::class
+                ]
+            ],
+            [
+                'concurrencylimit' => 2,
+                'limit' => 2,
+                'pertasklimits' => [],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test2_task(time() - 20, null),
+                ],
+                'expected' => [
+                    adhoc_test2_task::class
+                ]
+            ],
+            [
+                'concurrencylimit' => 3,
+                'limit' => 100,
+                'pertasklimits' => [],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test2_task(time() - 20, time()),
+                    new adhoc_test2_task(time() - 20, time()),
+                    new adhoc_test2_task(time() - 20, null),
+                    new adhoc_test3_task(time() - 20, time()),
+                    new adhoc_test3_task(time() - 20, time()),
+                    new adhoc_test3_task(time() - 20, null),
+                    new adhoc_test4_task(time() - 20, time()),
+                    new adhoc_test4_task(time() - 20, time()),
+                    new adhoc_test4_task(time() - 20, null),
+                    new adhoc_test5_task(time() - 20, time()),
+                    new adhoc_test5_task(time() - 20, time()),
+                    new adhoc_test5_task(time() - 20, null),
+                ],
+                'expected' => [
+                    adhoc_test_task::class,
+                    adhoc_test2_task::class,
+                    adhoc_test3_task::class,
+                    adhoc_test4_task::class,
+                    adhoc_test5_task::class
+                ]
+            ],
+            [
+                'concurrencylimit' => 3,
+                'limit' => 100,
+                'pertasklimits' => [
+                    'adhoc_test_task' => 2,
+                    'adhoc_test2_task' => 2,
+                    'adhoc_test3_task' => 2,
+                    'adhoc_test4_task' => 2,
+                    'adhoc_test5_task' => 2
+                ],
+                'tasks' => [
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, time()),
+                    new adhoc_test_task(time() - 20, null),
+                    new adhoc_test2_task(time() - 20, time()),
+                    new adhoc_test2_task(time() - 20, time()),
+                    new adhoc_test2_task(time() - 20, null),
+                    new adhoc_test3_task(time() - 20, time()),
+                    new adhoc_test3_task(time() - 20, time()),
+                    new adhoc_test3_task(time() - 20, null),
+                    new adhoc_test4_task(time() - 20, time()),
+                    new adhoc_test4_task(time() - 20, time()),
+                    new adhoc_test4_task(time() - 20, null),
+                    new adhoc_test5_task(time() - 20, time()),
+                    new adhoc_test5_task(time() - 20, time()),
+                    new adhoc_test5_task(time() - 20, null),
+                ],
+                'expected' => []
+            ]
+        ];
+    }
+
+    /**
+     * Test that the candidate adhoc tasks are returned in the right order.
+     *
+     * @dataProvider test_get_candidate_adhoc_tasks_provider
+     *
+     * @param int $concurrencylimit The max number of runners each task can consume
+     * @param int $limit SQL limit
+     * @param array $pertasklimits Per-task limits
+     * @param array $tasks Array of tasks to put in DB and retrieve
+     * @param array $expected Array of expected classnames
+     * @return void
+     * @covers \manager::get_candidate_adhoc_tasks
+     */
+    public function test_get_candidate_adhoc_tasks(
+        int $concurrencylimit,
+        int $limit,
+        array $pertasklimits,
+        array $tasks,
+        array $expected
+    ): void {
+        $this->resetAfterTest();
+
+        foreach ($tasks as $task) {
+            manager::queue_adhoc_task($task);
+        }
+
+        $candidates = manager::get_candidate_adhoc_tasks(time(), $limit, $concurrencylimit, $pertasklimits);
+        $this->assertEquals(
+            array_map(
+                function(string $classname): string {
+                    return '\\' . $classname;
+                },
+                $expected
+            ),
+            array_column($candidates, 'classname')
+        );
+    }
+}