// 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;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.IO;
using System.IO.Compression;
using System.Security.Cryptography;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Xml;
using System.Xml.Linq;
using Microsoft.CodeAnalysis.AddImport;
using Microsoft.CodeAnalysis.Elfie.Model;
using Microsoft.CodeAnalysis.Shared.Utilities;
using Roslyn.Utilities;
using static System.FormattableString;

namespace Microsoft.CodeAnalysis.SymbolSearch;

/// <summary>
/// A service which enables searching for packages matching certain criteria.
/// It works against a <see cref="Microsoft.CodeAnalysis.Elfie"/> database to find results.
/// 
/// This implementation also spawns a task which will attempt to keep that database up to
/// date by downloading patches on a daily basis.
/// </summary>
internal sealed partial class SymbolSearchUpdateEngine
{
    // Internal for testing purposes.
    internal const string ContentAttributeName = "content";
    internal const string ChecksumAttributeName = "checksum";
    internal const string UpToDateAttributeName = "upToDate";
    internal const string TooOldAttributeName = "tooOld";

    public const string HostId = "RoslynNuGetSearch";
    private const string MicrosoftAssemblyReferencesName = "MicrosoftAssemblyReferences";

    private const int AddReferenceDatabaseTextFileFormatVersion = 1;

    /// <summary>
    /// Logged messages kept in memory to help us diagnose what was going on previously if a crash occurs.
    /// </summary>
    private static readonly LinkedList<string> s_logs = new();

    private readonly ConcurrentDictionary<string, object> _sourceToUpdateSentinel = [];

    // Interfaces that abstract out the external functionality we need.  Used so we can easily
    // mock behavior during tests.
    private readonly IDelayService _delayService;
    private readonly IIOService _ioService;
    private readonly IFileDownloaderFactory _fileDownloaderFactory;
    private readonly IPatchService _patchService;
    private readonly IDatabaseFactoryService _databaseFactoryService;
    private readonly Func<Exception, CancellationToken, bool> _reportAndSwallowExceptionUnlessCanceled;

    /// <param name="cancellationToken">
    /// Cancellation support for the task we use to keep the local database up to date.
    /// Currently used only in tests so we can shutdown gracefully.  In normal VS+OOP scenarios
    /// we don't care about this and we just get torn down when the OOP process goes down.
    /// </param>
    public ValueTask UpdateContinuouslyAsync(string source, string localSettingsDirectory, CancellationToken cancellationToken)
    {
        // Only the first thread to try to update this source should succeed
        // and cause us to actually begin the update loop. 
        var ourSentinel = new object();
        var currentSentinel = _sourceToUpdateSentinel.GetOrAdd(source, ourSentinel);

        if (ourSentinel != currentSentinel)
        {
            // We already have an update loop for this source.  Nothing for us to do.
            return default;
        }

        // We were the first ones to try to update this source.  Spawn off a task to do
        // the updating.
        return new Updater(this, source, localSettingsDirectory).UpdateInBackgroundAsync(cancellationToken);
    }

    private static void LogInfo(string text)
        => Log(text);

    private static void LogException(Exception exception, string text)
        => Log(text + ". " + exception.ToString());

    private static void Log(string text)
    {
        // Keep a running in memory log as well for debugging purposes.
        s_logs.AddLast(text);
        while (s_logs.Count > 100)
            s_logs.RemoveFirst();
    }

    private sealed class Updater(SymbolSearchUpdateEngine service, string source, string localSettingsDirectory)
    {
        private readonly SymbolSearchUpdateEngine _service = service;
        private readonly string _source = source;
        private readonly DirectoryInfo _cacheDirectoryInfo = new(Path.Combine(
                localSettingsDirectory, "PackageCache", string.Format(Invariant($"Format{AddReferenceDatabaseTextFileFormatVersion}"))));

        /// <summary>
        /// Internal for testing purposes.
        /// </summary>
        internal async ValueTask UpdateInBackgroundAsync(CancellationToken cancellationToken)
        {
            // We only support this single source currently.
            if (_source != PackageSourceHelper.NugetOrgSourceName)
            {
                return;
            }

            // Keep on looping until we're told to shut down.
            while (!cancellationToken.IsCancellationRequested)
            {
                LogInfo("Starting update");
                try
                {
                    var delayUntilNextUpdate = await UpdateDatabaseInBackgroundWorkerAsync(cancellationToken).ConfigureAwait(false);

