Cron - Not running multiple instances of a job

首先定位 scheduler tick 的处理逻辑、ICronJob 入口点以及对 job.Run(stoppingToken) 的调用。跟踪计划任务的选择和跟踪方式。完成标准是：计划任务在其上一次 task 仍在运行时不会再次启动，但在完成后可以启动后续运行。

This is not an elaborate solution, but it works (in theory). You can't choose to have multiple instances running or not. I just assumed not to have multiple instances running regardless. I assumed this is not required unless you are running a job every minute or two.

I added a field to the scheduler that tracks the Task returned by job.Run(stoppingToken);. On ever tick check the list for completed tasks and clear them. Prior to running a job skip if it is on the list of running jobs.

I'm sure this might need some tweaking this was my first attempt at it.

private Dictionary<string, Task> _runningJobs;

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// Clear completed jobs
foreach (var key in _runningJobs.Keys)
{
    if (_runningJobs[key].IsCompleted)
    {
        _runningJobs[key].Dispose();
        _runningJobs.Remove(key);
    }
}

// Run jobs that are in the map
RunActiveJobs(runMap, now, stoppingToken);

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foreach (var run in currentRuns)
{

    // We are sure (thanks to our extension method)
    // that the service is of type ICronJob
    var job = (ICronJob)_serviceProvider.GetRequiredService(run);
    string jobName = job.GetType().Name;

    // Continue to next job if job is already running
    if (_runningJobs.ContainsKey(jobName))
    {
        continue;
    }

    // We don't want to await jobs explicitly because that
    // could interfere with other job runs
    var task = job.Run(stoppingToken);

    // Add task to running jobs
    _runningJobs.Add(jobName, task);
}