TaskRouter.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Collections.Concurrent;

namespace Microsoft.Build.BackEnd
{
    /// <summary>
    /// Determines where a task should be executed in multi-threaded mode.
    /// In multi-threaded execution mode, tasks marked with MSBuildMultiThreadableTaskAttribute run
    /// in-process within thread nodes, while all other tasks are routed to sidecar TaskHost processes
    /// for isolation.
    /// </summary>
    /// <remarks>
    /// This class should only be used when in multi-threaded mode. Traditional multi-proc builds
    /// have different semantics and should not use this routing logic.
    /// <para>
    /// The attribute is the only routing signal. Implementing <see cref="Microsoft.Build.Framework.IMultiThreadableTask"/>
    /// does not opt a task into in-process execution: <c>Microsoft.Build.Utilities.ToolTask</c> implements that
    /// interface, so honoring it here would silently opt in every ToolTask-derived task in the ecosystem, none of which
    /// have been reviewed for thread safety. The interface instead controls whether the engine injects a
    /// TaskEnvironment into the task, which TaskExecutionHost handles separately.
    /// </para>
    /// </remarks>
    internal static class TaskRouter
    {
        /// <summary>
        /// Cache of task types to their multi-threadable attribute status.
        /// This avoids repeated reflection calls for the same task types.
        /// </summary>
        private static readonly ConcurrentDictionary<Type, bool> s_multiThreadableTaskCache = new();

    /// <summary>
    /// Determines if a task needs to be routed to an out-of-process TaskHost sidecar
    /// in multi-threaded mode based on its thread-safety characteristics.
    /// </summary>
    /// <param name="taskType">The type of the task to evaluate.</param>
    /// <returns>
    /// True if the task should be executed in an out-of-process TaskHost sidecar;
    /// false if it can safely run in-process within a thread node.
    /// </returns>
    /// <remarks>
    /// This method only considers the task's thread-safety indicators.
    /// The caller is responsible for:
    /// - Only calling this in multi-threaded mode
    /// - Handling explicit out-of-proc requests (via TaskHostFactory or parameters)
    /// - Handling the isAlreadyOutOfProc scenario
    /// 
    /// In multi-threaded mode:
    /// - Tasks marked with MSBuildMultiThreadableTaskAttribute (non-inheritable) are considered
    ///   thread-safe and can run in-process (returns false)
    /// - Tasks without the attribute must run in a sidecar TaskHost for isolation (returns true)
    /// </remarks>
    public static bool NeedsTaskHostInMultiThreadedMode(Type taskType)
    {
        ArgumentNullException.ThrowIfNull(taskType);

        // Tasks without the thread-safety attribute need isolation in a TaskHost sidecar
        return !HasMultiThreadableTaskAttribute(taskType);
    }

        /// <summary>
        /// Checks if a task type is marked with MSBuildMultiThreadableTaskAttribute.
        /// Detection is based on namespace and name only, ignoring the defining assembly,
        /// which allows customers to define the attribute in their own assemblies.
        /// Results are cached to avoid repeated reflection calls.
        /// </summary>
        /// <param name="taskType">The task type to check.</param>
        /// <returns>True if the task has the attribute; false otherwise.</returns>
        private static bool HasMultiThreadableTaskAttribute(Type taskType)
        {
            return s_multiThreadableTaskCache.GetOrAdd(
                taskType,
                static t =>
                {
                    const string attributeFullName = "Microsoft.Build.Framework.MSBuildMultiThreadableTaskAttribute";

                    // Check for the attribute by full name, not by type identity
                    // This allows custom-defined attributes from different assemblies
                    foreach (object attr in t.GetCustomAttributes(inherit: false))
                    {
                        if (attr.GetType().FullName == attributeFullName)
                        {
                            return true;
                        }
                    }

                    return false;
                });
        }

        /// <summary>
        /// Clears the thread-safety cache. Used primarily for testing.
        /// </summary>
        internal static void ClearCache()
        {
            s_multiThreadableTaskCache.Clear();
        }
    }
}