[C#] FileDescriptor.GetAllDependedExtensions' extension cache is not thread-safe, causes Dictionary corruption under concurrent FromGeneratedCode calls

What version of protobuf and what language are you using?

3.36.0

Version: main/v33.0/etc (NOTE: please update to the
latest supported version of
protoc/runtime possible beforehand to attempt to resolve your problem)

Language: C#

What
supported operating system version
are you using (e.g. Linux, Windows) ?

This happened in Linux and was reproduced in a mac machine

What
supported runtime / compiler version
are you using (e.g. python version, gcc version)

.Net8

What did you do?

Steps to reproduce the behavior:
Demo application

public class ProtobufExtensionCacheRaceReproTests
{
    private const int MaxAttempts = 15;
    private static readonly TimeSpan PerAttemptTimeout = TimeSpan.FromSeconds(20);

    private readonly ITestOutputHelper _output;

    public ProtobufExtensionCacheRaceReproTests(ITestOutputHelper output)
    {
        _output = output;
    }

    [Fact]
    [Trait("Category", "ProtobufRaceRepro")]
    public async Task ConcurrentFirstTouchOfSharedProtoDependencies_CorruptsExtensionCache()
    {
        var reproDllPath = ResolveReproDllPath();
        File.Exists(reproDllPath).Should().BeTrue(
            $"the repro harness must be built alongside the test project at '{reproDllPath}'");

        for (var attempt = 1; attempt <= MaxAttempts; attempt++)
        {
            var (exitCode, stdout) = await RunReproProcessAsync(reproDllPath);
            _output.WriteLine($"Attempt {attempt}/{MaxAttempts}: exit={exitCode}");
            _output.WriteLine(stdout);

            if (exitCode == 1 && stdout.Contains("RACE_REPRODUCED"))
            {
                return;
            }
        }

        Assert.Fail(
            $"Did not reproduce the Google.Protobuf extension-cache race in {MaxAttempts} attempts. " +
            "This is a timing-dependent upstream bug (protobuf 3.36.0 FileDescriptor.GetAllDependedExtensions " +
            "writes to a shared static Dictionary without a lock) - if this consistently fails to reproduce, " +
            "either the dependency has been downgraded/patched, or the machine is too fast/slow for the current " +
            "thread count to collide reliably.");
    }

    private static string ResolveReproDllPath()
    {
        var baseDir = AppContext.BaseDirectory;
        var reproDir = baseDir.Replace(
            $"{Path.DirectorySeparatorChar}TTD.Infra.Janus.UT{Path.DirectorySeparatorChar}",
            $"{Path.DirectorySeparatorChar}TTD.Infra.Janus.ProtobufRaceRepro{Path.DirectorySeparatorChar}");
        return Path.Combine(reproDir, "TTD.Infra.Janus.ProtobufRaceRepro.dll");
    }

    private static async Task<(int ExitCode, string Stdout)> RunReproProcessAsync(string reproDllPath)
    {
        using var process = new Process
        {
            StartInfo = new ProcessStartInfo
            {
                FileName = "dotnet",
                Arguments = $"exec \"{reproDllPath}\"",
                RedirectStandardOutput = true,
                RedirectStandardError = true,
                UseShellExecute = false,
            },
        };

        process.Start();
        var stdoutTask = process.StandardOutput.ReadToEndAsync();
        var stderrTask = process.StandardError.ReadToEndAsync();

        using var cts = new CancellationTokenSource(PerAttemptTimeout);
        await process.WaitForExitAsync(cts.Token);

        var stdout = await stdoutTask;
        var stderr = await stderrTask;
        return (process.ExitCode, stdout + stderr);
    }
}

UT to run the application and check for repro

public class ProtobufExtensionCacheRaceReproTests
{
    private const int MaxAttempts = 15;
    private static readonly TimeSpan PerAttemptTimeout = TimeSpan.FromSeconds(20);

    private readonly ITestOutputHelper _output;

    public ProtobufExtensionCacheRaceReproTests(ITestOutputHelper output)
    {
        _output = output;
    }

    [Fact]
    [Trait("Category", "ProtobufRaceRepro")]
    public async Task ConcurrentFirstTouchOfSharedProtoDependencies_CorruptsExtensionCache()
    {
        var reproDllPath = ResolveReproDllPath();
        File.Exists(reproDllPath).Should().BeTrue(
            $"the repro harness must be built alongside the test project at '{reproDllPath}'");

        for (var attempt = 1; attempt <= MaxAttempts; attempt++)
        {
            var (exitCode, stdout) = await RunReproProcessAsync(reproDllPath);
            _output.WriteLine($"Attempt {attempt}/{MaxAttempts}: exit={exitCode}");
            _output.WriteLine(stdout);

            if (exitCode == 1 && stdout.Contains("RACE_REPRODUCED"))
            {
                return;
            }
        }

