fix(libc): switch update streams to reading in vfscanf

vfscanf, which vscanf and scanf also go through, read without switching
the stream to reading first. On Windows a vfscanf directly after fwrite
on an r+ stream returned bytes of the write buffer, as fscanf did before
a7e6e597d. It now calls SwitchToReading as fscanf does.

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

@@ -653,6 +653,7 @@ extern FileStream* __stdinp_nid_postfix;
 int APS5_VABI vfscanf_nid_postfix(FileStream* stream, const char* format, VaList* args) {
     if (stream && !stream->IsOpen()) { errno = 9; return -1; }
     auto* native = GetNativeStream(stream);
+    if (!stream->SwitchToReading()) return EOF;
 #ifdef _WIN32
     const int result = LibcDetail::ScanFileWindows_nid_no_patch(native, format, args);
 #else

core/libs/tests/FileStreams.cpp

@@ -4,6 +4,7 @@
 #include <array>
 #include <barrier>
 #include <cerrno>
+#include <cstdarg>
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
@@ -102,6 +103,23 @@ static void CheckExclusiveCppOpen(const std::string& base) {
     }
 }
 
+static int APS5_VABI ScanList(FileStream* stream, const char* format, ...) {
+#ifdef _WIN32
+    __builtin_sysv_va_list args;
+    __builtin_sysv_va_start(args, format);
+#else
+    std::va_list args;
+    va_start(args, format);
+#endif
+    const int result = vfscanf_nid_postfix(stream, format, reinterpret_cast<VaList*>(args));
+#ifdef _WIN32
+    __builtin_sysv_va_end(args);
+#else
+    va_end(args);
+#endif
+    return result;
+}
+
 static std::string ReadWhole(const std::string& path) {
     auto* stream = fopen_nid_postfix(path.c_str(), "rb");
     Require(stream != nullptr);
@@ -158,6 +176,11 @@ static void CheckUpdateStreamReadsAfterWrites(const std::string& base) {
         Require(fscanf_nid_postfix(stream, "%3s", buffer.data()) == 1, "fscanf after fwrite on an r+ stream");
         Require(std::strcmp(buffer.data(), "234") == 0, "fscanf after fwrite on an r+ stream read other bytes");
     });
+    CheckReadAfterWrite(path, "r+b", overwritten, [](FileStream* stream) {
+        std::array<char, 4> buffer{};
+        Require(ScanList(stream, "%3s", buffer.data()) == 1, "vfscanf after fwrite on an r+ stream");
+        Require(std::strcmp(buffer.data(), "234") == 0, "vfscanf after fwrite on an r+ stream read other bytes");
+    });
     CheckReadAfterWrite(path, "w+b", "AB", [](FileStream* stream) {
         std::array<char, 4> buffer{};
         Require(fread_nid_postfix(buffer.data(), 1, buffer.size(), stream) == 0 && feof_nid_postfix(stream) != 0, "fread after fwrite on a w+ stream");