fix(actions): ensure all waiting jobs get runners in large workflows (#38200)

## Summary

Fixes two related bugs that cause jobs in large workflows (50+ parallel
jobs) to never get a runner assigned even though runners are free.

### Bug 1 — Concurrent runner race

When N runners all poll `FetchTask` with a stale `tasksVersion`
simultaneously, they all query the same waiting job list sorted by
`(updated, id)` and all pick **job #1**. Only one wins the `UPDATE WHERE
task_id=0` optimistic lock; the rest return empty-handed but still
receive `latestVersion` in the response. They then consider themselves
"up to date" and skip `PickTask` on every subsequent poll, leaving jobs
#2–50 permanently unassigned.

**Fix:** `CreateTaskForRunner` now iterates through all matching waiting
jobs. When the optimistic lock fails on job #1, it immediately tries job
#2, then #3, etc., each in its own independent transaction so a failed
attempt rolls back cleanly before the next candidate is tried.
`PickTask` no longer wraps this call in an outer `db.WithTx` (which
caused `halfCommitter` entanglement that prevented per-attempt
rollbacks).

### Bug 2 — Idle runner doesn't re-check after finishing a task

`tasks_version` only bumps when a job transitions **to** waiting (new
workflow triggered, blocked→unblocked). After a runner finishes its
current task it polls `FetchTask` with `tasksVersion == latestVersion`,
so the server skips `PickTask` entirely — the remaining 45 waiting jobs
are invisible to the now-idle runner.

**Fix:** Also call `IncreaseTaskVersion` in `UpdateRunJob` when a
(non-reusable-caller) job transitions to a **done** state. Idle runners
then see a version mismatch on their next poll and attempt `PickTask`,
picking up the remaining jobs.

models/actions/run_job.go

@@ -361,6 +361,14 @@ func CollectAllDescendantJobs(parent *ActionRunJob, allJobs []*ActionRunJob) []*
 	return out
 }
 
+// hasWaitingJobsToPick reports whether any waiting, unclaimed, non-reusable job
+// remains in the repo, i.e. work that an idle runner could still pick up.
+func hasWaitingJobsToPick(ctx context.Context, repoID int64) (bool, error) {
+	return db.GetEngine(ctx).
+		Where("repo_id = ? AND task_id = ? AND status = ? AND is_reusable_caller = ?", repoID, 0, StatusWaiting, false).
+		Exist(&ActionRunJob{})
+}
+
 func UpdateRunJob(ctx context.Context, job *ActionRunJob, cond builder.Cond, cols ...string) (int64, error) {
 	e := db.GetEngine(ctx)
 
@@ -385,14 +393,6 @@ func UpdateRunJob(ctx context.Context, job *ActionRunJob, cond builder.Cond, col
 		return affected, nil
 	}
 
-	// Reusable workflow caller jobs are never picked up by runners, so they don't need a task-version bump.
-	if statusUpdated && job.Status.IsWaiting() && !job.IsReusableCaller {
-		// if the status of job changes to waiting again, increase tasks version.
-		if err := IncreaseTaskVersion(ctx, job.OwnerID, job.RepoID); err != nil {
-			return 0, err
-		}
-	}
-
 	if job.RunID == 0 {
 		var err error
 		if job, err = GetRunJobByRepoAndID(ctx, job.RepoID, job.ID); err != nil {
@@ -400,6 +400,37 @@ func UpdateRunJob(ctx context.Context, job *ActionRunJob, cond builder.Cond, col
 		}
 	}
 
+	// Reusable workflow caller jobs are never picked up by runners, so they don't need a task-version bump.
+	if statusUpdated && !job.IsReusableCaller {
+		switch {
+		case job.Status.IsWaiting():
+			// A job returning to the waiting queue is work a runner can pick up, so bump the
+			// version to wake idle runners whose tasksVersion already equals latestVersion.
+			if err := IncreaseTaskVersion(ctx, job.OwnerID, job.RepoID); err != nil {
+				return 0, err
+			}
+		case job.Status.IsDone():
+			// When a job finishes, bump the version so that idle runners — whose
+			// tasksVersion already equals the current latestVersion — learn that
+			// remaining waiting jobs are still available and attempt PickTask again.
+			// Without this bump, runners that completed their tasks would see
+			// tasksVersion==latestVersion and skip PickTask, leaving the other jobs
+			// permanently unassigned until the version changes for another reason.
+			// Only bump when waiting work actually remains for this repo, otherwise
+			// every job completion would needlessly bump the global version and wake
+			// every idle runner instance-wide for nothing.
+			hasWaiting, err := hasWaitingJobsToPick(ctx, job.RepoID)
+			if err != nil {
+				return 0, err
+			}
+			if hasWaiting {
+				if err := IncreaseTaskVersion(ctx, job.OwnerID, job.RepoID); err != nil {
+					return 0, err
+				}
+			}
+		}
+	}
+
 	if statusUpdated && job.ParentJobID > 0 {
 		// Reusable workflow caller's children cascade their status changes upward to the parent caller.
 		parent, err := GetRunJobByRunAndID(ctx, job.RunID, job.ParentJobID)