                    LogInfo($"Waiting {delayUntilNextUpdate} until next update");
                    await Task.Delay(delayUntilNextUpdate, cancellationToken).ConfigureAwait(false);
                }
                catch (OperationCanceledException)
                {
                    LogInfo("Update canceled. Ending update loop");
                    return;
                }
            }
        }

        private static string ConvertToFileName(string source)
        {
            // Replace all occurrences of a single underscore with a double underscore.
            // Now we know any single underscores in the text come from escaping some
            // character.
            source = source.Replace("_", "__");

            var invalidChars = new HashSet<char>(Path.GetInvalidFileNameChars());
            var builder = new StringBuilder();

            // Escape any character not allowed in a path.  Escaping is simple, we just
            // use a single underscore followed by the ASCII numeric value of the character,
            // followed by another underscore.
            // i.e. ":" is 58 is ASCII, and becomes _58_ in the final name.
            foreach (var c in source)
            {
                if (invalidChars.Contains(c))
                {
                    builder.Append("_" + (int)c + "_");
                }
                else
                {
                    builder.Append(c);
                }
            }

            return builder.ToString();
        }

        /// <returns>The timespan the caller should wait until calling this method again.</returns>
        private async Task<TimeSpan> UpdateDatabaseInBackgroundWorkerAsync(CancellationToken cancellationToken)
        {
            // Attempt to update the local db if we have one, or download a full db
            // if we don't.  In the event of any error back off a minute and try 
            // again.  Lot of errors are possible here as IO/network/other-libraries
            // are involved.  For example, we might get errors trying to write to 
            // disk.
            try
            {
                CleanCacheDirectory(cancellationToken);

                // If we have a local database, then see if it needs to be patched. Otherwise download the full
                // database.
                //
                // (intentionally not wrapped in IOUtilities.  If this throws we want to restart).
                //
                // Ensure we get a fresh FileInfo for the db.  We need to make sure that the data we're querying
                // (like .Exists) is up to date and isn't the a cached value from a prior run.
                var databaseFileInfo = new FileInfo(
                    Path.Combine(_cacheDirectoryInfo.FullName, ConvertToFileName(_source) + ".txt"));

                if (_service._ioService.Exists(databaseFileInfo))
                {
                    LogInfo("Local database file exists. Patching local database");
                    return await PatchLocalDatabaseAsync(databaseFileInfo, cancellationToken).ConfigureAwait(false);
                }
                else
                {
                    LogInfo("Local database file does not exist. Downloading full database");
                    return await DownloadFullDatabaseAsync(databaseFileInfo, cancellationToken).ConfigureAwait(false);
                }
            }
            catch (Exception e) when (_service._reportAndSwallowExceptionUnlessCanceled(e, cancellationToken))
            {
                // Something bad happened (IO Exception, network exception etc.).
                // ask our caller to try updating again a minute from now.
                //
                // Note: we skip OperationCanceledException because it's not 'bad'.
                // It's the standard way to indicate that we've been asked to shut
                // down.
                var delay = _service._delayService.ExpectedFailureDelay;
                LogException(e, $"Error occurred updating. Retrying update in {delay}");
                return delay;
            }
        }

        private void CleanCacheDirectory(CancellationToken cancellationToken)
        {
            LogInfo("Cleaning cache directory");

            // (intentionally not wrapped in IOUtilities.  If this throws we want to restart).
            if (!_service._ioService.Exists(_cacheDirectoryInfo))
            {
                LogInfo("Creating cache directory");

                // (intentionally not wrapped in IOUtilities.  If this throws we want to restart).
                _service._ioService.Create(_cacheDirectoryInfo);
                LogInfo("Cache directory created");
            }

            cancellationToken.ThrowIfCancellationRequested();
        }

        private async Task<TimeSpan> DownloadFullDatabaseAsync(FileInfo databaseFileInfo, CancellationToken cancellationToken)
        {
            var (_, delay) = await DownloadFullDatabaseWorkerAsync(databaseFileInfo, cancellationToken).ConfigureAwait(false);
            return delay;
        }

        private async Task<(bool succeeded, TimeSpan delay)> DownloadFullDatabaseWorkerAsync(FileInfo databaseFileInfo, CancellationToken cancellationToken)
        {
            // Will hit https://aka.ms/vssettings/pub/RoslynNuGetSearch/Elfie_V1/Latest.xml. Providing this
            // link in the source to make it easy for maintainers to hit the endpoint to see if it succeeds and that
            // the data and http headers are what are expected.
            var serverPath = Invariant($"Elfie_V{AddReferenceDatabaseTextFileFormatVersion}/Latest.xml");

