feat: add response routing for service import request-reply patterns

src/NATS.Server/Imports/ResponseRouter.cs

@@ -0,0 +1,64 @@
+using System.Security.Cryptography;
+using NATS.Server.Auth;
+
+namespace NATS.Server.Imports;
+
+/// <summary>
+/// Handles response routing for service imports.
+/// Maps to Go's service reply prefix generation and response cleanup.
+/// Reference: golang/nats-server/server/accounts.go — addRespServiceImport, removeRespServiceImport
+/// </summary>
+public static class ResponseRouter
+{
+    private static readonly char[] Base62 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".ToCharArray();
+
+    /// <summary>
+    /// Generates a unique reply prefix for response routing.
+    /// Format: "_R_.{10 random base62 chars}."
+    /// </summary>
+    public static string GenerateReplyPrefix()
+    {
+        Span<byte> bytes = stackalloc byte[10];
+        RandomNumberGenerator.Fill(bytes);
+        var chars = new char[10];
+        for (int i = 0; i < 10; i++)
+            chars[i] = Base62[bytes[i] % 62];
+        return $"_R_.{new string(chars)}.";
+    }
+
+    /// <summary>
+    /// Creates a response service import that maps the generated reply prefix
+    /// back to the original reply subject on the requesting account.
+    /// </summary>
+    public static ServiceImport CreateResponseImport(
+        Account exporterAccount,
+        ServiceImport originalImport,
+        string originalReply)
+    {
+        var replyPrefix = GenerateReplyPrefix();
+
+        var responseSi = new ServiceImport
+        {
+            DestinationAccount = exporterAccount,
+            From = replyPrefix + ">",
+            To = originalReply,
+            IsResponse = true,
+            ResponseType = originalImport.ResponseType,
+            Export = originalImport.Export,
+            TimestampTicks = DateTime.UtcNow.Ticks,
+        };
+
+        exporterAccount.Exports.Responses[replyPrefix] = responseSi;
+        return responseSi;
+    }
+
+    /// <summary>
+    /// Removes a response import from the account's export map.
+    /// For Singleton responses, this is called after the first reply is delivered.
+    /// For Streamed/Chunked, it is called when the response stream ends.
+    /// </summary>
+    public static void CleanupResponse(Account account, string replyPrefix, ServiceImport responseSi)
+    {
+        account.Exports.Responses.Remove(replyPrefix);
+    }
+}

tests/NATS.Server.Tests/ResponseRoutingTests.cs

@@ -0,0 +1,127 @@
+using NATS.Server.Auth;
+using NATS.Server.Imports;
+
+namespace NATS.Server.Tests;
+
+public class ResponseRoutingTests
+{
+    [Fact]
+    public void GenerateReplyPrefix_creates_unique_prefix()
+    {
+        var prefix1 = ResponseRouter.GenerateReplyPrefix();
+        var prefix2 = ResponseRouter.GenerateReplyPrefix();
+
+        prefix1.ShouldStartWith("_R_.");
+        prefix2.ShouldStartWith("_R_.");
+        prefix1.ShouldNotBe(prefix2);
+        prefix1.Length.ShouldBeGreaterThan(4);
+    }
+
+    [Fact]
+    public void GenerateReplyPrefix_ends_with_dot()
+    {
+        var prefix = ResponseRouter.GenerateReplyPrefix();
+
+        prefix.ShouldEndWith(".");
+        // Format: "_R_." + 10 chars + "." = 15 chars
+        prefix.Length.ShouldBe(15);
+    }
+
+    [Fact]
+    public void Singleton_response_import_removed_after_delivery()
+    {
+        var exporter = new Account("exporter");
+        exporter.AddServiceExport("api.test", ServiceResponseType.Singleton, null);
+
+        var replyPrefix = ResponseRouter.GenerateReplyPrefix();
+        var responseSi = new ServiceImport
+        {
+            DestinationAccount = exporter,
+            From = replyPrefix + ">",
+            To = "_INBOX.original.reply",
+            IsResponse = true,
+            ResponseType = ServiceResponseType.Singleton,
+        };
+        exporter.Exports.Responses[replyPrefix] = responseSi;
+
+        exporter.Exports.Responses.ShouldContainKey(replyPrefix);
+
+        // Simulate singleton delivery cleanup
+        ResponseRouter.CleanupResponse(exporter, replyPrefix, responseSi);
+
+        exporter.Exports.Responses.ShouldNotContainKey(replyPrefix);
+    }
+
+    [Fact]
+    public void CreateResponseImport_registers_in_exporter_responses()
+    {
+        var exporter = new Account("exporter");
+        var importer = new Account("importer");
+        exporter.AddServiceExport("api.test", ServiceResponseType.Singleton, null);
+
+        var originalSi = new ServiceImport
+        {
+            DestinationAccount = exporter,
+            From = "api.test",
+            To = "api.test",
+            Export = exporter.Exports.Services["api.test"],
+            ResponseType = ServiceResponseType.Singleton,
+        };
+
+        var responseSi = ResponseRouter.CreateResponseImport(exporter, originalSi, "_INBOX.abc123");
+
+        responseSi.IsResponse.ShouldBeTrue();
+        responseSi.ResponseType.ShouldBe(ServiceResponseType.Singleton);
+        responseSi.To.ShouldBe("_INBOX.abc123");
+        responseSi.DestinationAccount.ShouldBe(exporter);
+        responseSi.From.ShouldEndWith(">");
+        responseSi.Export.ShouldBe(originalSi.Export);
+
+        // Should be registered in the exporter's response map
+        exporter.Exports.Responses.Count.ShouldBe(1);
+    }
+
+    [Fact]
+    public void CreateResponseImport_preserves_streamed_response_type()
+    {
+        var exporter = new Account("exporter");
+        exporter.AddServiceExport("api.stream", ServiceResponseType.Streamed, null);
+
+        var originalSi = new ServiceImport
+        {
+            DestinationAccount = exporter,
+            From = "api.stream",
+            To = "api.stream",
+            Export = exporter.Exports.Services["api.stream"],
+            ResponseType = ServiceResponseType.Streamed,
+        };
+
+        var responseSi = ResponseRouter.CreateResponseImport(exporter, originalSi, "_INBOX.xyz789");
+
+        responseSi.ResponseType.ShouldBe(ServiceResponseType.Streamed);
+    }
+
+    [Fact]
+    public void Multiple_response_imports_each_get_unique_prefix()
+    {
+        var exporter = new Account("exporter");
+        exporter.AddServiceExport("api.test", ServiceResponseType.Singleton, null);
+
+        var originalSi = new ServiceImport
+        {
+            DestinationAccount = exporter,
+            From = "api.test",
+            To = "api.test",
+            Export = exporter.Exports.Services["api.test"],
+            ResponseType = ServiceResponseType.Singleton,
+        };
+
+        var resp1 = ResponseRouter.CreateResponseImport(exporter, originalSi, "_INBOX.reply1");
+        var resp2 = ResponseRouter.CreateResponseImport(exporter, originalSi, "_INBOX.reply2");
+
+        exporter.Exports.Responses.Count.ShouldBe(2);
+        resp1.To.ShouldBe("_INBOX.reply1");
+        resp2.To.ShouldBe("_INBOX.reply2");
+        resp1.From.ShouldNotBe(resp2.From);
+    }
+}