FileTextLoader.cs

// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.

using System;
using System.Collections.Immutable;
using System.Composition;
using System.Diagnostics;
using System.IO;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.Host;
using Microsoft.CodeAnalysis.Internal.Log;
using Microsoft.CodeAnalysis.Options;
using Microsoft.CodeAnalysis.Options.Providers;
using Microsoft.CodeAnalysis.Text;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis
{
    internal static class FileTextLoaderOptions
    {
        /// <summary>
        /// Hidden registry key to control maximum size of a text file we will read into memory. 
        /// we have this option to reduce a chance of OOM when user adds massive size files to the solution.
        /// Default threshold is 100MB which came from some internal data on big files and some discussion.
        /// 
        /// User can override default value by setting DWORD value on FileLengthThreshold in 
        /// "[VS HIVE]\Roslyn\Internal\Performance\Text"
        /// </summary>
        internal static readonly Option<long> FileLengthThreshold = new Option<long>(nameof(FileTextLoaderOptions), nameof(FileLengthThreshold), defaultValue: 100 * 1024 * 1024,
            storageLocations: new LocalUserProfileStorageLocation(@"Roslyn\Internal\Performance\Text\FileLengthThreshold"));
    }

    [ExportOptionProvider, Shared]
    internal class FileTextLoaderOptionsProvider : IOptionProvider
    {
        [ImportingConstructor]
        public FileTextLoaderOptionsProvider()
        {
        }

        public ImmutableArray<IOption> Options { get; } = ImmutableArray.Create<IOption>(
            FileTextLoaderOptions.FileLengthThreshold);
    }

    [DebuggerDisplay("{GetDebuggerDisplay(), nq}")]
    public class FileTextLoader : TextLoader
    {
        private readonly string _path;
        private readonly Encoding _defaultEncoding;

        /// <summary>
        /// Creates a content loader for specified file.
        /// </summary>
        /// <param name="path">An absolute file path.</param>
        /// <param name="defaultEncoding">
        /// Specifies an encoding to be used if the actual encoding can't be determined from the stream content (the stream doesn't start with Byte Order Mark).
        /// If not specified auto-detect heuristics are used to determine the encoding.
        /// Note that if the stream starts with Byte Order Mark the value of <paramref name="defaultEncoding"/> is ignored.
        /// </param>
        /// <exception cref="ArgumentNullException"><paramref name="path"/> is null.</exception>
        /// <exception cref="ArgumentException"><paramref name="path"/> is not an absolute path.</exception>
        public FileTextLoader(string path, Encoding defaultEncoding)
        {
            CompilerPathUtilities.RequireAbsolutePath(path, "path");

            _path = path;
            _defaultEncoding = defaultEncoding;
        }

        /// <summary>
        /// Absolute path of the file.
        /// </summary>
        public string Path
        {
            get { return _path; }
        }

        /// <summary>
        /// Specifies an encoding to be used if the actual encoding of the file 
        /// can't be determined from the stream content (the stream doesn't start with Byte Order Mark).
        /// If <c>null</c> auto-detect heuristics are used to determine the encoding. 
        /// Note that if the stream starts with Byte Order Mark the value of <see cref="DefaultEncoding"/> is ignored.
        /// </summary>
        public Encoding DefaultEncoding
        {
            get { return _defaultEncoding; }
        }

        protected virtual SourceText CreateText(Stream stream, Workspace workspace)
        {
            var factory = workspace.Services.GetService<ITextFactoryService>();
            return factory.CreateText(stream, _defaultEncoding);
        }

        /// <summary>
        /// Load a text and a version of the document in the workspace.
        /// </summary>
        /// <exception cref="IOException"></exception>
        /// <exception cref="InvalidDataException"></exception>
        public override async Task<TextAndVersion> LoadTextAndVersionAsync(Workspace workspace, DocumentId documentId, CancellationToken cancellationToken)
        {
            ValidateFileLength(workspace, _path);

            var prevLastWriteTime = FileUtilities.GetFileTimeStamp(_path);

            TextAndVersion textAndVersion;

