feat(ffi): unwind safety: shield FFI entrypoints with panic guard (#546)

This PR implements widely accepted Rust programming practices for
dealing with panics across ABI (programming language) boundaries.

- Add panic_guard.rs to wrap FFI calls and prevent panic across FFI/ABI boundary (undefined behavior).
- Capture per-thread backtraces via a temporary panic hook
- After a panic, subsequent invocations are poisoned.
- Integrate with_unwind_guard across the engine, schema registry, and target registry exportis

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

bindings/csharp/Regorus.Tests/PanicGuardTests.cs

@@ -0,0 +1,84 @@
+#if REGORUS_FFI_TEST_HOOKS
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System;
+using System.Runtime.InteropServices;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Regorus.Internal;
+
+namespace Regorus.Tests;
+
+[TestClass]
+public sealed class PanicGuardTests
+{
+    [TestInitialize]
+    public void Initialize()
+    {
+        API.regorus_engine_test_reset_poison();
+    }
+
+    [TestCleanup]
+    public void Cleanup()
+    {
+        API.regorus_engine_test_reset_poison();
+    }
+
+    [TestMethod]
+    public void Panic_produces_invalid_operation_exception()
+    {
+        var panic = Assert.ThrowsException<InvalidOperationException>(TriggerPanic);
+        StringAssert.Contains(panic.Message, "panicked", "panic message should capture payload");
+    }
+
+    [TestMethod]
+    public void Poison_flag_blocks_subsequent_calls()
+    {
+        _ = Assert.ThrowsException<InvalidOperationException>(TriggerPanic);
+        var poisoned = Assert.ThrowsException<InvalidOperationException>(TriggerPanic);
+        StringAssert.Contains(poisoned.Message, "poisoned", "poisoned message should explain guard state");
+    }
+
+    private static unsafe void TriggerPanic()
+    {
+        var result = API.regorus_engine_test_trigger_panic();
+        try
+        {
+            if (result.status == RegorusStatus.Ok)
+            {
+                return;
+            }
+
+            var message = PtrToStringUtf8((IntPtr)result.error_message);
+            throw result.status.CreateException(message);
+        }
+        finally
+        {
+            API.regorus_result_drop(result);
+        }
+    }
+
+    private static string? PtrToStringUtf8(IntPtr ptr)
+    {
+#if NETSTANDARD2_1
+        return Marshal.PtrToStringUTF8(ptr);
+#else
+        if (ptr == IntPtr.Zero)
+        {
+            return null;
+        }
+
+        var len = 0;
+        while (Marshal.ReadByte(ptr, len) != 0)
+        {
+            len++;
+        }
+
+        var buffer = new byte[len];
+        Marshal.Copy(ptr, buffer, 0, buffer.Length);
+        return System.Text.Encoding.UTF8.GetString(buffer);
+#endif
+    }
+}
+
+#endif

bindings/csharp/Regorus.Tests/Regorus.Tests.csproj

@@ -6,11 +6,17 @@
     <!-- More info about dotnet test integration https://learn.microsoft.com/dotnet/core/testing/unit-testing-platform-integration-dotnet-test -->
     <TestingPlatformDotnetTestSupport>true</TestingPlatformDotnetTestSupport>
     <TestingPlatformShowTestsFailure>true</TestingPlatformShowTestsFailure>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   
   <PropertyGroup>
     <!-- If the environment variable is set (such as in a Github Action run), append the suffix to the version number -->
     <RegorusPackageVersionSuffix Condition="'$(VersionSuffix)' != ''">-$(VersionSuffix)</RegorusPackageVersionSuffix>
+    <UseLocalRegorus Condition="'$(UseLocalRegorus)' == ''">true</UseLocalRegorus>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(UseLocalRegorus)' == 'true'">
+    <DefineConstants>$(DefineConstants);REGORUS_FFI_TEST_HOOKS</DefineConstants>
   </PropertyGroup>
 
   <ItemGroup>
@@ -21,7 +27,13 @@
     <PackageReference Include="MSTest" Version="3.8.2" />
   </ItemGroup>
 
-  <ItemGroup>
-    <PackageReference Include="Regorus" Version="0.8.0$(RegorusPackageVersionSuffix)"/>
+  <ItemGroup Condition="'$(UseLocalRegorus)' == 'true'">
+    <ProjectReference Include="../Regorus/Regorus.csproj">
+      <AdditionalProperties>EnableRegorusTestHooks=true</AdditionalProperties>
+    </ProjectReference>
+  </ItemGroup>
+
+  <ItemGroup Condition="'$(UseLocalRegorus)' != 'true'">
+    <PackageReference Include="Regorus" Version="0.8.0$(RegorusPackageVersionSuffix)" />
   </ItemGroup>
 </Project>