models/actions/task.go

@@ -227,13 +227,18 @@ func makeTaskStepDisplayName(step *jobparser.Step, limit int) (name string) {
 	return util.EllipsisDisplayString(name, limit) // database column has a length limit
 }
 
-func CreateTaskForRunner(ctx context.Context, runner *ActionRunner) (*ActionTask, bool, error) {
-	ctx, committer, err := db.TxContext(ctx)
-	if err != nil {
-		return nil, false, err
-	}
-	defer committer.Close()
+// errJobAlreadyClaimed is a sentinel used inside claimJobForRunner to signal that
+// another runner won the optimistic-lock race; it is never returned to callers.
+var errJobAlreadyClaimed = errors.New("job already claimed by another runner")
 
+// CreateTaskForRunner finds a waiting job that matches the runner's labels and
+// atomically claims it. It iterates through all matching jobs so that a
+// concurrent claim by another runner (which would lose the optimistic lock on
+// job #1) does not leave the remaining jobs permanently unassigned.
+func CreateTaskForRunner(ctx context.Context, runner *ActionRunner) (*ActionTask, bool, error) {
+	if db.InTransaction(ctx) {
+		return nil, false, errors.New("CreateTaskForRunner must not be called within a database transaction")
+	}
 	e := db.GetEngine(ctx)
 
 	jobCond := builder.NewCond()
@@ -254,83 +259,144 @@ func CreateTaskForRunner(ctx context.Context, runner *ActionRunner) (*ActionTask
 	}
 
 	// TODO: a more efficient way to filter labels
-	var job *ActionRunJob
 	log.Trace("runner labels: %v", runner.AgentLabels)
 	for _, v := range jobs {
-		if runner.CanMatchLabels(v.RunsOn) {
-			job = v
-			break
+		if !runner.CanMatchLabels(v.RunsOn) {
+			continue
 		}
-	}
-	if job == nil {
-		return nil, false, nil
-	}
-	if err := job.LoadAttributes(ctx); err != nil {
-		return nil, false, err
-	}
-
-	now := timeutil.TimeStampNow()
-	job.Started = now
-	job.Status = StatusRunning
-
-	task := &ActionTask{
-		JobID:             job.ID,
-		Attempt:           job.Attempt,
-		RunnerID:          runner.ID,
-		Started:           now,
-		Status:            StatusRunning,
-		RepoID:            job.RepoID,
-		OwnerID:           job.OwnerID,
-		CommitSHA:         job.CommitSHA,
-		IsForkPullRequest: job.IsForkPullRequest,
-	}
-	task.GenerateAndFillToken()
-
-	workflowJob, err := job.ParseJob()
-	if err != nil {
-		return nil, false, fmt.Errorf("load job %d: %w", job.ID, err)
-	}
-
-	if _, err := e.Insert(task); err != nil {
-		return nil, false, err
-	}
-
-	task.LogFilename = logFileName(job.Run.Repo.FullName(), task.ID)
-	if err := UpdateTask(ctx, task, "log_filename"); err != nil {
-		return nil, false, err
-	}
-
-	if len(workflowJob.Steps) > 0 {
-		steps := make([]*ActionTaskStep, len(workflowJob.Steps))
-		for i, v := range workflowJob.Steps {
-			steps[i] = &ActionTaskStep{
-				Name:   makeTaskStepDisplayName(v, 255),
-				TaskID: task.ID,
-				Index:  int64(i),
-				RepoID: task.RepoID,
-				Status: StatusWaiting,
-			}
-		}
-		if _, err := e.Insert(steps); err != nil {
+		task, ok, err := claimJobForRunner(ctx, runner, v)
+		if err != nil {
 			return nil, false, err
 		}
-		task.Steps = steps
+		if ok {
+			return task, true, nil
+		}
+		// Another runner claimed this job concurrently; try the next one.
 	}
+	return nil, false, nil
+}
 
