Fix native candidate and schema list handling

[amorphobia]

背景

在跨 DLL 调用 librime 候选迭代 API 时，RimeCandidateListIterator 直接使用包含托管字符串字段的 RimeCandidate 接收原生结构。

候选项中的字符串指针由 librime 管理，并可能在迭代过程中被替换或在迭代结束后失效。直接依赖自动封送会导致生命周期边界不明确，也可能在原生指针失效后读取数据。

此外，Switcher 的 Schema 列表处理存在释放路径不完整和 selected list 调用错误的问题。

修改内容

• 新增与原生布局一致的 NativeRimeCandidate。
• 在候选迭代期间立即将原生字符串指针复制为托管字符串。
• 使用 try/finally 保证始终调用 CandidateListEnd。
• 使用 try/finally 保证 Switcher Schema 列表始终通过 SchemaListDestroy 释放。
• 修复 GetSelectedSchemaList 错误调用 GetAvailableSchemaList 的问题。

影响

• 消除候选数据跨 DLL 边界时对原生指针生命周期的依赖。
• 避免候选迭代或 Schema 转换异常时泄漏原生资源。
• GetSelectedSchemaList 现在返回正确的已选 Schema 列表。
• RimeCandidateListIterator.Candidate 从公共字段调整为只读属性。现有源码读取方式不变，但已经针对旧程序集编译的消费者需要重新编译。

[amorphobia]

让 AI 写了个例子，看看上次 new 出来的 text 地址是不是下次 delete 的地址

probe.cpp

#include <cstdint>
#include <cstdio>
#include <cstring>

struct Candidate {
    char* text;
    char* comment;
    void* reserved;
};

struct Iterator {
    std::uintptr_t ptr;
    int index;
    Candidate candidate;
};

static const char* const kTexts[] = { "first", "second", "third" };
static const char* const kComments[] = { "one", "two", "three" };
static constexpr int kCount = sizeof(kTexts) / sizeof(kTexts[0]);

static char* g_last_owned_text = nullptr;

static char* copy_string(const char* source) {
    const size_t length = std::strlen(source) + 1;
    auto* copy = new char[length];
    std::memcpy(copy, source, length);
    return copy;
}

extern "C" __declspec(dllexport)
int candidate_begin(Iterator* iterator) {
    std::memset(iterator, 0, sizeof(*iterator));
    iterator->index = -1;
    return 1;
}

extern "C" __declspec(dllexport)
int candidate_next(Iterator* iterator) {
    ++iterator->index;

    if (iterator->index >= kCount) {
        return 0;
    }

    // Mirrors librime: the iterator owns and replaces these pointers.
    std::fprintf(
        stderr,
        "[next] deleting text=%p, comment=%p\n",
        static_cast<void*>(iterator->candidate.text),
        static_cast<void*>(iterator->candidate.comment));
    std::fflush(stderr);

    delete[] iterator->candidate.text;
    delete[] iterator->candidate.comment;

    iterator->candidate.text = copy_string(kTexts[iterator->index]);
    iterator->candidate.comment = copy_string(kComments[iterator->index]);
    iterator->candidate.reserved = nullptr;
    g_last_owned_text = iterator->candidate.text;

    std::fprintf(
        stderr,
        "[next] allocated text=%p\n",
        static_cast<void*>(iterator->candidate.text));
    std::fflush(stderr);

    return 1;
}

extern "C" __declspec(dllexport)
void candidate_end(Iterator* iterator) {
    std::fprintf(
        stderr,
        "[end] deleting text=%p, comment=%p\n",
        static_cast<void*>(iterator->candidate.text),
        static_cast<void*>(iterator->candidate.comment));
    std::fflush(stderr);

    delete[] iterator->candidate.text;
    delete[] iterator->candidate.comment;
    std::memset(iterator, 0, sizeof(*iterator));
    g_last_owned_text = nullptr;
}

extern "C" __declspec(dllexport)
void* candidate_last_owned_text() {
    return g_last_owned_text;
}

probe.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <None Update="probe.dll" CopyToOutputDirectory="PreserveNewest" />
  </ItemGroup>

</Project>

Program.cs

using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]
public struct Candidate
{
    [MarshalAs(UnmanagedType.LPUTF8Str)]
    public string? Text;

    [MarshalAs(UnmanagedType.LPUTF8Str)]
    public string? Comment;

    public IntPtr Reserved;
}

[StructLayout(LayoutKind.Sequential)]
public struct Iterator
{
    public UIntPtr Ptr;
    public int Index;

    [MarshalAs(UnmanagedType.Struct)]
    public Candidate Candidate;
}

internal static class Native
{
    [DllImport("probe", CallingConvention = CallingConvention.Cdecl)]
    internal static extern int candidate_begin(ref Iterator iterator);

    [DllImport("probe", CallingConvention = CallingConvention.Cdecl)]
    internal static extern int candidate_next(ref Iterator iterator);

    [DllImport("probe", CallingConvention = CallingConvention.Cdecl)]
    internal static extern void candidate_end(ref Iterator iterator);

    [DllImport("probe", CallingConvention = CallingConvention.Cdecl)]
    internal static extern IntPtr candidate_last_owned_text();
}

internal static class Program
{
    private static void Main()
    {
        var iterator = default(Iterator);

        if (Native.candidate_begin(ref iterator) == 0)
        {
            throw new InvalidOperationException("begin failed");
        }

        try
        {
            Console.WriteLine("Calling first next...");
            Native.candidate_next(ref iterator);

            Console.WriteLine($"Managed text: {iterator.Candidate.Text}");
            Console.WriteLine(
                $"Native-owned pointer: 0x{Native.candidate_last_owned_text().ToInt64():X}");

            Console.WriteLine();
            Console.WriteLine("Calling second next...");
            Native.candidate_next(ref iterator);

            Console.WriteLine($"Managed text: {iterator.Candidate.Text}");
            Console.WriteLine(
                $"Native-owned pointer: 0x{Native.candidate_last_owned_text().ToInt64():X}");
        }
        finally
        {
            Native.candidate_end(ref iterator);
        }
    }
}

编译 DLL

cl /LD /EHsc probe.cpp /Fe:probe.dll

运行

dotnet run

我的结果

Calling first next...
[next] deleting text=0000000000000000, comment=0000000000000000
[next] allocated text=000001F9000C9B90
Managed text: first
Native-owned pointer: 0x1F9000C9B90

Calling second next...
[next] deleting text=000001F9000C9DB0, comment=000001F9000C9D70
[next] allocated text=000001F9000C9D00
Managed text: second
Native-owned pointer: 0x1F9000C9D00
[end] deleting text=000001F9000C9B70, comment=000001F9000C9C00

C++ 一开始 new 出来的地址是 0x1F9000C9B90, 调用 next 的时候 delete 的地址是 0x1F9000C9DB0；然后第二次 new 出来的地址是 0x1F9000C9D00，最终 delete 的地址是 0x1F9000C9B70。