            LogInfo($"Downloading and processing full database: {serverPath}");

            var element = await DownloadFileAsync(serverPath, cancellationToken).ConfigureAwait(false);
            var result = await ProcessFullDatabaseXElementAsync(databaseFileInfo, element, cancellationToken).ConfigureAwait(false);

            LogInfo("Downloading and processing full database completed");
            return result;
        }

        private async Task<(bool succeeded, TimeSpan delay)> ProcessFullDatabaseXElementAsync(
            FileInfo databaseFileInfo, XElement element, CancellationToken cancellationToken)
        {
            LogInfo("Processing full database element");

            // Convert the database contents in the XML to a byte[].
            var (succeeded, contentBytes) = await TryParseDatabaseElementAsync(element, cancellationToken).ConfigureAwait(false);

            if (!succeeded)
            {
                // Something was wrong with the full database.  Trying again soon after won't
                // really help.  We'll just get the same busted XML from the remote service
                // cache.  So we want to actually wait some long enough amount of time so that
                // we can retrieve good data the next time around.

                var failureDelay = _service._delayService.CatastrophicFailureDelay;
                LogInfo($"Unable to parse full database element. Update again in {failureDelay}");
                return (succeeded: false, failureDelay);
            }

            var bytes = contentBytes!;
            AddReferenceDatabase database;

            // Make a database out of that and set it to our in memory database that we'll be 
            // searching.
            try
            {
                using var stream = new MemoryStream(bytes);
                database = CreateAndSetInMemoryDatabase(stream, isBinary: false);
            }
            catch (Exception e) when (_service._reportAndSwallowExceptionUnlessCanceled(e, cancellationToken))
            {
                // We retrieved bytes from the server, but we couldn't make a DB
                // out of it.  That's very bad.  Just trying again one minute later
                // isn't going to help.  We need to wait until there is good data
                // on the server for us to download.
                var failureDelay = _service._delayService.CatastrophicFailureDelay;
                LogInfo($"Unable to create database from full database element. Update again in {failureDelay}");
                return (succeeded: false, failureDelay);
            }

            // Write the file out to disk so we'll have it the next time we launch VS.  Do this
            // after we set the in-memory instance so we at least have something to search while
            // we're waiting to write. It's ok if either of these writes don't succeed. If the txt
            // file fails to persist, a subsequent VS session will redownload the index and again try
            // to persist. If the binary file fails to persist, a subsequent VS session will just load
            // the index from the txt file and again try to persist the binary file.
            await WriteDatabaseTextFileAsync(databaseFileInfo, bytes, cancellationToken).ConfigureAwait(false);
            await WriteDatabaseBinaryFileAsync(database, databaseFileInfo, cancellationToken).ConfigureAwait(false);

            var delay = _service._delayService.UpdateSucceededDelay;
            LogInfo($"Processing full database element completed. Update again in {delay}");
            return (succeeded: true, delay);
        }

        private async Task WriteDatabaseTextFileAsync(FileInfo databaseFileInfo, byte[] bytes, CancellationToken cancellationToken)
        {
            LogInfo("Writing database file");

            await WriteDatabaseFileAsync(databaseFileInfo, new ArraySegment<byte>(bytes), cancellationToken).ConfigureAwait(false);

            LogInfo("Writing database file completed");
        }

        private async Task WriteDatabaseBinaryFileAsync(AddReferenceDatabase database, FileInfo databaseFileInfo, CancellationToken cancellationToken)
        {
            using var memoryStream = new MemoryStream();
            using var writer = new BinaryWriter(memoryStream);

            LogInfo("Writing database binary file");

            database.WriteBinary(writer);
            writer.Flush();

            // Obtain the underlying array from the memory stream. If for some reason this isn't available,
            // fall back to reading the stream into a new byte array.
            if (!memoryStream.TryGetBuffer(out var arraySegmentBuffer))
            {
                memoryStream.Position = 0;

