Use UTC as default timezone when schedule Actions cron tasks (#31742)

Fix #31657.

According to the
[doc](https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#onschedule)
of GitHub Actions, The timezone for cron should be UTC, not the local
timezone. And Gitea Actions doesn't have any reasons to change this, so
I think it's a bug.

However, Gitea Actions has extended the syntax, as it supports
descriptors like `@weekly` and `@every 5m`, and supports specifying the
timezone like `TZ=UTC 0 10 * * *`. So we can make it use UTC only when
the timezone is not specified, to be compatible with GitHub Actions, and
also respect the user's specified.

It does break the feature because the times to run tasks would be
changed, and it may confuse users. So I don't think we should backport
this.

## ⚠️ BREAKING ⚠️

If the server's local time zone is not UTC, a scheduled task would run
at a different time after upgrading Gitea to this version.

3 files changed, 104 insertions, 12 deletions

diff --git a/models/actions/schedule.go b/models/actions/schedule.go
index 3646a046a0..c751ef51ca 100644
--- a/models/actions/schedule.go
+++ b/models/actions/schedule.go
@@ -13,8 +13,6 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/timeutil"
 	webhook_module "code.gitea.io/gitea/modules/webhook"
-
-	"github.com/robfig/cron/v3"
 )
 
 // ActionSchedule represents a schedule of a workflow file
@@ -53,8 +51,6 @@ func GetReposMapByIDs(ctx context.Context, ids []int64) (map[int64]*repo_model.R
 	return repos, db.GetEngine(ctx).In("id", ids).Find(&repos)
 }
 
-var cronParser = cron.NewParser(cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor)
-
 // CreateScheduleTask creates new schedule task.
 func CreateScheduleTask(ctx context.Context, rows []*ActionSchedule) error {
 	// Return early if there are no rows to insert
@@ -80,19 +76,21 @@ func CreateScheduleTask(ctx context.Context, rows []*ActionSchedule) error {
 		now := time.Now()
 
 		for _, spec := range row.Specs {
+			specRow := &ActionScheduleSpec{
+				RepoID:     row.RepoID,
+				ScheduleID: row.ID,
+				Spec:       spec,
+			}
 			// Parse the spec and check for errors
-			schedule, err := cronParser.Parse(spec)
+			schedule, err := specRow.Parse()
 			if err != nil {
 				continue // skip to the next spec if there's an error
 			}
 
+			specRow.Next = timeutil.TimeStamp(schedule.Next(now).Unix())
+
 			// Insert the new schedule spec row
-			if err = db.Insert(ctx, &ActionScheduleSpec{
-				RepoID:     row.RepoID,
-				ScheduleID: row.ID,
-				Spec:       spec,
-				Next:       timeutil.TimeStamp(schedule.Next(now).Unix()),
-			}); err != nil {
+			if err = db.Insert(ctx, specRow); err != nil {
 				return err
 			}
 		}
diff --git a/models/actions/schedule_spec.go b/models/actions/schedule_spec.go
index 91240459a0..923e5f7807 100644
--- a/models/actions/schedule_spec.go
+++ b/models/actions/schedule_spec.go
@@ -5,6 +5,8 @@ package actions
 
 import (
 	"context"
+	"strings"
+	"time"
 
 	"code.gitea.io/gitea/models/db"
 	repo_model "code.gitea.io/gitea/models/repo"
@@ -32,8 +34,29 @@ type ActionScheduleSpec struct {
 	Updated timeutil.TimeStamp `xorm:"updated"`
 }
 
+// Parse parses the spec and returns a cron.Schedule
+// Unlike the default cron parser, Parse uses UTC timezone as the default if none is specified.
 func (s *ActionScheduleSpec) Parse() (cron.Schedule, error) {
-	return cronParser.Parse(s.Spec)
+	parser := cron.NewParser(cron.Minute | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor)
+	schedule, err := parser.Parse(s.Spec)
+	if err != nil {
+		return nil, err
+	}
+
+	// If the spec has specified a timezone, use it
+	if strings.HasPrefix(s.Spec, "TZ=") || strings.HasPrefix(s.Spec, "CRON_TZ=") {
+		return schedule, nil
+	}
+
+	specSchedule, ok := schedule.(*cron.SpecSchedule)
+	// If it's not a spec schedule, like "@every 5m", timezone is not relevant
+	if !ok {
+		return schedule, nil
+	}
+
+	// Set the timezone to UTC
+	specSchedule.Location = time.UTC
+	return specSchedule, nil
 }
 
 func init() {
diff --git a/models/actions/schedule_spec_test.go b/models/actions/schedule_spec_test.go
new file mode 100644
index 0000000000..0c26fce4b2
--- /dev/null
+++ b/models/actions/schedule_spec_test.go
@@ -0,0 +1,71 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package actions
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestActionScheduleSpec_Parse(t *testing.T) {
+	// Mock the local timezone is not UTC
+	local := time.Local
+	tz, err := time.LoadLocation("Asia/Shanghai")
+	require.NoError(t, err)
+	defer func() {
+		time.Local = local
+	}()
+	time.Local = tz
+
+	now, err := time.Parse(time.RFC3339, "2024-07-31T15:47:55+08:00")
+	require.NoError(t, err)
+
+	tests := []struct {
+		name    string
+		spec    string
+		want    string
+		wantErr assert.ErrorAssertionFunc
+	}{
+		{
+			name:    "regular",
+			spec:    "0 10 * * *",
+			want:    "2024-07-31T10:00:00Z",
+			wantErr: assert.NoError,
+		},
+		{
+			name:    "invalid",
+			spec:    "0 10 * *",
+			want:    "",
+			wantErr: assert.Error,
+		},
+		{
+			name:    "with timezone",
+			spec:    "TZ=America/New_York 0 10 * * *",
+			want:    "2024-07-31T14:00:00Z",
+			wantErr: assert.NoError,
+		},
+		{
+			name:    "timezone irrelevant",
+			spec:    "@every 5m",
+			want:    "2024-07-31T07:52:55Z",
+			wantErr: assert.NoError,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			s := &ActionScheduleSpec{
+				Spec: tt.spec,
+			}
+			got, err := s.Parse()
+			tt.wantErr(t, err)
+
+			if err == nil {
+				assert.Equal(t, tt.want, got.Next(now).UTC().Format(time.RFC3339))
+			}
+		})
+	}
+}