fix: explain why a workflow cannot be used to run a job (#666)

- report back to the Forgejo instance
- log Forgejo runner side in debug mode
- display the workflow with line number to facilitate matching with errors
- split the error into multiple lines if possible

![image](/attachments/db376308-2ebc-4b63-831f-1ccd782107fb)

Refs forgejo/act#170

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/666
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

internal/app/run/runner.go

@@ -183,6 +183,22 @@ func (r *Runner) Run(ctx context.Context, task *runnerv1.Task) error {
 	return nil
 }
 
+func logAndReport(reporter *report.Reporter, message string, args ...any) {
+	log.Debugf(message, args...)
+	reporter.Logf(message, args...)
+}
+
+func explainFailedGenerateWorkflow(task *runnerv1.Task, log func(message string, args ...any), err error) error {
+	for n, line := range strings.Split(string(task.WorkflowPayload), "\n") {
+		log("%d: %s", n+1, line)
+	}
+	log("Errors were found and although they tend to be cryptic the line number they refer to gives a hint as to where the problem might be.")
+	for _, line := range strings.Split(err.Error(), "\n") {
+		log("%s", line)
+	}
+	return fmt.Errorf("the workflow file is not usable")
+}
+
 func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.Reporter) (err error) {
 	defer func() {
 		if r := recover(); r != nil {
@@ -194,7 +210,9 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 
 	workflow, jobID, err := generateWorkflow(task)
 	if err != nil {
-		return err
+		return explainFailedGenerateWorkflow(task, func(message string, args ...any) {
+			logAndReport(reporter, message, args...)
+		}, err)
 	}
 
 	plan, err := model.CombineWorkflowPlanner(workflow).PlanJob(jobID)

internal/app/run/runner_test.go

@@ -2,13 +2,30 @@ package run
 
 import (
 	"context"
+	"errors"
+	"fmt"
 	"testing"
 
+	runnerv1 "code.forgejo.org/forgejo/actions-proto/runner/v1"
 	"runner.forgejo.org/internal/pkg/labels"
 
 	"github.com/stretchr/testify/assert"
 )
 
+func TestExplainFailedGenerateWorkflow(t *testing.T) {
+	logged := ""
+	log := func(message string, args ...any) {
+		logged += fmt.Sprintf(message, args...) + "\n"
+	}
+	task := &runnerv1.Task{
+		WorkflowPayload: []byte("on: [push]\njobs:\n"),
+	}
+	generateWorkflowError := errors.New("message 1\nmessage 2")
+	err := explainFailedGenerateWorkflow(task, log, generateWorkflowError)
+	assert.Error(t, err)
+	assert.Equal(t, "1: on: [push]\n2: jobs:\n3: \nErrors were found and although they tend to be cryptic the line number they refer to gives a hint as to where the problem might be.\nmessage 1\nmessage 2\n", logged)
+}
+
 func TestLabelUpdate(t *testing.T) {
 	ctx := context.Background()
 	ls := labels.Labels{}