            // In many .NET Framework versions (specifically the 4.5.* series, but probably much earlier
            // and also later) there is this particularly interesting bit in FileStream.BeginReadAsync:
            //
            //     // [ed: full comment clipped for brevity]
            //     //
            //     // If we did a sync read to fill the buffer, we could avoid the
            //     // problem, and any async read less than 64K gets turned into a
            //     // synchronous read by NT anyways...
            //     if (numBytes < _bufferSize)
            //     {
            //         if (_buffer == null) _buffer = new byte[_bufferSize];
            //         IAsyncResult bufferRead = BeginReadCore(_buffer, 0, _bufferSize, null, null, 0);
            //         _readLen = EndRead(bufferRead);
            //
            // In English, this means that if you do a asynchronous read for smaller than _bufferSize,
            // this is implemented by the framework by starting an asynchronous read, and then
            // blocking your thread until that read is completed. The comment implies this is "fine"
            // because the asynchronous read will actually be synchronous and thus EndRead won't do
            // any blocking -- it'll be an effective no-op. In theory, everything is fine here.
            //
            // In reality, this can end very poorly. That read in fact can be asynchronous, which means the
            // EndRead will enter a wait and block the thread. If we are running that call to ReadAsync on a
            // thread pool thread that completed a previous piece of IO, it means there has to be another
            // thread available to service the completion of that request in order for our thread to make
            // progress. Why is this worse than the claim about the operating system turning an
            // asynchronous read into a synchronous one? If the underlying native ReadFile completes
            // synchronously, that would mean just our thread is being blocked, and will be unblocked once
            // the kernel gets done with our work. In this case, if the OS does do the read asynchronously
            // we are now dependent on another thread being available to unblock us.
            //
            // So how does ths manifest itself? We have seen dumps from customers reporting hangs where
            // we have over a hundred thread pool threads all blocked on EndRead() calls as we read this stream.
            // In these cases, the user had just completed a build that had a bunch of XAML files, and
            // this resulted in many .g.i.cs files being written and updated. As a result, Roslyn is trying to
            // re-read them to provide a new compilation to the XAML language service that is asking for it.
            // Inspecting these dumps and sampling some of the threads made some notable discoveries:
            //
            // 1. When there was a read blocked, it was the _last_ chunk that we were reading in the file in
            //    the file that we were reading. This leads me to believe that it isn't simply very slow IO
            //    (like a network drive), because in that case I'd expect to see some threads in different
            //    places than others.
            // 2. Some stacks were starting by the continuation of a ReadAsync, and some were the first read
            //    of a file from the background parser. In the first case, all of those threads were if the
            //    files were over 4K in size. The ones with the BackgroundParser still on the stack were files
            //    less than 4K in size.
            // 3. The "time unresponsive" in seconds correlated with roughly the number of threads we had
            //    blocked, which makes me think we were impacted by the once-per-second hill climbing algorithm
            //    used by the thread pool.
            //
            // So what's my analysis? When the XAML language service updated all the files, we kicked off
            // background parses for all of them. If the file was over 4K the asynchronous read actually did
            // happen (see point #2), but we'd eventually block the thread pool reading the last chunk.
            // Point #1 confirms that it was always the last chunk. And in small file cases, we'd block on
            // the first chunk. But in either case, we'd be blocking off a thread pool thread until another
            // thread pool thread was available. Since we had enough requests going (over a hundred),
            // sometimes the user got unlucky and all the threads got blocked. At this point, the CLR
            // started slowly kicking off more threads, but each time it'd start a new thread rather than
            // starting work that would be needed to unblock a thread, it just handled an IO that resulted
            // in another file read hitting the end of the file and another thread would get blocked. The
            // CLR then must kick off another thread, rinse, repeat. Eventually it'll make progress once
            // there's no more pending IO requests, everything will complete, and life then continues.
            //
            // To work around this issue, we set bufferSize to 1, which means that all reads should bypass
            // this logic. This is tracked by https://github.com/dotnet/corefx/issues/6007, at least in
            // corefx. We also open the file for reading with FileShare mode read/write/delete so that
            // we do not lock this file.
            using (var stream = FileUtilities.RethrowExceptionsAsIOException(() => new FileStream(_path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete, bufferSize: 1, useAsync: true)))
            {
                var version = VersionStamp.Create(prevLastWriteTime);

