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dotnet: in-process FFI runtime hosting (InProcess transport) #1901

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40b9b7a
dotnet: add in-process FFI runtime hosting (InProcess transport)
SteveSandersonMS Jul 3, 2026
fed63c1
dotnet: FFI in-proc host spawns the bundled CLI, drops copilot-runtim…
SteveSandersonMS Jul 3, 2026
b673393
dotnet: emit the FFI cdylib as a natural platform shared library
SteveSandersonMS Jul 3, 2026
396870f
FFI: zero-copy send path and single-write framing
SteveSandersonMS Jul 3, 2026
ba941d4
JsonRpc: write frame header straight into the pooled buffer
SteveSandersonMS Jul 3, 2026
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136 changes: 123 additions & 13 deletions dotnet/src/Client.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -79,6 +79,7 @@
private readonly List<LifecycleSubscription> _lifecycleHandlers = [];

private Task<Connection>? _connectionTask;
private FfiRuntimeHost? _ffiHost;
private bool _disposed;
private int? _actualPort;
private int? _negotiatedProtocolVersion;
Expand Down Expand Up @@ -135,13 +136,16 @@
public CopilotClient(CopilotClientOptions? options = null)
{
_options = options ?? new();
_connection = _options.Connection ?? RuntimeConnection.ForStdio();
_connection = _options.Connection ?? ResolveDefaultConnection(_options);

switch (_connection)
{
case StdioRuntimeConnection:
break;

case InProcessRuntimeConnection:
break;

case TcpRuntimeConnection tcp:
if (tcp.ConnectionToken is { Length: 0 })
{
Expand Down Expand Up @@ -198,6 +202,38 @@
}
}

/// <summary>
/// Environment variable that overrides the transport used when the caller does not
/// specify <see cref="CopilotClientOptions.Connection"/>. Accepts <c>"inprocess"</c>
/// or <c>"stdio"</c> (case-insensitive); unset preserves the default stdio transport.
/// Any other value is an error. Ignored when a <see cref="RuntimeConnection"/> is set
/// explicitly.
/// </summary>
internal const string DefaultConnectionEnvVar = "COPILOT_SDK_DEFAULT_CONNECTION";

/// <summary>
/// Resolves the default <see cref="RuntimeConnection"/> for the no-Connection case,
/// honoring <see cref="DefaultConnectionEnvVar"/>.
/// </summary>
private static RuntimeConnection ResolveDefaultConnection(CopilotClientOptions options)
{
var value = options.Environment is not null
&& options.Environment.TryGetValue(DefaultConnectionEnvVar, out var fromOptions)
? fromOptions
: Environment.GetEnvironmentVariable(DefaultConnectionEnvVar);

if (string.IsNullOrEmpty(value) || string.Equals(value, "stdio", StringComparison.OrdinalIgnoreCase))
{
return RuntimeConnection.ForStdio();
}
if (string.Equals(value, "inprocess", StringComparison.OrdinalIgnoreCase))
{
return RuntimeConnection.ForInProcess();
}
throw new ArgumentException(
$"Invalid {DefaultConnectionEnvVar} value '{value}'. Expected 'inprocess', 'stdio', or unset.");
}

/// <summary>
/// Parses a runtime URL into a URI with host and port.
/// </summary>
Expand Down Expand Up @@ -251,7 +287,16 @@

try
{
if (_connection is UriRuntimeConnection)
if (_connection is InProcessRuntimeConnection)
{
// In-process FFI hosting: load the Rust cdylib and let it spawn
// the CLI worker, instead of the SDK launching a CLI child process.
var ffiHost = FfiRuntimeHost.Create(ResolveCliPathForFfi(), GetNapiPrebuildsFolderOrThrow(), _options.Environment, _logger);
_ffiHost = ffiHost;
await ffiHost.StartAsync(ct);
connection = await ConnectToServerAsync(null, null, null, null, ct, ffiHost);
}
else if (_connection is UriRuntimeConnection)
{
// External runtime
_actualPort = _optionsPort;
Expand Down Expand Up @@ -438,7 +483,7 @@

private async Task CleanupConnectionAsync(Connection ctx, List<Exception>? errors, bool gracefulRuntimeShutdown)
{
if (gracefulRuntimeShutdown && ctx.CliProcess is not null)
if (gracefulRuntimeShutdown && (ctx.CliProcess is not null || ctx.FfiHost is not null))
{
var runtimeShutdownTimestamp = Stopwatch.GetTimestamp();
try
Expand Down Expand Up @@ -478,6 +523,13 @@
{
await CleanupCliProcessAsync(childProcess, ctx.StderrPump, errors, _logger);
}

if (ctx.FfiHost is { } ffiHost)
{
try { ffiHost.Dispose(); }
catch (Exception ex) { AddCleanupError(errors, ex, _logger); }