-	job.TaskID = task.ID
-	if n, err := UpdateRunJob(ctx, job, builder.Eq{"task_id": 0}); err != nil {
-		return nil, false, err
-	} else if n != 1 {
+// claimJobForRunner attempts to atomically claim job for runner inside its own
+// transaction. Returns (task, true, nil) on success, or (nil, false, nil) when
+// another runner wins the optimistic-lock race (the caller should try the next
+// candidate job).
+func claimJobForRunner(ctx context.Context, runner *ActionRunner, job *ActionRunJob) (*ActionTask, bool, error) {
+	var resultTask *ActionTask
+
+	err := db.WithTx(ctx, func(ctx context.Context) error {
+		e := db.GetEngine(ctx)
+
+		if err := job.LoadAttributes(ctx); err != nil {
+			return err
+		}
+
+		now := timeutil.TimeStampNow()
+		job.Started = now
+		job.Status = StatusRunning
+
+		task := &ActionTask{
+			JobID:             job.ID,
+			Attempt:           job.Attempt,
+			RunnerID:          runner.ID,
+			Started:           now,
+			Status:            StatusRunning,
+			RepoID:            job.RepoID,
+			OwnerID:           job.OwnerID,
+			CommitSHA:         job.CommitSHA,
+			IsForkPullRequest: job.IsForkPullRequest,
+		}
+		task.GenerateAndFillToken()
+
+		workflowJob, err := job.ParseJob()
+		if err != nil {
+			return fmt.Errorf("load job %d: %w", job.ID, err)
+		}
+
+		if _, err := e.Insert(task); err != nil {
+			return err
+		}
+
+		task.LogFilename = logFileName(job.Run.Repo.FullName(), task.ID)
+		if err := UpdateTask(ctx, task, "log_filename"); err != nil {
+			return err
+		}
+
+		if len(workflowJob.Steps) > 0 {
+			steps := make([]*ActionTaskStep, len(workflowJob.Steps))
+			for i, v := range workflowJob.Steps {
+				steps[i] = &ActionTaskStep{
+					Name:   makeTaskStepDisplayName(v, 255),
+					TaskID: task.ID,
+					Index:  int64(i),
+					RepoID: task.RepoID,
+					Status: StatusWaiting,
+				}
+			}
+			if _, err := e.Insert(steps); err != nil {
+				return err
+			}
+			task.Steps = steps
+		}
+
+		job.TaskID = task.ID
+		n, err := UpdateRunJob(ctx, job, builder.And(builder.Eq{"task_id": 0}, builder.Eq{"status": StatusWaiting}))
+		if err != nil {
+			return err
+		}
+		if n != 1 {
+			// Another runner claimed this job between our scan and this update;
+			// signal the outer loop to move on without treating this as an error.
+			return errJobAlreadyClaimed
+		}
+
+		task.Job = job
+		resultTask = task
+		return nil
+	})
+
+	if errors.Is(err, errJobAlreadyClaimed) {
 		return nil, false, nil
 	}
-
-	task.Job = job
-
-	if err := committer.Commit(); err != nil {
+	if err != nil {
 		return nil, false, err
 	}
+	return resultTask, true, nil
+}
 
-	return task, true, nil
+// ReleaseTaskForRunner reverts a freshly-claimed but undelivered task: it deletes
+// the task together with its steps and returns the job to the waiting queue. It is
+// used when assembling the runner response fails after the job was already claimed,
+// so the job is not stranded in running state with no runner ever executing it.
+func ReleaseTaskForRunner(ctx context.Context, task *ActionTask) error {
+	return db.WithTx(ctx, func(ctx context.Context) error {
+		e := db.GetEngine(ctx)
+
+		job, err := GetRunJobByRepoAndID(ctx, task.RepoID, task.JobID)
+		if err != nil {
+			return err
+		}
+
+		job.Status = StatusWaiting
+		job.Started = 0
+		job.TaskID = 0
+		// Guard on task_id and status so we only release while the job still
+		// references this task and has not progressed past running.
+		n, err := UpdateRunJob(ctx, job, builder.Eq{"task_id": task.ID, "status": StatusRunning}, "status", "started", "task_id")
+		if err != nil {
+			return err
+		}
+		if n != 1 {
+			return fmt.Errorf("release task %d: job %d no longer references it", task.ID, task.JobID)
+		}
+
+		if _, err := e.Delete(&ActionTaskStep{TaskID: task.ID}); err != nil {
+			return err
+		}
+		if _, err := e.ID(task.ID).Delete(&ActionTask{}); err != nil {
+			return err
+		}
+		return nil
+	})
 }
 