                // we do this so that we asynchronously read from file. and this should allocate less for IDE case. 
                // but probably not for command line case where it doesn't use more sophisticated services.
                using (var readStream = await SerializableBytes.CreateReadableStreamAsync(stream, cancellationToken: cancellationToken).ConfigureAwait(false))
                {
                    var text = CreateText(readStream, workspace);
                    textAndVersion = TextAndVersion.Create(text, version, _path);
                }
            }

            // Check if the file was definitely modified and closed while we were reading. In this case, we know the read we got was
            // probably invalid, so throw an IOException which indicates to our caller that we should automatically attempt a re-read.
            // If the file hasn't been closed yet and there's another writer, we will rely on file change notifications to notify us
            // and reload the file.
            var newLastWriteTime = FileUtilities.GetFileTimeStamp(_path);
            if (!newLastWriteTime.Equals(prevLastWriteTime))
            {
                var message = string.Format(WorkspacesResources.File_was_externally_modified_colon_0, _path);
                throw new IOException(message);
            }

            return textAndVersion;
        }

        /// <summary>
        /// Load a text and a version of the document in the workspace.
        /// </summary>
        /// <exception cref="IOException"></exception>
        /// <exception cref="InvalidDataException"></exception>
        internal override TextAndVersion LoadTextAndVersionSynchronously(Workspace workspace, DocumentId documentId, CancellationToken cancellationToken)
        {
            ValidateFileLength(workspace, _path);

            var prevLastWriteTime = FileUtilities.GetFileTimeStamp(_path);

            TextAndVersion textAndVersion;

            // Open file for reading with FileShare mode read/write/delete so that we do not lock this file.
            using (var stream = FileUtilities.RethrowExceptionsAsIOException(() => new FileStream(_path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete, bufferSize: 4096, useAsync: false)))
            {
                var version = VersionStamp.Create(prevLastWriteTime);
                var text = CreateText(stream, workspace);
                textAndVersion = TextAndVersion.Create(text, version, _path);
            }

            // Check if the file was definitely modified and closed while we were reading. In this case, we know the read we got was
            // probably invalid, so throw an IOException which indicates to our caller that we should automatically attempt a re-read.
            // If the file hasn't been closed yet and there's another writer, we will rely on file change notifications to notify us
            // and reload the file.
            var newLastWriteTime = FileUtilities.GetFileTimeStamp(_path);
            if (!newLastWriteTime.Equals(prevLastWriteTime))
            {
                var message = string.Format(WorkspacesResources.File_was_externally_modified_colon_0, _path);
                throw new IOException(message);
            }

            return textAndVersion;
        }

        private string GetDebuggerDisplay()
        {
            return nameof(Path) + " = " + Path;
        }

        private static void ValidateFileLength(Workspace workspace, string path)
        {
            // Validate file length is under our threshold. 
            // Otherwise, rather than reading the content into the memory, we will throw
            // InvalidDataException to caller of FileTextLoader.LoadText to deal with 
            // the situation.
            // 
            // check this (http://source.roslyn.io/#Microsoft.CodeAnalysis.Workspaces/Workspace/Solution/TextDocumentState.cs,132)
            // to see how workspace deal with exception from FileTextLoader. other consumer can handle the exception differently
            var fileLength = FileUtilities.GetFileLength(path);
            var threshold = workspace.Options.GetOption(FileTextLoaderOptions.FileLengthThreshold);
            if (fileLength > threshold)
            {
                // log max file length which will log to VS telemetry in VS host
                Logger.Log(FunctionId.FileTextLoader_FileLengthThresholdExceeded, KeyValueLogMessage.Create(m =>
                {
                    m["FileLength"] = fileLength;
                    m["Ext"] = PathUtilities.GetExtension(path);
                }));

                var message = string.Format(WorkspacesResources.File_0_size_of_1_exceeds_maximum_allowed_size_of_2, path, fileLength, threshold);
                throw new InvalidDataException(message);
            }
        }
    }
}