[llvm] [libc] Refactor scanf reader to match printf (PR #66023)

[Joseph Huber via llvm-commits]

================
@@ -9,15 +9,63 @@
 #ifndef LLVM_LIBC_SRC_STDIO_SCANF_CORE_VFSCANF_INTERNAL_H
 #define LLVM_LIBC_SRC_STDIO_SCANF_CORE_VFSCANF_INTERNAL_H
 
+#include "src/__support/File/file.h"
 #include "src/__support/arg_list.h"
+#include "src/stdio/scanf_core/reader.h"
+#include "src/stdio/scanf_core/scanf_main.h"
 
 #include <stdio.h>
 
 namespace __llvm_libc {
+
+namespace internal {
+
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
+
+LIBC_INLINE int getc(void *f) {
+  unsigned char c;
+  auto result = reinterpret_cast<__llvm_libc::File *>(f)->read_unlocked(&c, 1);
+  size_t r = result.value;
+  if (result.has_error() || r != 1) {
+    return '\0';
+  }
+  return c;
+}
+
+LIBC_INLINE void ungetc(int c, void *f) {
+  reinterpret_cast<__llvm_libc::File *>(f)->ungetc(c);
+}
+
+LIBC_INLINE int ferror_unlocked(FILE *f) {
+  return reinterpret_cast<__llvm_libc::File *>(f)->error_unlocked();
+}
+
+#else // defined(LIBC_COPT_STDIO_USE_SYSTEM_FILE)
+
+LIBC_INLINE int getc(void *f) { return ::getc(reinterpret_cast<::FILE *>(f)); }
+
+LIBC_INLINE void ungetc(int c, void *f) {
+  ::ungetc(c, reinterpret_cast<::FILE *>(f));
+}
+
+LIBC_INLINE int ferror_unlocked(::FILE *f) { return ::ferror_unlocked(f); }
+
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
+
+} // namespace internal
+
 namespace scanf_core {
 
-int vfscanf_internal(::FILE *__restrict stream, const char *__restrict format,
-                     internal::ArgList &args);
+LIBC_INLINE int vfscanf_internal(::FILE *__restrict stream,
+                                 const char *__restrict format,
+                                 internal::ArgList &args) {
+  scanf_core::Reader reader(stream, &internal::getc, internal::ungetc);
+  int retval = scanf_core::scanf_main(&reader, format, args);
+  if (retval == 0 && internal::ferror_unlocked(stream)) {
+    return EOF;
+  }
----------------
jhuber6 wrote:

```suggestion
  if (retval == 0 && internal::ferror_unlocked(stream))
    return EOF;
```
LLVM style omits braces like these

https://github.com/llvm/llvm-project/pull/66023