 func UpdateTask(ctx context.Context, task *ActionTask, cols ...string) error {

models/actions/task_test.go

@@ -306,3 +306,68 @@ func TestStopTaskCancellingFallsBackForMissingRunner(t *testing.T) {
 	assert.Equal(t, StatusCancelled, jobAfterStop.Status)
 	assert.NotZero(t, jobAfterStop.Stopped)
 }
+
+// TestReleaseTaskForRunner verifies that releasing a freshly-claimed task returns
+// its job to the waiting queue and deletes the task and its steps, so a failure
+// while assembling the runner response cannot strand the job in running state.
+func TestReleaseTaskForRunner(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+
+	run := &ActionRun{
+		Title:         "release-task-test-run",
+		RepoID:        1,
+		OwnerID:       2,
+		WorkflowID:    "test.yaml",
+		Index:         9902,
+		TriggerUserID: 2,
+		Ref:           "refs/heads/main",
+		CommitSHA:     "c2d72f548424103f01ee1dc02889c1e2bff816b0",
+		Event:         "push",
+		TriggerEvent:  "push",
+		Status:        StatusWaiting,
+	}
+	require.NoError(t, db.Insert(t.Context(), run))
+
+	job := &ActionRunJob{
+		RunID:           run.ID,
+		RepoID:          run.RepoID,
+		OwnerID:         run.OwnerID,
+		CommitSHA:       run.CommitSHA,
+		Name:            "release-job",
+		Attempt:         1,
+		JobID:           "release-job",
+		Status:          StatusWaiting,
+		RunsOn:          []string{"ubuntu-latest"},
+		WorkflowPayload: []byte("on: push\njobs:\n  release-job:\n    runs-on: ubuntu-latest\n    steps:\n      - run: echo hi\n"),
+	}
+	require.NoError(t, db.Insert(t.Context(), job))
+
+	runner := &ActionRunner{
+		UUID:        "release-runner-uuid",
+		Name:        "release-runner",
+		AgentLabels: []string{"ubuntu-latest"},
+	}
+	runner.GenerateAndFillToken()
+	require.NoError(t, db.Insert(t.Context(), runner))
+
+	task, ok, err := CreateTaskForRunner(t.Context(), runner)
+	require.NoError(t, err)
+	require.True(t, ok)
+	require.NotNil(t, task)
+
+	claimed := unittest.AssertExistsAndLoadBean(t, &ActionRunJob{ID: job.ID})
+	require.Equal(t, StatusRunning, claimed.Status)
+	require.Equal(t, task.ID, claimed.TaskID)
+
+	require.NoError(t, ReleaseTaskForRunner(t.Context(), task))
+
+	// Job is back in the waiting queue with no task assigned.
+	released := unittest.AssertExistsAndLoadBean(t, &ActionRunJob{ID: job.ID})
+	assert.Equal(t, StatusWaiting, released.Status)
+	assert.Zero(t, released.TaskID)
+	assert.Zero(t, released.Started)
+
+	// The task and its steps are gone.
+	unittest.AssertNotExistsBean(t, &ActionTask{ID: task.ID})
+	unittest.AssertNotExistsBean(t, &ActionTaskStep{TaskID: task.ID})
+}

services/actions/task.go

@@ -12,6 +12,7 @@ import (
 	actions_model "gitea.dev/models/actions"
 	"gitea.dev/models/db"
 	secret_model "gitea.dev/models/secret"
+	"gitea.dev/modules/log"
 
 	"google.golang.org/protobuf/types/known/structpb"
 )
@@ -47,59 +48,26 @@ func PickTask(ctx context.Context, runner *actions_model.ActionRunner) (*runnerv
 		}
 	}
 
