feat(libc): reopen guest streams while preserving identity

core/libs/prx/libc/include/FileStream.hpp

@@ -65,6 +65,20 @@ public:
     }
 
     GuestFilePrefix& GuestState() { return _guest; }
+    bool Reopen(const char* filename, const char* mode) {
+        auto* previous = GetHandle();
+        _guest = {};
+        _handle = std::freopen(filename, mode, previous);
+        if (!_handle) return false;
+        _guest.flags = 0x10;
+#ifdef _WIN32
+        const int descriptor = _fileno(_handle);
+#else
+        const int descriptor = ::fileno(_handle);
+#endif
+        _guest.descriptor = descriptor >= 0 && descriptor <= 32767 ? static_cast<std::int16_t>(descriptor) : -1;
+        return true;
+    }
     void SyncStatus() {
         _guest.readRemaining = 0;
         _guest.writeRemaining = 0;

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

@@ -16,6 +16,25 @@ extern "C" {
 
 [[noreturn]] void APS5_VABI _ZSt11_Xbad_allocv_nid_postfix();
 
+FileStream* APS5_VABI freopen_nid_postfix(const char* filename, const char* mode, FileStream* stream) {
+    if (!stream || !mode) { errno = 22; return nullptr; }
+    if (!filename) { errno = 45; return nullptr; } // Mode-only reopening is not supported.
+    const char* supported[] = {"r", "w", "a", "rb", "wb", "ab", "r+", "w+", "a+",
+        "rb+", "wb+", "ab+", "r+b", "w+b", "a+b"};
+    bool valid = false;
+    for (const auto* candidate : supported) if (std::strcmp(mode, candidate) == 0) valid = true;
+    if (!valid) { errno = 22; return nullptr; }
+    try {
+        const auto path = *filename ? ResolvePath_nid_no_patch(filename).string() : std::string{};
+        if (stream->Reopen(path.c_str(), mode)) return stream;
+        const int error = errno;
+        if (stream->IsDynamic()) delete stream;
+        errno = error;
+        return nullptr;
+    } catch (const std::bad_alloc&) { errno = 12; return nullptr; }
+      catch (const std::filesystem::filesystem_error&) { errno = 5; return nullptr; }
+}
+
 FileStream* APS5_VABI fopen_nid_postfix(const char* filename, const char* mode) {
     if (!filename || !mode) throw std::runtime_error(std::string(__func__) + ": " + FOPEN_MSG_NULL_ARG);
     const std::filesystem::path fpath = ResolvePath_nid_no_patch(filename);

core/libs/tests/GuestStandardStreams.cpp

@@ -4,7 +4,11 @@
 #include <cstring>
 #include <cstdarg>
 #include <cstdint>
+#include <filesystem>
+#include <fstream>
+#include <chrono>
 extern "C" {
+FileStream* APS5_VABI freopen_nid_postfix(const char*, const char*, FileStream*);
 int APS5_VABI fseeko_nid_postfix(FileStream*, std::int64_t, int);
 std::int64_t APS5_VABI ftello_nid_postfix(FileStream*);
 int APS5_VABI fseek_nid_postfix(FileStream*, std::int64_t, int);
@@ -102,4 +106,24 @@ int main() {
     Require(fseeko_nid_postfix(&positioned, 0, SEEK_SET) == 0);
     Require(!feof_nid_postfix(&positioned));
     positioned.Close();
+
+    const auto filename = "anyps5-reopen-test-" + std::to_string(std::chrono::steady_clock::now().time_since_epoch().count());
+    FileStream redirected(std::tmpfile());
+    Require(fgetc_nid_postfix(&redirected) == EOF && feof_nid_postfix(&redirected));
+    Require(freopen_nid_postfix(filename.c_str(), "w+b", &redirected) == &redirected);
+    Require(!feof_nid_postfix(&redirected));
+    Require(fputc_nid_postfix('R', &redirected) == 'R');
+    Require(fseeko_nid_postfix(&redirected, 0, SEEK_SET) == 0);
+    Require(fgetc_nid_postfix(&redirected) == 'R');
+    Require(freopen_nid_postfix(filename.c_str(), "ab", &redirected) == &redirected);
+    Require(fputc_nid_postfix('S', &redirected) == 'S');
+    Require(freopen_nid_postfix(nullptr, "r", &redirected) == nullptr && *__error_nid_postfix() == 45);
+    redirected.Close();
+    { std::ifstream input(filename, std::ios::binary); std::string contents; std::getline(input, contents);
+      Require(contents == "RS"); }
+    Require(std::filesystem::remove(filename));
+    FileStream failed(std::tmpfile());
+    Require(freopen_nid_postfix(filename.c_str(), "rb", &failed) == nullptr);
+    Require(*__error_nid_postfix() == 2);
+    Require(failed.GuestState().flags == 0 && failed.GuestState().descriptor == -1);
 }