Add a task control service to facilitate a new structure for tasks (#12657)

* Add a task control service to facilitate a new structure for tasks

task/backend/task.go

@@ -0,0 +1,31 @@
+package backend
+
+import (
+	"context"
+	"time"
+
+	"github.com/influxdata/influxdb"
+)
+
+// TaskControlService is a low-level controller interface, intended to be passed to
+// task executors and schedulers, which allows creation, completion, and status updates of runs.
+type TaskControlService interface {
+	// CreateNextRun attempts to create a new run.
+	// The new run's ScheduledFor is assigned the earliest possible time according to task's cron,
+	// that is later than any in-progress run and LatestCompleted run.
+	// If the run's ScheduledFor would be later than the passed-in now, CreateNextRun returns a RunNotYetDueError.
+	CreateNextRun(ctx context.Context, taskID influxdb.ID, now int64) (RunCreation, error)
+
+	// FinishRun removes runID from the list of running tasks and if its `ScheduledFor` is later then last completed update it.
+	FinishRun(ctx context.Context, taskID, runID influxdb.ID) (*influxdb.Run, error)
+
+	// NextDueRun returns the Unix timestamp of when the next call to CreateNextRun will be ready.
+	// The returned timestamp reflects the task's offset, so it does not necessarily exactly match the schedule time.
+	NextDueRun() (int64, error)
+
+	// UpdateRunState sets the run state at the respective time.
+	UpdateRunState(ctx context.Context, taskID, runID influxdb.ID, when time.Time, state RunStatus) error
+
+	// AddRunLog adds a log line to the run.
+	AddRunLog(ctx context.Context, taskID, runID influxdb.ID, when time.Time, log string) error
+}