-	if err := db.WithTx(ctx, func(ctx context.Context) error {
-		t, ok, err := actions_model.CreateTaskForRunner(ctx, runner)
-		if err != nil {
-			return fmt.Errorf("CreateTaskForRunner: %w", err)
-		}
-		if !ok {
-			return nil
-		}
-
-		if err := t.LoadAttributes(ctx); err != nil {
-			return fmt.Errorf("task LoadAttributes: %w", err)
-		}
-		job = t.Job
-		actionTask = t
-
-		secrets, err := secret_model.GetSecretsOfTask(ctx, t)
-		if err != nil {
-			return fmt.Errorf("GetSecretsOfTask: %w", err)
-		}
-
-		vars, err := actions_model.GetVariablesOfRun(ctx, t.Job.Run)
-		if err != nil {
-			return fmt.Errorf("GetVariablesOfRun: %w", err)
-		}
-
-		needs, err := findTaskNeeds(ctx, job)
-		if err != nil {
-			return fmt.Errorf("findTaskNeeds: %w", err)
-		}
-
-		taskContext, err := generateTaskContext(ctx, t)
-		if err != nil {
-			return fmt.Errorf("generateTaskContext: %w", err)
-		}
-
-		task = &runnerv1.Task{
-			Id:              t.ID,
-			WorkflowPayload: t.Job.WorkflowPayload,
-			Context:         taskContext,
-			Secrets:         secrets,
-			Vars:            vars,
-			Needs:           needs,
-		}
-
-		return nil
-	}); err != nil {
-		return nil, false, err
+	t, ok, err := actions_model.CreateTaskForRunner(ctx, runner)
+	if err != nil {
+		return nil, false, fmt.Errorf("CreateTaskForRunner: %w", err)
 	}
-
-	if task == nil {
+	if !ok {
 		return nil, false, nil
 	}
 
+	task, job, err = buildRunnerTask(ctx, t)
+	if err != nil {
+		// The job was already claimed but assembling its payload failed; release the
+		// claim so the job returns to the waiting queue instead of being stranded in
+		// running state with no runner ever executing it.
+		if relErr := actions_model.ReleaseTaskForRunner(ctx, t); relErr != nil {
+			log.Error("ReleaseTaskForRunner [task_id: %d]: %v", t.ID, relErr)
+		}
+		return nil, false, err
+	}
+	actionTask = t
+
 	CreateCommitStatusForRunJobs(ctx, job.Run, job)
 	NotifyWorkflowJobStatusUpdateWithTask(ctx, job, actionTask)
 	// job.Run is loaded inside the transaction before UpdateRunJob sets run.Started,
@@ -111,6 +79,44 @@ func PickTask(ctx context.Context, runner *actions_model.ActionRunner) (*runnerv
 	return task, true, nil
 }
 
+// buildRunnerTask assembles the runner-facing task payload for an already-claimed
+// task. All operations are read-only; on error the caller releases the claim.
+func buildRunnerTask(ctx context.Context, t *actions_model.ActionTask) (*runnerv1.Task, *actions_model.ActionRunJob, error) {
+	if err := t.LoadAttributes(ctx); err != nil {
+		return nil, nil, fmt.Errorf("task LoadAttributes: %w", err)
+	}
+	job := t.Job
+
+	secrets, err := secret_model.GetSecretsOfTask(ctx, t)
+	if err != nil {
+		return nil, nil, fmt.Errorf("GetSecretsOfTask: %w", err)
+	}
+
+	vars, err := actions_model.GetVariablesOfRun(ctx, t.Job.Run)
+	if err != nil {
+		return nil, nil, fmt.Errorf("GetVariablesOfRun: %w", err)
+	}
+
+	needs, err := findTaskNeeds(ctx, job)
+	if err != nil {
+		return nil, nil, fmt.Errorf("findTaskNeeds: %w", err)
+	}
+
+	taskContext, err := generateTaskContext(ctx, t)
+	if err != nil {
+		return nil, nil, fmt.Errorf("generateTaskContext: %w", err)
+	}
+
+	return &runnerv1.Task{
+		Id:              t.ID,
+		WorkflowPayload: t.Job.WorkflowPayload,
+		Context:         taskContext,
+		Secrets:         secrets,
+		Vars:            vars,
+		Needs:           needs,
+	}, job, nil
+}
+
 func generateTaskContext(ctx context.Context, t *actions_model.ActionTask) (*structpb.Struct, error) {
 	giteaRuntimeToken, err := CreateAuthorizationToken(t.ID, t.Job.RunID, t.JobID)
 	if err != nil {