                // Read the buffer directly from the memory stream.
                arraySegmentBuffer = new ArraySegment<byte>(memoryStream.ReadAllBytes());
            }

            var databaseBinaryFileInfo = GetBinaryFileInfo(databaseFileInfo);
            await WriteDatabaseFileAsync(databaseBinaryFileInfo, arraySegmentBuffer, cancellationToken).ConfigureAwait(false);

            LogInfo("Writing database binary file completed");
        }

        private async Task WriteDatabaseFileAsync(FileInfo databaseFileInfo, ArraySegment<byte> bytes, CancellationToken cancellationToken)
        {
            await RepeatIOAsync(
                cancellationToken =>
                {
                    var guidString = Guid.NewGuid().ToString();
                    var tempFilePath = Path.Combine(_cacheDirectoryInfo.FullName, guidString + ".tmp");

                    LogInfo($"Temp file path: {tempFilePath}");

                    try
                    {
                        // First, write to a temporary file next to the actual database file.
                        // Note that we explicitly use FileStream so that we can call .Flush to ensure the
                        // file has been completely written to disk (at least as well as the OS can guarantee
                        // things).

                        LogInfo("Writing temp file");

                        // (intentionally not wrapped in IOUtilities.  If this throws we want to retry writing).
                        _service._ioService.WriteAndFlushAllBytes(tempFilePath, bytes);
                        LogInfo("Writing temp file completed");

                        // If we have an existing db file, try to replace it file with the temp file.
                        // Otherwise, just move the temp file into place.
                        if (_service._ioService.Exists(databaseFileInfo))
                        {
                            LogInfo("Replacing database file");
                            _service._ioService.Replace(tempFilePath, databaseFileInfo.FullName, destinationBackupFileName: null, ignoreMetadataErrors: true);
                            LogInfo("Replace database file completed");
                        }
                        else
                        {
                            LogInfo("Moving database file");
                            _service._ioService.Move(tempFilePath, databaseFileInfo.FullName);
                            LogInfo("Moving database file completed");
                        }
                    }
                    finally
                    {
                        // Try to delete the temp file if it is still around.
                        // If this fails, that's unfortunately, but just proceed.
                        IOUtilities.PerformIO(() => _service._ioService.Delete(new FileInfo(tempFilePath)));
                    }
                }, cancellationToken).ConfigureAwait(false);
        }

        private static FileInfo GetBinaryFileInfo(FileInfo databaseFileInfo)
            => new(Path.ChangeExtension(databaseFileInfo.FullName, ".bin"));

        private async Task<TimeSpan> PatchLocalDatabaseAsync(FileInfo databaseFileInfo, CancellationToken cancellationToken)
        {
            LogInfo("Patching local database");

            LogInfo("Reading in local database");

            using var stream = GetDatabaseStream(databaseFileInfo, out var isBinary);

            LogInfo($"Reading in local database completed. isBinary={isBinary}");

            // Make a database instance from the stream and set it as the current in memory database
            // that searches will run against. If we can't make a database instance from the stream
            // then our local database is corrupt and we need to download the full database to get back
            // into a good state.
            AddReferenceDatabase database;
            try
            {
                database = CreateAndSetInMemoryDatabase(stream, isBinary);
            }
            catch (Exception e) when (_service._reportAndSwallowExceptionUnlessCanceled(e, cancellationToken))
            {
                LogException(e, "Error creating database from local copy. Downloading full database");
                return await DownloadFullDatabaseAsync(databaseFileInfo, cancellationToken).ConfigureAwait(false);
            }

            // Now attempt to download and apply patch file.
            //
            // Will hit https://aka.ms/vssettings/pub/RoslynNuGetSearch/Elfie_V1/{db_version}_Patch.xml.
            // Providing this link in the source to make it easy for maintainers to hit the endpoint to see if it
            // succeeds and that the data and http headers are what are expected.
            var serverPath = Invariant($"Elfie_V{AddReferenceDatabaseTextFileFormatVersion}/{database.DatabaseVersion}_Patch.xml");

            LogInfo("Downloading and processing patch file: " + serverPath);

            var element = await DownloadFileAsync(serverPath, cancellationToken).ConfigureAwait(false);
            var delayUntilUpdate = await ProcessPatchXElementAsync(
                databaseFileInfo,
                element,
                // We pass a delegate to get the database bytes so that we can avoid reading the bytes when we don't need them due to no patch to apply.
                // Note: ApplyPatch requires byte[], so we must read into memory here.
                getDatabaseBytes: () => _service._ioService.ReadAllBytes(isBinary ? GetBinaryFileInfo(databaseFileInfo).FullName : databaseFileInfo.FullName),
                cancellationToken).ConfigureAwait(false);

