feat: cron arithmetic and persisted workflow schedules

server/internal/services/workflows.go

@@ -8,6 +8,7 @@ import (
 
 	"gitea.hostxtra.co.uk/mrhid6/vantage/server/internal/db"
 	"gitea.hostxtra.co.uk/mrhid6/vantage/server/internal/models"
+	"gitea.hostxtra.co.uk/mrhid6/vantage/server/internal/workflowsched"
 	"github.com/google/uuid"
 	"go.mongodb.org/mongo-driver/v2/bson"
 	"go.mongodb.org/mongo-driver/v2/mongo"
@@ -60,6 +61,11 @@ func EnsureWorkflowIndexes() error {
 	}); err != nil {
 		return err
 	}
+	if _, err := db.Col("workflows").Indexes().CreateOne(ctx, mongo.IndexModel{
+		Keys: bson.D{{Key: "next_run_at", Value: 1}},
+	}); err != nil {
+		return err
+	}
 	if err := EnsureLogIndexes(); err != nil {
 		return err
 	}
@@ -321,3 +327,45 @@ func DeleteWorkflow(instanceID, id string) error {
 	_, err := db.Col("workflows").DeleteOne(ctx, bson.M{"workflow_id": id, "instance_id": instanceID})
 	return err
 }
+
+// SetSchedule validates and stores a workflow's schedule, computing the first
+// occurrence. next_run_at is persisted rather than held in memory: a leader
+// handover between computing an occurrence and firing it would otherwise lose
+// it or fire it twice.
+//
+// Passing s == nil, or a disabled schedule, clears next_run_at so the
+// scheduler's query stops matching the document at all.
+func SetSchedule(instanceID, workflowID string, s *models.Schedule) (*time.Time, error) {
+	ctx, cancel := wfCtx()
+	defer cancel()
+
+	set := bson.M{"schedule": s, "updated_at": time.Now()}
+	unset := bson.M{}
+
+	var next *time.Time
+	if s != nil && s.Enabled {
+		at, err := workflowsched.NextOccurrence(s.Cron, s.TZ, time.Now())
+		if err != nil {
+			return nil, err
+		}
+		next = &at
+		set["next_run_at"] = at
+	} else {
+		unset["next_run_at"] = ""
+	}
+
+	update := bson.M{"$set": set}
+	if len(unset) > 0 {
+		update["$unset"] = unset
+	}
+
+	res, err := db.Col("workflows").UpdateOne(ctx,
+		bson.M{"workflow_id": workflowID, "instance_id": instanceID}, update)
+	if err != nil {
+		return nil, err
+	}
+	if res.MatchedCount == 0 {
+		return nil, mongo.ErrNoDocuments
+	}
+	return next, nil
+}