Check notice

Code scanning / CodeQL

Generic catch clause Note

Generic catch clause.
_ffiHost = null;
}
}

private static async Task CleanupCliProcessAsync(Process childProcess, ProcessStderrPump? stderrPump, List<Exception>? errors, ILogger? logger)
Expand Down Expand Up @@ -1795,12 +1847,14 @@
int? serverVersion;
try
{
var token = _connection switch
{
TcpRuntimeConnection tcp => tcp.ConnectionToken,
UriRuntimeConnection uri => uri.ConnectionToken,
_ => null,
};
var token = _ffiHost is not null
? null // FFI hosting is an ungated in-process connection; no token.
: _connection switch
{
TcpRuntimeConnection tcp => tcp.ConnectionToken,
UriRuntimeConnection uri => uri.ConnectionToken,
_ => null,
};
var connectResponse = await InvokeRpcAsync<ConnectResult>(
connection.Rpc, "connect", [new ConnectRequest { Token = token }], connection.StderrBuffer, cancellationToken);
serverVersion = (int)connectResponse.ProtocolVersion;
Expand Down Expand Up @@ -2077,6 +2131,55 @@
return arch != null ? $"{os}-{arch}" : null;
}

private string ResolveCliPathForFfi()
{
var envCliPath = _options.Environment is not null && _options.Environment.TryGetValue("COPILOT_CLI_PATH", out var envValue)
? envValue
: System.Environment.GetEnvironmentVariable("COPILOT_CLI_PATH");
if (!string.IsNullOrEmpty(envCliPath))
{
return envCliPath;
}

// Fall back to the bundled single-file CLI the same way stdio discovers it.
// It embeds its own Node and is spawned directly as `copilot --embedded-host`,
// with the sibling `prebuilds/<rid>/runtime.node` cdylib loaded in-process.
var bundled = GetBundledCliPath(out var searchedPath);
return bundled
?? throw new InvalidOperationException(
"In-process FFI hosting requires the Copilot CLI. Set the COPILOT_CLI_PATH "
+ $"environment variable, or ensure the bundled CLI is present (looked in '{searchedPath}').");
}

/// <summary>
/// Returns the napi-rs prebuilds folder name for the current host — the
/// <c>&lt;node-platform&gt;-&lt;arch&gt;</c> convention (e.g. <c>win32-x64</c>,
/// <c>darwin-arm64</c>, <c>linux-x64</c>) under which the runtime ships
/// <c>prebuilds/&lt;folder&gt;/runtime.node</c>. This differs from the .NET RID
/// (<c>win-x64</c>/<c>osx-x64</c>) for Windows and macOS.
/// </summary>
private static string? GetNapiPrebuildsFolder()
{
string platform;
if (OperatingSystem.IsWindows()) platform = "win32";
else if (OperatingSystem.IsLinux()) platform = "linux";
else if (OperatingSystem.IsMacOS()) platform = "darwin";
else return null;

var arch = System.Runtime.InteropServices.RuntimeInformation.OSArchitecture switch
{
System.Runtime.InteropServices.Architecture.X64 => "x64",
System.Runtime.InteropServices.Architecture.Arm64 => "arm64",
_ => null,
};

return arch != null ? $"{platform}-{arch}" : null;
}

private static string GetNapiPrebuildsFolderOrThrow() =>
GetNapiPrebuildsFolder()
?? throw new InvalidOperationException("Could not determine a napi-rs prebuilds folder for FFI hosting.");

private static (string FileName, IEnumerable<string> Args) ResolveCliCommand(string cliPath, IEnumerable<string> args)
{
var isJsFile = cliPath.EndsWith(".js", StringComparison.OrdinalIgnoreCase);
Expand All @@ -2089,7 +2192,7 @@
return (cliPath, args);
}

private async Task<Connection> ConnectToServerAsync(Process? cliProcess, string? tcpHost, int? tcpPort, ProcessStderrPump? stderrPump, CancellationToken cancellationToken)
private async Task<Connection> ConnectToServerAsync(Process? cliProcess, string? tcpHost, int? tcpPort, ProcessStderrPump? stderrPump, CancellationToken cancellationToken, FfiRuntimeHost? ffiHost = null)
{
var setupTimestamp = Stopwatch.GetTimestamp();
NetworkStream? networkStream = null;
Expand All @@ -2099,7 +2202,12 @@
{
Stream inputStream, outputStream;

if (_connection is StdioRuntimeConnection)
if (ffiHost is not null)
{
inputStream = ffiHost.ReceiveStream;
outputStream = ffiHost.SendStream;
}
else if (_connection is StdioRuntimeConnection)
{
if (cliProcess == null)
{
Expand Down Expand Up @@ -2165,7 +2273,7 @@
"CopilotClient.ConnectToServerAsync transport setup complete. Elapsed={Elapsed}",
setupTimestamp);

var connection = new Connection(rpc, cliProcess, networkStream, stderrPump);
var connection = new Connection(rpc, cliProcess, networkStream, stderrPump, ffiHost);
_serverRpc = connection.Server;

return connection;
Expand Down Expand Up @@ -2368,14 +2476,16 @@
JsonRpc rpc,
Process? cliProcess, // Set if we created the child process
NetworkStream? networkStream, // Set if using TCP
ProcessStderrPump? stderrPump = null) // Captures stderr for error messages
ProcessStderrPump? stderrPump = null, // Captures stderr for error messages
FfiRuntimeHost? ffiHost = null) // Set if using in-process FFI hosting
{
public Process? CliProcess => cliProcess;
public JsonRpc Rpc => rpc;
public ServerRpc Server => field ?? Interlocked.CompareExchange(ref field, new(rpc), null) ?? field;
public NetworkStream? NetworkStream => networkStream;
public ProcessStderrPump? StderrPump => stderrPump;
public StringBuilder? StderrBuffer => stderrPump?.Buffer;
public FfiRuntimeHost? FfiHost => ffiHost;
}

private sealed class ProcessStderrPump
Expand Down
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