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Fix torn write of struct causing crash in symbol finder. #69927

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merged 10 commits into from
Sep 13, 2023
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Fix torn write of struct causing crash in symbol finder. #69927

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f2b038a
Add contract calls to help trace down an internally reported crash
CyrusNajmabadi Sep 13, 2023
ae7cb03
Simplify
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09eafaf
Add asserts
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Simplify
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2f69fab
Switch to strongbox
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Whitespace
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Comment
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31 changes: 14 additions & 17 deletions src/Workspaces/Core/Portable/FindSymbols/SymbolTree/SymbolTreeInfo.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,16 +3,15 @@
// See the LICENSE file in the project root for more information.

using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.Collections;
using Microsoft.CodeAnalysis.PooledObjects;
using Microsoft.CodeAnalysis.Shared.Collections;
using Microsoft.CodeAnalysis.Utilities;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis.FindSymbols
Expand Down Expand Up @@ -70,16 +69,10 @@ public MultiDictionary<string, ExtensionMethodInfo>.ValueSet GetExtensionMethodI

public bool ContainsExtensionMethod => _receiverTypeNameToExtensionMethodMap?.Count > 0;

private SpellChecker? _spellChecker;
private SpellChecker SpellChecker
{
get
{
_spellChecker ??= CreateSpellChecker(Checksum, _nodes);
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SpellChecker? was a Nullable<SpellChecker>. This is just a pure struct which can have torn writes. By moving to a StrongBox<SpellChecker> we have teh runtime guarantee that if _spllChecker is not null then _spellChecker.Value is 'completely written'.


return _spellChecker.Value;
}
}
/// <summary>
/// Explicitly boxed so that we can safely initialize/read this across threads without the need for a lock.
/// </summary>
private StrongBox<SpellChecker>? _spellChecker;
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❗ Since _spellChecker.Value is never written after construction, and there are no other instantiations of the SpellChecker type, this change is functionally equivalent to (but much more complicated than) just making SpellChecker a reference type. You could further simplify things by using the helpers from #69017 to initialize the field on first use.

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@sharwell sharwell Sep 13, 2023 •
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The other option if you want to keep this a value type is to use this helper:

roslyn/src/Compilers/Core/Portable/InternalUtilities/RoslynLazyInitializer.cs

Lines 26 to 27 in bb804d5

public static T EnsureInitialized<T>([NotNull] ref T? target, ref bool initialized, [NotNullIfNotNull(nameof(syncLock))] ref object? syncLock, Func<T> valueFactory)
=> LazyInitializer.EnsureInitialized<T>(ref target!, ref initialized, ref syncLock, valueFactory);

The implementation of EnsureInitialized contains a lock-free fast path for the case where initialized is true (set at the end of initialization). You may need to use PooledDelegates to avoid a delegate allocation on the fast path as well.

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Thanks. Moved to just be a refernece type. That fits best here for me :)


private SymbolTreeInfo(
Checksum checksum,
Expand All @@ -96,7 +89,7 @@ private SymbolTreeInfo(
private SymbolTreeInfo(
Checksum checksum,
ImmutableArray<Node> sortedNodes,
SpellChecker? spellChecker,
StrongBox<SpellChecker>? spellChecker,
OrderPreservingMultiDictionary<int, int> inheritanceMap,
MultiDictionary<string, ExtensionMethodInfo>? receiverTypeNameToExtensionMethodMap)
{
Expand Down Expand Up @@ -181,11 +174,15 @@ private async Task<ImmutableArray<ISymbol>> FuzzyFindAsync(
using var similarNames = TemporaryArray<string>.Empty;
using var result = TemporaryArray<ISymbol>.Empty;

SpellChecker.FindSimilarWords(ref similarNames.AsRef(), name, substringsAreSimilar: false);
// Ensure the spell checker is initialized. This is concurrency safe. Technically multiple threads may end
// up overwriting the field, but even if that happens, we are sure to see a fully written spell checker as
// the runtime guarantees that the strongbox .Value field will be completely written when we read out field.
_spellChecker ??= CreateSpellChecker(Checksum, _nodes);
_spellChecker.Value.FindSimilarWords(ref similarNames.AsRef(), name, substringsAreSimilar: false);

foreach (var similarName in similarNames)
{
var symbols = await FindAsync(lazyAssembly, similarName, ignoreCase: true, cancellationToken: cancellationToken).ConfigureAwait(false);
var symbols = await FindAsync(lazyAssembly, similarName, ignoreCase: true, cancellationToken).ConfigureAwait(false);
result.AddRange(symbols);
}

Expand Down Expand Up @@ -302,8 +299,8 @@ private static int BinarySearch(ImmutableArray<Node> nodes, string name)

#region Construction

private static SpellChecker CreateSpellChecker(Checksum checksum, ImmutableArray<Node> sortedNodes)
=> new(checksum, sortedNodes.Select(n => n.Name));
private static StrongBox<SpellChecker> CreateSpellChecker(Checksum checksum, ImmutableArray<Node> sortedNodes)
=> new(new(checksum, sortedNodes.Select(n => n.Name)));

private static ImmutableArray<Node> SortNodes(ImmutableArray<BuilderNode> unsortedNodes)
{
Expand Down
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