Nonemm/src/Nonemm.Digital/ProcessBridgeChannel.cs

using System.Diagnostics;

namespace Nonemm.Digital;

/// Runs the bridge and talks to it over its standard input and output. On
/// Linux the bridge runs under Wine, which writes its own diagnostics to
/// standard error, so the protocol has standard output to itself. On Windows
/// `wine` is null and the bridge is started directly.
public sealed class ProcessBridgeChannel : BridgeChannel
{
    private const int KeptErrorLines = 20;

    private readonly string? wine;
    private readonly string bridgePath;
    private readonly string? prefix;
    private readonly string? logFolder;
    private readonly Queue<string> lastErrors = new();
    private readonly SemaphoreSlim writing = new(1, 1);
    private readonly Lock writingLog = new();
    private Process? bridge;
    private StreamWriter? log;
    private DateTime started;
    private int lastCount = int.MinValue;

    /// `prefix` is the WINEPREFIX to run in. Left null, Wine uses its default,
    /// which is what a station with one prefix wants. `wine` is the Wine
    /// command, or null to run the bridge without Wine. `logFolder`, when it is
    /// given, is where the protocol log for this run is written.
    public ProcessBridgeChannel(
        string bridgePath,
        string? prefix = null,
        string? wine = "wine",
        string? logFolder = null)
    {
        this.bridgePath = bridgePath;
        this.prefix = prefix;
        this.wine = wine;
        this.logFolder = logFolder;
    }

    public event EventHandler<string>? LineReceived;

    public event EventHandler<string>? Failed;

    public Task StartAsync(CancellationToken cancellation = default)
    {
        OpenLog();
        bridge = Start(bridgePath);
        _ = ReadOutputAsync(bridge);
        _ = ReadErrorsAsync(bridge);
        return Task.CompletedTask;
    }

    public async Task SendAsync(string line, CancellationToken cancellation = default)
    {
        Process running = bridge ?? throw new InvalidOperationException("the bridge is not started");
        await writing.WaitAsync(cancellation).ConfigureAwait(false);
        try
        {
            await running.StandardInput.WriteLineAsync(line.AsMemory(), cancellation)
                .ConfigureAwait(false);
            await running.StandardInput.FlushAsync(cancellation).ConfigureAwait(false);
            Log(">", line);
        }
        finally
        {
            writing.Release();
        }
    }

    public async Task<string> ToWindowsPathAsync(
        string path,
        CancellationToken cancellation = default)
    {
        if (wine is null)
        {
            return Path.GetFullPath(path);
        }
        using Process winepath = Start("winepath", "-w", path);
        string converted = await winepath.StandardOutput.ReadToEndAsync(cancellation)
            .ConfigureAwait(false);
        await winepath.WaitForExitAsync(cancellation).ConfigureAwait(false);
        if (winepath.ExitCode != 0 || converted.Trim().Length == 0)
        {
            throw new InvalidOperationException(
                $"winepath -w {path} failed with exit code {winepath.ExitCode}");
        }
        return converted.Trim();
    }

    public void Dispose()
    {
        try
        {
            if (bridge is { HasExited: false })
            {
                bridge.Kill(entireProcessTree: true);
            }
        }
        catch (InvalidOperationException)
        {
            // the bridge stopped on its own, so there is nothing left to kill
        }
        bridge?.Dispose();
        writing.Dispose();
        lock (writingLog)
        {
            log?.Dispose();
            log = null;
        }
    }

    /// One file per run of the engine, named for when it started. It is the
    /// whole protocol with a millisecond stamp on every line, which is what
    /// says who keyed the transmitter and when.
    private void OpenLog()
    {
        if (logFolder is null)
        {
            return;
        }
        try
        {
            Directory.CreateDirectory(logFolder);
            started = DateTime.Now;
            log = new StreamWriter(
                Path.Combine(logFolder, $"digital-{started:yyyyMMdd-HHmmss}.log"),
                append: false)
            {
                AutoFlush = true,
            };
        }
        catch (IOException)
        {
            // a log that cannot be written stops nothing
            log = null;
        }
        catch (UnauthorizedAccessException)
        {
            log = null;
        }
    }

    /// The buffer question goes out every 50 ms and is answered just as often,
    /// which would bury everything else. The question is left out and the
    /// answer is written only when the count has changed.
    private void Log(string direction, string line)
    {
        if (log is null)
        {
            return;
        }
        (string verb, string[] fields) = BridgeLine.Read(line);
        if (verb == "buffer")
        {
            if (direction == ">" || fields.Length == 0)
            {
                return;
            }
            if (!int.TryParse(fields[0], out int count) || count == lastCount)
            {
                return;
            }
            lastCount = count;
        }
        lock (writingLog)
        {
            log?.WriteLine(
                $"{(DateTime.Now - started).TotalMilliseconds,9:0} ms  {direction} {line.Replace('\t', ' ')}");
        }
    }

    private Process Start(string program, params string[] arguments)
    {
        ProcessStartInfo start = new()
        {
            FileName = wine ?? program,
            RedirectStandardInput = true,
            RedirectStandardOutput = true,
            RedirectStandardError = true,
            UseShellExecute = false,
            CreateNoWindow = true,
        };
        if (wine is not null)
        {
            start.ArgumentList.Add(program);
        }
        foreach (string argument in arguments)
        {
            start.ArgumentList.Add(argument);
        }
        if (prefix is not null)
        {
            start.Environment["WINEPREFIX"] = prefix;
        }
        return Process.Start(start)
            ?? throw new InvalidOperationException($"{wine} {program} did not start");
    }

    private async Task ReadOutputAsync(Process running)
    {
        while (await running.StandardOutput.ReadLineAsync().ConfigureAwait(false) is { } line)
        {
            Log("<", line);
            LineReceived?.Invoke(this, line);
        }
        await running.WaitForExitAsync().ConfigureAwait(false);
        Failed?.Invoke(this, $"the bridge exited with code {running.ExitCode}: {ErrorTail()}");
    }

    private async Task ReadErrorsAsync(Process running)
    {
        while (await running.StandardError.ReadLineAsync().ConfigureAwait(false) is { } line)
        {
            lock (lastErrors)
            {
                lastErrors.Enqueue(line);
                if (lastErrors.Count > KeptErrorLines)
                {
                    lastErrors.Dequeue();
                }
            }
        }
    }

    private string ErrorTail()
    {
        lock (lastErrors)
        {
            return string.Join(" / ", lastErrors);
        }
    }
}