            LogInfo("Downloading and processing patch file completed");
            LogInfo("Patching local database completed");

            return delayUntilUpdate;

            Stream GetDatabaseStream(FileInfo databaseFileInfo, out bool isBinary)
            {
                var databaseBinaryFileInfo = GetBinaryFileInfo(databaseFileInfo);

                try
                {
                    // First attempt to read from the binary file. If that fails, fall back to the text file.
                    isBinary = true;
                    return _service._ioService.OpenRead(databaseBinaryFileInfo.FullName);
                }
                catch (Exception e) when (IOUtilities.IsNormalIOException(e))
                {
                }

                // (intentionally not wrapped in IOUtilities.  If this throws we want to restart).
                isBinary = false;
                return _service._ioService.OpenRead(databaseFileInfo.FullName);
            }
        }

        /// <summary>
        /// Creates a database instance with the stream passed in.  If creating the database succeeds,
        /// then it will be set as the current in memory version.  In the case of failure (which 
        /// indicates that our data is corrupt), the exception will bubble up and must be appropriately
        /// dealt with by the caller.
        /// </summary>
        private AddReferenceDatabase CreateAndSetInMemoryDatabase(Stream stream, bool isBinary)
        {
            var database = CreateDatabaseFromStream(stream, isBinary);
            _service._sourceToDatabase[_source] = new AddReferenceDatabaseWrapper(database);
            return database;
        }

        private async Task<TimeSpan> ProcessPatchXElementAsync(
            FileInfo databaseFileInfo, XElement patchElement, Func<byte[]> getDatabaseBytes, CancellationToken cancellationToken)
        {
            try
            {
                LogInfo("Processing patch element");
                var delayUntilUpdate = await TryProcessPatchXElementAsync(databaseFileInfo, patchElement, getDatabaseBytes, cancellationToken).ConfigureAwait(false);
                if (delayUntilUpdate != null)
                {
                    LogInfo($"Processing patch element completed. Update again in {delayUntilUpdate.Value}");
                    return delayUntilUpdate.Value;
                }

                // Fall through and download full database.
            }
            catch (Exception e) when (_service._reportAndSwallowExceptionUnlessCanceled(e, cancellationToken))
            {
                LogException(e, "Error occurred while processing patch element. Downloading full database");
                // Fall through and download full database.
            }

            return await DownloadFullDatabaseAsync(databaseFileInfo, cancellationToken).ConfigureAwait(false);
        }

        private async Task<TimeSpan?> TryProcessPatchXElementAsync(
            FileInfo databaseFileInfo, XElement patchElement, Func<byte[]> getDatabaseBytes, CancellationToken cancellationToken)
        {
            ParsePatchElement(patchElement, out var upToDate, out var tooOld, out var patchBytes);
            AddReferenceDatabase database;

            if (upToDate)
            {
                LogInfo("Local version is up to date");

                var databaseBinaryFileInfo = GetBinaryFileInfo(databaseFileInfo);
                if (!_service._ioService.Exists(databaseBinaryFileInfo))
                    await WriteDatabaseBinaryFileAsync(_service._sourceToDatabase[_source].Database, databaseFileInfo, cancellationToken).ConfigureAwait(false);

                return _service._delayService.UpdateSucceededDelay;
            }

            if (tooOld)
            {
                LogInfo("Local version too old");
                return null;
            }

            var databaseBytes = getDatabaseBytes();
            LogInfo($"Got patch. databaseBytes.Length={databaseBytes.Length} patchBytes.Length={patchBytes!.Length}.");

            // We have patch data.  Apply it to our current database bytes to produce the new
            // database.
            LogInfo("Applying patch");
            var finalBytes = _service._patchService.ApplyPatch(databaseBytes, patchBytes);
            LogInfo($"Applying patch completed. finalBytes.Length={finalBytes.Length}");

            // finalBytes is generated from the current database and the patch, not from the binary file.
            using var stream = new MemoryStream(finalBytes);
            database = CreateAndSetInMemoryDatabase(stream, isBinary: false);