        Assert.Fail(
            $"Did not reproduce the Google.Protobuf extension-cache race in {MaxAttempts} attempts. " +
            "This is a timing-dependent upstream bug (protobuf 3.36.0 FileDescriptor.GetAllDependedExtensions " +
            "writes to a shared static Dictionary without a lock) - if this consistently fails to reproduce, " +
            "either the dependency has been downgraded/patched, or the machine is too fast/slow for the current " +
            "thread count to collide reliably.");
    }

    private static string ResolveReproDllPath()
    {
        var baseDir = AppContext.BaseDirectory;
        var reproDir = baseDir.Replace(
            $"{Path.DirectorySeparatorChar}TTD.Infra.Janus.UT{Path.DirectorySeparatorChar}",
            $"{Path.DirectorySeparatorChar}TTD.Infra.Janus.ProtobufRaceRepro{Path.DirectorySeparatorChar}");
        return Path.Combine(reproDir, "TTD.Infra.Janus.ProtobufRaceRepro.dll");
    }

    private static async Task<(int ExitCode, string Stdout)> RunReproProcessAsync(string reproDllPath)
    {
        using var process = new Process
        {
            StartInfo = new ProcessStartInfo
            {
                FileName = "dotnet",
                Arguments = $"exec \"{reproDllPath}\"",
                RedirectStandardOutput = true,
                RedirectStandardError = true,
                UseShellExecute = false,
            },
        };

        process.Start();
        var stdoutTask = process.StandardOutput.ReadToEndAsync();
        var stderrTask = process.StandardError.ReadToEndAsync();

        using var cts = new CancellationTokenSource(PerAttemptTimeout);
        await process.WaitForExitAsync(cts.Token);

        var stdout = await stdoutTask;
        var stderr = await stderrTask;
        return (process.ExitCode, stdout + stderr);
    }
}

Output

Attempt 1/15: exit=1
RACE_REPRODUCED
--- System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.set_Item(TKey key, TValue value)
   at Google.Protobuf.Reflection.FileDescriptor.GetAllDependedExtensions(FileDescriptor descriptor)
   at System.Linq.Enumerable.SelectManySingleSelectorIterator`2.MoveNext()
   at System.Linq.Enumerable.DistinctIterator`1.MoveNext()
   at System.Collections.Generic.List`1.AddRange(IEnumerable`1 collection)
   at System.Linq.Enumerable.ConcatIterator`1.ToList()
   at Google.Protobuf.Reflection.FileDescriptor.GetAllExtensions(FileDescriptor[] dependencies, GeneratedClrTypeInfo generatedInfo)
   at Google.Protobuf.Reflection.FileDescriptor.FromGeneratedCode(Byte[] descriptorData, FileDescriptor[] dependencies, GeneratedClrTypeInfo generatedCodeInfo)
   at Program.<>c__DisplayClass0_0.<<Main>$>g__BuildFile|0(String name, FileDescriptor[] dependencies) in /Users/menna.elmesalmy/dev/workspace/TTD.Infra.Janus/src/TTD.Infra.Janus.ProtobufRaceRepro/Program.cs:line 58
   at Program.<>c__DisplayClass0_0.<<Main>$>b__4() in /Users/menna.elmesalmy/dev/workspace/TTD.Infra.Janus/src/TTD.Infra.Janus.ProtobufRaceRepro/Program.cs:line 83

What did you expect to see
No exceptions in the application.

What did you see instead?

Our redis polling keeps failing

[08/09/2026 11:54:38 ERR 0003]  <RedisStreamTelemetry.StreamReadError> Error reading updates from Redis stream stream-1
ERROR:The type initializer for 'ProtoConfigReflection' threw an exception.
System.TypeInitializationException: The type initializer for 'ProtoConfigReflection' threw an exception.
 ---> System.InvalidOperationException: Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection's state is no longer correct.
   at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
   at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
   at Google.Protobuf.Reflection.FileDescriptor.GetAllDependedExtensions(FileDescriptor descriptor)
   at System.Linq.Enumerable.SelectManySingleSelectorIterator`2.MoveNext()
   at System.Collections.Generic.List`1.AddRange(IEnumerable`1 collection)
   at System.Linq.Enumerable.ConcatIterator`1.ToList()
   at Google.Protobuf.Reflection.FileDescriptor.GetAllDependedExtensions(FileDescriptor descriptor)
   at System.Linq.Enumerable.SelectManySingleSelectorIterator`2.MoveNext()
   at System.Linq.Enumerable.DistinctIterator`1.MoveNext()
   at System.Collections.Generic.List`1.AddRange(IEnumerable`1 collection)
   at System.Linq.Enumerable.ConcatIterator`1.ToList()
   at Google.Protobuf.Reflection.FileDescriptor.GetAllExtensions(FileDescriptor[] dependencies, GeneratedClrTypeInfo generatedInfo)
   at Google.Protobuf.Reflection.FileDescriptor.FromGeneratedCode(Byte[] descriptorData, FileDescriptor[] dependencies, GeneratedClrTypeInfo generatedCodeInfo)
   at ProtoConfigReflection..cctor() in /ProtoConfigReflection/obj/Release/net8.0/Protos/ConfigUpdatesStream.cs:line 45
   --- End of inner exception stack trace ---
   at ProtoConfigMessage.get_Descriptor() in Contract/obj/Release/net8.0/Protos/ConfigUpdatesStream.cs:line 98
   at ConfigStreamMessage.ToString() in /Contract/obj/Release/net8.0/Protos/ConfigUpdatesStream.cs:line ...

Anything else we should know about your project / environment

N/A

This seems to be solvable with a lock or ConcurrentDictionary, any concern with me raising a PR to do that?
Changing

        private static readonly Dictionary<string, List<Extension>> allDependedExtensionsCache = new();

to

       private static readonly ConcurrentDictionary<string, List<Extension>> allDependedExtensionsCache = new();

does the trick in my intial testing

Start by locating Google.Protobuf.Reflection.FileDescriptor.GetAllDependedExtensions and inspect its allDependedExtensionsCache access during concurrent FromGeneratedCode calls. Run the supplied ProtobufExtensionCacheRaceReproTests or equivalent repro, then add coverage that exercises concurrent first use and confirms no collection-corruption exceptions occur.