tests/integration/actions_concurrent_claim_test.go

@@ -0,0 +1,118 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"sync"
+	"testing"
+
+	actions_model "gitea.dev/models/actions"
+	"gitea.dev/models/db"
+	"gitea.dev/models/unittest"
+	"gitea.dev/tests"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// minimalWorkflowPayload returns the minimal YAML for a single-job workflow with no steps.
+func minimalConcurrentWorkflowPayload(jobID string) []byte {
+	return []byte("on: push\njobs:\n  " + jobID + ":\n    runs-on: ubuntu-latest\n")
+}
+
+// TestCreateTaskForRunnerConcurrentClaim verifies that when multiple runners
+// poll simultaneously and all initially see the same first waiting job,
+// each runner claims a distinct job rather than all but one being left
+// empty-handed. This is the regression test for the race condition where
+// runners losing the optimistic-lock on job #1 would receive latestVersion
+// and never retry the remaining 49+ jobs.
+//
+// It lives in tests/integration rather than a unit test because SQLite
+// serializes write transactions, so the contended optimistic-lock path this
+// guards only runs concurrently against MySQL/PostgreSQL in CI.
+func TestCreateTaskForRunnerConcurrentClaim(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	const numJobs = 3
+
+	run := &actions_model.ActionRun{
+		Title:         "concurrent-claim-test-run",
+		RepoID:        1,
+		OwnerID:       2,
+		WorkflowID:    "test.yaml",
+		Index:         9901,
+		TriggerUserID: 2,
+		Ref:           "refs/heads/main",
+		CommitSHA:     "c2d72f548424103f01ee1dc02889c1e2bff816b0",
+		Event:         "push",
+		TriggerEvent:  "push",
+		Status:        actions_model.StatusWaiting,
+	}
+	require.NoError(t, db.Insert(t.Context(), run))
+
+	jobs := make([]*actions_model.ActionRunJob, numJobs)
+	for i := range numJobs {
+		jobID := "concurrent-job-" + string(rune('a'+i))
+		jobs[i] = &actions_model.ActionRunJob{
+			RunID:           run.ID,
+			RepoID:          run.RepoID,
+			OwnerID:         run.OwnerID,
+			CommitSHA:       run.CommitSHA,
+			Name:            jobID,
+			Attempt:         1,
+			JobID:           jobID,
+			Status:          actions_model.StatusWaiting,
+			RunsOn:          []string{"ubuntu-latest"},
+			WorkflowPayload: minimalConcurrentWorkflowPayload(jobID),
+		}
+		require.NoError(t, db.Insert(t.Context(), jobs[i]))
+	}
+
+	runners := make([]*actions_model.ActionRunner, numJobs)
+	for i := range numJobs {
+		r := &actions_model.ActionRunner{
+			UUID:        "concurrent-runner-uuid-" + string(rune('a'+i)),
+			Name:        "concurrent-runner-" + string(rune('a'+i)),
+			AgentLabels: []string{"ubuntu-latest"},
+		}
+		r.GenerateAndFillToken()
+		runners[i] = r
+		require.NoError(t, db.Insert(t.Context(), runners[i]))
+	}
+
+	// Simulate the burst: all runners call CreateTaskForRunner concurrently,
+	// as happens when all see the same stale tasksVersion simultaneously.
+	type result struct {
+		task *actions_model.ActionTask
+		ok   bool
+		err  error
+	}
+	results := make([]result, numJobs)
+	var wg sync.WaitGroup
+	for i := range numJobs {
+		wg.Go(func() {
+			task, ok, err := actions_model.CreateTaskForRunner(t.Context(), runners[i])
+			results[i] = result{task, ok, err}
+		})
+	}
+	wg.Wait()
+
+	// Every runner must have received a task without error.
+	claimedJobIDs := make(map[int64]bool)
+	for i, r := range results {
+		require.NoError(t, r.err, "runner %d got an unexpected error", i)
+		require.True(t, r.ok, "runner %d did not get a task even though free jobs exist", i)
+		require.NotNil(t, r.task)
+		assert.False(t, claimedJobIDs[r.task.JobID], "job %d was claimed by more than one runner", r.task.JobID)
+		claimedJobIDs[r.task.JobID] = true
+	}
+	assert.Len(t, claimedJobIDs, numJobs, "expected %d distinct jobs to be claimed", numJobs)
+
+	// All jobs must now be running with a task assigned.
+	for _, j := range jobs {
+		updated := unittest.AssertExistsAndLoadBean(t, &actions_model.ActionRunJob{ID: j.ID})
+		assert.Equal(t, actions_model.StatusRunning, updated.Status)
+		assert.NotZero(t, updated.TaskID)
+	}
+}