[libc] fix scanf error handling

[michaelrj]

Summary

Scanf is supposed to return EOF when it fails to make any conversions
and there is an input failure. Previously it would return EOF on a
matching failure, which may be an input failure but can also be a
parsing error.

libc/src/stdio/scanf_core/file_reader.h

FileReader(::FILE *init_file) {
     file = reinterpret_cast<__llvm_libc::File *>(init_file);
     file->lock();
   }
 
   ~FileReader() { file->unlock(); }
 
   char get_char();
   void unget_char(char c);
+  bool has_error() { return file->error_unlocked(); }
 };
 
 } // namespace scanf_core
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_STDIO_SCANF_CORE_FILE_READER_H

libc/src/stdio/scanf_core/reader.h

public:
   // signal to stop parsing.
   char getc();
 
   // This moves the input back by one character, placing c into the buffer if
   // this is a file reader, else c is ignored.
   void ungetc(char c);
 
   size_t chars_read() { return cur_chars_read; }
+
+  bool has_error();
 };
 
 } // namespace scanf_core
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_STDIO_SCANF_CORE_READER_H

libc/src/stdio/scanf_core/reader.cpp

if (reader_type == ReaderType::String) {
     // need to place anything back into a buffer, and modifying the source
     // string would be dangerous.
     return string_reader->unget_char();
   } else {
     return file_reader->unget_char(c);
   }
 }
 
+bool Reader::has_error() {
+  if (reader_type == ReaderType::File) {
+    return file_reader->has_error();
+  }
+  return false;
+}
+
 } // namespace scanf_core
 } // namespace __llvm_libc

libc/src/stdio/scanf_core/scanf_main.cpp

for (FormatSection cur_section = parser.get_next_section();
     if (cur_section.has_conv) {
       ret_val = convert(reader, cur_section);
       conversions += ret_val == READ_OK ? 1 : 0;
     } else {
       ret_val = raw_match(reader, cur_section.raw_string);
     }
   }
 
-  if (conversions == 0 && ret_val != READ_OK) {
+  if (conversions == 0 && reader->has_error()) {
     // This is intended to be converted to EOF in the client call to avoid
     // including stdio.h in this internal file.
     return -1;
   }
   return conversions;
 }
 
 } // namespace scanf_core
 } // namespace __llvm_libc