75 lines
2.7 KiB
C#
75 lines
2.7 KiB
C#
 
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using MongoDB.Driver;
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using MongoDB.Entities;
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namespace PrivaPub.Infrastructure.Backup
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{
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// One backup or restore at a time, whoever starts it (the CLI, the nightly schedule, the administrator's page): a
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// document held with a heartbeat. A holder that died (no heartbeat for two minutes) is taken over. The collection is
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// never backed up, nor dropped by a restore.
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public class MaintenanceLock : Entity
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{
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public const string Name = "maintenance";
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static readonly TimeSpan Stale = TimeSpan.FromMinutes(2);
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static readonly TimeSpan Beat = TimeSpan.FromSeconds(30);
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public string Owner { get; set; }
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public string What { get; set; }
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public DateTime Since { get; set; }
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public DateTime Heartbeat { get; set; }
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/// <summary>Takes the lock for what is said, or null when someone else holds it; disposing the result frees it.</summary>
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public static async Task<Held> Take(string what, CancellationToken token)
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{
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var owner = $"{Environment.MachineName}:{Environment.ProcessId}:{Guid.NewGuid():N}";
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var now = DateTime.UtcNow;
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var stale = now - Stale;
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try
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{
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var taken = await DB.Default.UpdateAndGet<MaintenanceLock>()
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.Match(l => l.ID == Name && (l.Heartbeat < stale || l.Owner == null))
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.Modify(l => l.Owner, owner)
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.Modify(l => l.What, what)
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.Modify(l => l.Since, now)
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.Modify(l => l.Heartbeat, now)
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.Option(o => o.IsUpsert = true)
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.ExecuteAsync(token);
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return taken?.Owner == owner ? new Held(owner) : default;
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}
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catch (MongoCommandException ex) when (ex.Code == 11000)
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{
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return default;//held: the upsert met the live lock's id
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}
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catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
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{
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return default;
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}
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}
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/// <summary>Who holds it now and for what, or null.</summary>
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public static async Task<MaintenanceLock> Current(CancellationToken token)
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{
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var held = await DB.Default.Find<MaintenanceLock>().Match(l => l.ID == Name).ExecuteFirstAsync(token);
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return held?.Owner != null && held.Heartbeat >= DateTime.UtcNow - Stale ? held : default;
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}
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public sealed class Held : IAsyncDisposable
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{
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readonly string _owner;
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readonly Timer _heartbeat;
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public Held(string owner)
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{
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_owner = owner;
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_heartbeat = new Timer(_ => _ = DB.Default.Update<MaintenanceLock>().Match(l => l.ID == Name && l.Owner == owner)
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.Modify(l => l.Heartbeat, DateTime.UtcNow).ExecuteAsync(), default, Beat, Beat);
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}
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public async ValueTask DisposeAsync()
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{
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await _heartbeat.DisposeAsync();
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await DB.Default.Update<MaintenanceLock>().Match(l => l.ID == Name && l.Owner == _owner)
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.Modify(l => l.Owner, null).Modify(l => l.Heartbeat, DateTime.MinValue).ExecuteAsync();
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}
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}
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}
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}
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