            // Attempt to persist the txt and binary forms of the index. It's ok if either of these writes
            // don't succeed. If the txt file fails to persist, a subsequent VS session will redownload the
            // index and again try to persist. If the binary file fails to persist, a subsequent VS session
            // will just load the index from the txt file and again try to persist the binary file.
            await WriteDatabaseTextFileAsync(databaseFileInfo, finalBytes, cancellationToken).ConfigureAwait(false);
            await WriteDatabaseBinaryFileAsync(database, databaseFileInfo, cancellationToken).ConfigureAwait(false);

            return _service._delayService.UpdateSucceededDelay;
        }

        private static void ParsePatchElement(XElement patchElement, out bool upToDate, out bool tooOld, out byte[]? patchBytes)
        {
            patchBytes = null;

            var upToDateAttribute = patchElement.Attribute(UpToDateAttributeName);
            upToDate = upToDateAttribute != null && (bool)upToDateAttribute;

            var tooOldAttribute = patchElement.Attribute(TooOldAttributeName);
            tooOld = tooOldAttribute != null && (bool)tooOldAttribute;

            var contentsAttribute = patchElement.Attribute(ContentAttributeName);
            if (contentsAttribute != null)
            {
                var contents = contentsAttribute.Value;
                patchBytes = Convert.FromBase64String(contents);
            }

            var hasPatchBytes = patchBytes != null;

            var value = (upToDate ? 1 : 0) +
                        (tooOld ? 1 : 0) +
                        (hasPatchBytes ? 1 : 0);
            if (value != 1)
            {
                throw new FormatException($"Patch format invalid. {nameof(upToDate)}={upToDate} {nameof(tooOld)}={tooOld} {nameof(hasPatchBytes)}={hasPatchBytes}");
            }
        }

        private AddReferenceDatabase CreateDatabaseFromStream(Stream stream, bool isBinary)
        {
            LogInfo("Creating database from stream");
            var result = _service._databaseFactoryService.CreateDatabaseFromStream(stream, isBinary);
            LogInfo("Creating database from stream completed");
            return result;
        }

        private async Task<XElement> DownloadFileAsync(string serverPath, CancellationToken cancellationToken)
        {
            LogInfo("Creating download client: " + serverPath);

            // Create a client that will attempt to download the specified file.  The client works
            // in the following manner:
            //
            //      1) If the file is not cached locally it will download it in the background.
            //         Until the file is downloaded, null will be returned from client.ReadFile.
            //      2) If the file is cached locally and was downloaded less than (24 * 60) 
            //         minutes ago, then the client will do nothing (until that time has elapsed).
            //         Calls to client.ReadFile will return the cached file.
            //      3) If the file is cached locally and was downloaded more than (24 * 60) 
            //         minutes ago, then the client will attempt to download the file.
            //         In the interim period null will be returned from client.ReadFile.
            var pollingMinutes = (int)TimeSpan.FromDays(1).TotalMinutes;
            using var client = _service._fileDownloaderFactory.CreateClient(HostId, serverPath, pollingMinutes);

            LogInfo("Creating download client completed");

            // Poll the client every minute until we get the file.
            while (true)
            {
                cancellationToken.ThrowIfCancellationRequested();

                var (element, delay) = await TryDownloadFileAsync(client, cancellationToken).ConfigureAwait(false);
                if (element == null)
                {
                    LogInfo($"File not downloaded. Trying again in {delay}");
                    await Task.Delay(delay, cancellationToken).ConfigureAwait(false);
                }
                else
                {
                    // File was downloaded.  
                    return element;
                }
            }
        }

        /// <summary>Returns 'null' if download is not available and caller should keep polling.</summary>
        private async Task<(XElement? element, TimeSpan delay)> TryDownloadFileAsync(IFileDownloader fileDownloader, CancellationToken cancellationToken)
        {
            LogInfo("Read file from client");

            using var stream = await fileDownloader.ReadFileAsync().ConfigureAwait(false);

            if (stream == null)
            {
                LogInfo("Read file completed. Client returned no data");
                return (element: null, _service._delayService.CachePollDelay);
            }

            LogInfo("Read file completed. Client returned data");
            LogInfo("Converting data to XElement");

            // We're reading in our own XML file, but even so, use conservative settings
            // just to be on the safe side.  First, disallow DTDs entirely (we will never
            // have one ourself).  And also, prevent any external resolution of files when
            // processing the XML.
            var settings = new XmlReaderSettings
            {
                DtdProcessing = DtdProcessing.Prohibit,
                XmlResolver = null
            };

