fix(libc): append through a descriptor opened by fdopen in an append mode on Windows

core/libs/prx/libc/src/FileAndHeap.cpp

@@ -87,6 +87,31 @@ static std::FILE* OpenNative(const std::filesystem::path& path, const std::strin
 #endif
 }
 
+#ifdef _WIN32
+static bool AppendThroughDescriptor(int descriptor) {
+    const auto handle = reinterpret_cast<HANDLE>(::_get_osfhandle(descriptor));
+    if (handle == INVALID_HANDLE_VALUE) return false;
+    if (descriptor <= 2 || GetFileType(handle) != FILE_TYPE_DISK) return true;
+    const int translation = ::_setmode(descriptor, _O_BINARY);
+    if (translation == -1 || ::_setmode(descriptor, translation) == -1) return false;
+    HANDLE copy = nullptr;
+    if (!DuplicateHandle(GetCurrentProcess(), handle, GetCurrentProcess(), &copy, 0, TRUE, DUPLICATE_SAME_ACCESS)) {
+        errno = EBADF;
+        return false;
+    }
+    const int appending = ::_open_osfhandle(reinterpret_cast<std::intptr_t>(copy), _O_APPEND | translation);
+    if (appending < 0) {
+        CloseHandle(copy);
+        return false;
+    }
+    const int moved = ::_dup2(appending, descriptor);
+    const int error = errno;
+    ::_close(appending);
+    errno = error;
+    return moved == 0;
+}
+#endif
+
 using GuestCompare = int (APS5_VABI *)(const void*, const void*);
 
 static void SwapElements(unsigned char* left, unsigned char* right, size_t size) {
@@ -196,6 +221,7 @@ FileStream* APS5_VABI fdopen_nid_postfix(int descriptor, const char* mode) {
         const auto release = [](void* pointer) { ::operator delete(pointer); };
         std::unique_ptr<void, decltype(release)> storage(::operator new(sizeof(FileStream)), release);
 #ifdef _WIN32
+        if (parsed->append && !AppendThroughDescriptor(descriptor)) return nullptr;
         auto* native = ::_fdopen(descriptor, nativeMode.c_str());
 #else
         auto* native = ::fdopen(descriptor, nativeMode.c_str());

core/libs/tests/GuestStandardStreams.cpp

@@ -392,6 +392,31 @@ static void CheckDescriptorModes(std::FILE* original) {
 #endif
 }
 
+static void CheckDescriptorAppend(std::FILE* original) {
+    std::string expected = "retained";
+    for (const char* mode : {"a", "ab", "a+", "ae"}) {
+        const int descriptor = DuplicateDescriptor(original);
+#ifdef _WIN32
+        Require(descriptor >= 0 && _lseek(descriptor, 2, SEEK_SET) == 2);
+#else
+        Require(descriptor >= 0 && ::lseek(descriptor, 2, SEEK_SET) == 2);
+#endif
+        auto* wrapped = fdopen_nid_postfix(descriptor, mode);
+        Require(wrapped != nullptr && fileno_nid_postfix(wrapped) == descriptor);
+        Require(fputc_nid_postfix('+', wrapped) == '+' && fflush_nid_postfix(wrapped) == 0);
+        Require(fseek_nid_postfix(wrapped, 0, SEEK_SET) == 0 && fputc_nid_postfix('+', wrapped) == '+');
+        Require(fclose_nid_postfix(wrapped) == 0);
+        expected += "++";
+        const int reader = DuplicateDescriptor(original);
+        Require(reader >= 0);
+        auto* written = fdopen_nid_postfix(reader, "r");
+        Require(written != nullptr && fseek_nid_postfix(written, 0, SEEK_SET) == 0);
+        char contents[32]{};
+        Require(fgets_nid_postfix(contents, sizeof(contents), written) == contents && expected == contents);
+        Require(fclose_nid_postfix(written) == 0);
+    }
+}
+
 int main() {
     Require(_Getmbcurmax_nid_postfix() == 1 && _Getmbcurmax_nid_postfix() == ___mb_cur_max_nid_postfix());
     CheckFlushWhileReading();
@@ -421,6 +446,7 @@ int main() {
     Require(::close(descriptor) == -1);
 #endif
     CheckDescriptorModes(original);
+    CheckDescriptorAppend(original);
     Require(std::fclose(original) == 0);
     char stringOutput[256];
     std::memset(stringOutput, '!', sizeof(stringOutput));