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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
using System.Collections.Immutable;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.LanguageServer.Handler.Diagnostics;
using Roslyn.Utilities;
namespace Microsoft.CodeAnalysis.LanguageServer.Handler;
internal abstract partial class VersionedPullCache<TCheapVersion, TExpensiveVersion, TState, TComputedData>
{
/// <summary>
/// Internal cache item that updates state for a particular <see cref="Workspace"/> and <see cref="ProjectOrDocumentId"/> in <see cref="VersionedPullCache{TCheapVersion, TExpensiveVersion, TState, TComputedData}"/>
/// This type ensures that the state for a particular key is never updated concurrently for the same key (but different key states can be concurrent).
/// </summary>
private sealed class CacheItem(string uniqueKey)
{
/// <summary>
/// Guards access to <see cref="_lastResult"/>.
/// This ensures that a cache entry is fully updated in a single transaction.
/// </summary>
private readonly SemaphoreSlim _gate = new(initialCount: 1);
/// <summary>
/// Stores the current state associated with this cache item.
/// Guarded by <see cref="_gate"/>
///
/// <list type="bullet">
/// <item>The resultId reported to the client.</item>
/// <item>The TCheapVersion of the data that was used to calculate results.
/// <para>
/// Note that this version can change even when nothing has actually changed (for example, forking the
/// LSP text, reloading the same project). So we additionally store:</para></item>
/// <item>A TExpensiveVersion (normally a checksum) checksum that will still allow us to reuse data even when
/// unimportant changes happen that trigger the cheap version change detection.</item>
/// <item>The checksum of the data that was computed when the resultId was generated.
/// <para>
/// When the versions above change, we must recalculate the data. However sometimes that data ends up being exactly the same as the prior request.
/// When that happens, this allows us to send back an unchanged result instead of reserializing data the client already has.
/// </para>
/// </item>
/// </list>
///
/// </summary>
private (string resultId, TCheapVersion cheapVersion, TExpensiveVersion expensiveVersion, Checksum dataChecksum)? _lastResult;
/// <summary>
/// Updates the values for this cache entry. Guarded by <see cref="_gate"/>
///
/// Returns <see langword="null"/> if the previousPullResult can be re-used, otherwise returns a new resultId and the new data associated with it.
/// </summary>
public async Task<(string, TComputedData)?> UpdateCacheItemAsync(
VersionedPullCache<TCheapVersion, TExpensiveVersion, TState, TComputedData> cache,
PreviousPullResult? previousPullResult,
bool isFullyLoaded,
TState state,
CancellationToken cancellationToken)
{
// Ensure that we only update the cache item one at a time.
// This means that the computation of new data for this item only occurs sequentially.
using (await _gate.DisposableWaitAsync(cancellationToken).ConfigureAwait(false))
{
TCheapVersion cheapVersion;
TExpensiveVersion expensiveVersion;
// Check if the version we have in the cache matches the request version. If so we can re-use the resultId.
if (isFullyLoaded &&
_lastResult is not null &&
_lastResult.Value.resultId == previousPullResult?.PreviousResultId)
{
cheapVersion = await cache.ComputeCheapVersionAsync(state, cancellationToken).ConfigureAwait(false);
if (cheapVersion != null && cheapVersion.Equals(_lastResult.Value.cheapVersion))
{
// The client's resultId matches our cached resultId and the cheap version is an
// exact match for our current cheap version. We return early here to avoid calculating
// expensive versions as we know nothing is changed.
return null;
}
// The current cheap version does not match the last reported. This may be because we've forked
// or reloaded a project, so fall back to calculating the full expensive version to determine if
// anything is actually changed.
expensiveVersion = await cache.ComputeExpensiveVersionAsync(state, cancellationToken).ConfigureAwait(false);
if (expensiveVersion != null && expensiveVersion.Equals(_lastResult.Value.expensiveVersion))
{
return null;
}
}
else
{
// The versions we have in our cache (if any) do not match the ones provided by the client (if any).
// We need to calculate new results.
cheapVersion = await cache.ComputeCheapVersionAsync(state, cancellationToken).ConfigureAwait(false);
expensiveVersion = await cache.ComputeExpensiveVersionAsync(state, cancellationToken).ConfigureAwait(false);
}
// Compute the new result for the request.
var data = await cache.ComputeDataAsync(state, cancellationToken).ConfigureAwait(false);
var dataChecksum = cache.ComputeChecksum(data);
string newResultId;
if (_lastResult is not null && _lastResult?.resultId == previousPullResult?.PreviousResultId && _lastResult?.dataChecksum == dataChecksum)
{
// The new data we've computed is exactly the same as the data we computed last time even though the versions have changed.
// Instead of reserializing everything, we can return the same result id back to the client.
// Ensure we store the updated versions we calculated against old resultId. If we do not do this,
// subsequent requests will always fail the version comparison check (the resultId is still associated with the older version even
// though we reused it here for a newer version) and will trigger re-computation.
// By storing the updated version with the resultId we can short circuit earlier in the version checks.
_lastResult = (_lastResult.Value.resultId, cheapVersion, expensiveVersion, dataChecksum);
return null;
}
else
{
// Keep track of the results we reported here so that we can short-circuit producing results for
// the same state of the world in the future. Use a custom result-id per type (doc requests or workspace
// requests) so that clients of one don't errantly call into the other.
//
// For example, a client getting document diagnostics should not ask for workspace diagnostics with the result-ids it got for
// doc-diagnostics. The two systems are different and cannot share results, or do things like report
// what changed between each other.
//
// Note that we can safely update the map before computation as any cancellation or exception
// during computation means that the client will never recieve this resultId and so cannot ask us for it.
newResultId = $"{uniqueKey}:{cache.GetNextResultId()}";
_lastResult = (newResultId, cheapVersion, expensiveVersion, dataChecksum);
return (newResultId, data);
}
}
}
}
}
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