            // This code must always succeed.  If it does not, that means that either the server reported bogus data
            // to the file-downloader, or the file-downloader is serving us bogus data.  In other event, there is
            // something wrong with those components, and we should both report the issue to Watson, and stop doing
            // the update.
            try
            {
                using var reader = XmlReader.Create(stream, settings);

                var element = XElement.Load(reader);
                LogInfo("Converting data to XElement completed");
                return (element, delay: default);
            }
            catch (Exception e) when (_service._reportAndSwallowExceptionUnlessCanceled(e, cancellationToken))
            {
                // We retrieved bytes from the server, but we couldn't make parse it an xml. out of it.  That's very
                // bad.  Just trying again one minute later isn't going to help.  We need to wait until there is
                // good data on the server for us to download.
                LogInfo($"Unable to parse file as XElement");
                return (element: null, _service._delayService.CatastrophicFailureDelay);
            }
        }

        private async Task RepeatIOAsync(Action<CancellationToken> action, CancellationToken cancellationToken)
        {
            const int repeat = 6;
            for (var i = 0; i < repeat; i++)
            {
                cancellationToken.ThrowIfCancellationRequested();

                try
                {
                    action(cancellationToken);
                    return;
                }
                catch (Exception e) when (IOUtilities.IsNormalIOException(e) || _service._reportAndSwallowExceptionUnlessCanceled(e, cancellationToken))
                {
                    // The exception filter above might be a little funny looking. We always
                    // want to enter this catch block, but if we ran into a normal IO exception
                    // we shouldn't bother reporting it. We don't want to get lots of hits just
                    // because something like an anti-virus tool locked the file and we
                    // couldn't write to it. The call to IsNormalIOException will shortcut
                    // around the reporting in this case.

                    var delay = _service._delayService.FileWriteDelay;
                    LogException(e, $"Operation failed. Trying again after {delay}");
                    await Task.Delay(delay, cancellationToken).ConfigureAwait(false);
                }
            }
        }

        private async Task<(bool succeeded, byte[]? contentBytes)> TryParseDatabaseElementAsync(XElement element, CancellationToken cancellationToken)
        {
            LogInfo("Parsing database element");
            var contentsAttribute = element.Attribute(ContentAttributeName);
            if (contentsAttribute == null)
            {
                _service._reportAndSwallowExceptionUnlessCanceled(new FormatException($"Database element invalid. Missing '{ContentAttributeName}' attribute"), CancellationToken.None);

                return (succeeded: false, null);
            }

            var contentBytes = await Updater.ConvertContentAttributeAsync(contentsAttribute, cancellationToken).ConfigureAwait(false);

            var checksumAttribute = element.Attribute(ChecksumAttributeName);
            if (checksumAttribute != null)
            {
                var expectedChecksum = checksumAttribute.Value;
                string actualChecksum;
                using (var sha256 = SHA256.Create())
                {
                    actualChecksum = Convert.ToBase64String(sha256.ComputeHash(contentBytes));
                }

                if (!StringComparer.Ordinal.Equals(expectedChecksum, actualChecksum))
                {
                    _service._reportAndSwallowExceptionUnlessCanceled(new FormatException($"Checksum mismatch: expected != actual. {expectedChecksum} != {actualChecksum}"), CancellationToken.None);

                    return (succeeded: false, null);
                }
            }

            return (succeeded: true, contentBytes);
        }

        private static async Task<byte[]> ConvertContentAttributeAsync(XAttribute contentsAttribute, CancellationToken cancellationToken)
        {
            var text = contentsAttribute.Value;
            var compressedBytes = Convert.FromBase64String(text);

            using var outStream = new MemoryStream();

            using (var inStream = new MemoryStream(compressedBytes))
            using (var deflateStream = new DeflateStream(inStream, CompressionMode.Decompress))
            {
#if NET
                await deflateStream.CopyToAsync(outStream, cancellationToken).ConfigureAwait(false);
#else
                await deflateStream.CopyToAsync(outStream).ConfigureAwait(false);
#endif
            }

            var bytes = outStream.ToArray();

            LogInfo($"Parsing complete. bytes.length={bytes.Length}");
            return bytes;
        }
    }
}
