duplicati/Duplicati/WebserverCore/Services/DatabaseLockTracker.cs

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using System.Collections.Concurrent;
using Duplicati.WebserverCore.Abstractions;

namespace Duplicati.WebserverCore.Services;

/// <summary>
/// Tracks which local backup database files are currently in use by running operations,
/// preventing concurrent access that would cause SQLite "database is locked" errors.
/// </summary>
/// <remarks>
/// Each database path gets its own <see cref="SemaphoreSlim"/> (initial count 1).
/// Queued tasks call <see cref="AcquireAsync"/> which waits for the lock.
/// Immediate tasks call <see cref="TryAcquire"/> which fails instantly if locked.
/// <para>
/// Semaphore entries are intentionally not removed on release. Removing them is racy: a
/// concurrent acquirer could take the lock between a release and a dictionary removal,
/// after which a new acquirer would create a fresh semaphore for the same path, breaking
/// mutual exclusion. The dictionary is bounded by the number of unique database paths
/// (one per backup), which is a small, long-lived set, so the lack of cleanup does not
/// cause unbounded growth in practice.
/// </para>
/// </remarks>
public class DatabaseLockTracker : IDatabaseLockTracker
{
    private const int MaxLockCount = 1;

    private readonly ConcurrentDictionary<string, SemaphoreSlim> _locks = new(Library.Utility.Utility.ClientFilenameStringComparer);

    /// <inheritdoc />
    public async Task<IAsyncDisposable> AcquireAsync(string dbPath, CancellationToken cancellationToken)
    {
        var semaphore = GetOrCreateSemaphore(dbPath);
        await semaphore.WaitAsync(cancellationToken).ConfigureAwait(false);
        return new LockRelease(semaphore);
    }

    /// <inheritdoc />
    public IAsyncDisposable? TryAcquire(string dbPath)
    {
        var semaphore = GetOrCreateSemaphore(dbPath);
        if (!semaphore.Wait(0))
            return null;
        return new LockRelease(semaphore);
    }

    /// <summary>
    /// Normalizes the database path and gets (or creates) the semaphore for it.
    /// </summary>
    private SemaphoreSlim GetOrCreateSemaphore(string dbPath)
    {
        var normalized = NormalizePath(dbPath);
        if (string.IsNullOrEmpty(normalized))
            throw new ArgumentException("Database path cannot be null or empty", nameof(dbPath));
        return _locks.GetOrAdd(normalized, _ => new SemaphoreSlim(MaxLockCount, MaxLockCount));
    }

    /// <summary>
    /// Normalizes a file path so that the same database file always maps to the same key,
    /// regardless of path separators or relative vs. absolute forms.
    /// </summary>
    private static string NormalizePath(string dbPath)
    {
        if (string.IsNullOrWhiteSpace(dbPath))
            return string.Empty;
        return Path.GetFullPath(dbPath);
    }

    /// <summary>
    /// A disposable handle that releases the semaphore when disposed.
    /// </summary>
    private sealed class LockRelease(SemaphoreSlim semaphore) : IAsyncDisposable
    {
        private int _released;

        public ValueTask DisposeAsync()
        {
            if (Interlocked.Exchange(ref _released, 1) == 0)
                semaphore.Release();
            return ValueTask.CompletedTask;
        }
    }
}