[libc][NFC] Add a separate flag for capturing the '+' in fopen mode string.

[sivachandra]

Summary

Having a separate flag helps in setting up proper flags when
implementing, say the Linux specialization of File.

Along the way, a signature for a function which is to be used to open
files has been added. The implementation of the function is to be
included in platform specializations.

libc/src/__support/File/file.h

 
 namespace __llvm_libc {
 
 // This a generic base class to encapsulate a platform independent file data
 // structure. Platform specific specializations should create a subclass as
 // suitable for their platform.
 class File {
 public:
+  static constexpr size_t DEFAULT_BUFFER_SIZE = 1024;
+
   using LockFunc = void(File *);
   using UnlockFunc = void(File *);
 
   using WriteFunc = size_t(File *, const void *, size_t);
   using ReadFunc = size_t(File *, void *, size_t);
   using SeekFunc = int(File *, long, int);
   using CloseFunc = int(File *);
   using FlushFunc = int(File *);
 
   using ModeFlags = uint32_t;
 
   // The three different types of flags below are to be used with '|' operator.
   // Their values correspond to mutually exclusive bits in a 32-bit unsigned
   // integer value. A flag set can include both READ and WRITE if the file
   // is opened in update mode (ie. if the file was opened with a '+' the mode
   // string.)
   enum class OpenMode : ModeFlags {
     READ = 0x1,
     WRITE = 0x2,
     APPEND = 0x4,
+    PLUS = 0x8,
   };
 
   // Denotes a file opened in binary mode (which is specified by including
   // the 'b' character in teh mode string.)
   enum class ContentType : ModeFlags {
     BINARY = 0x10,
   };
 
private:
   size_t read_limit;
 
   bool eof;
   bool err;
 
 protected:
   bool write_allowed() const {
     return mode & (static_cast<ModeFlags>(OpenMode::WRITE) |
-                   static_cast<ModeFlags>(OpenMode::APPEND));
+                   static_cast<ModeFlags>(OpenMode::APPEND) |
+                   static_cast<ModeFlags>(OpenMode::PLUS));
   }
 
   bool read_allowed() const {
-    return mode & static_cast<ModeFlags>(OpenMode::READ);
+    return mode & (static_cast<ModeFlags>(OpenMode::READ) |
+                   static_cast<ModeFlags>(OpenMode::PLUS));
   }
 
 public:
   // We want this constructor to be constexpr so that global file objects
   // like stdout do not require invocation of the constructor which can
   // potentially lead to static initialization order fiasco.
   constexpr File(WriteFunc *wf, ReadFunc *rf, SeekFunc *sf, CloseFunc *cf,
                  FlushFunc *ff, void *buffer, size_t buffer_size,
public:
   explicit FileLock(File *f) : file(f) { file->lock(); }
 
   ~FileLock() { file->unlock(); }
 
   FileLock(const FileLock &) = delete;
   FileLock(FileLock &&) = delete;
 };
 
+// The implementaiton of this function is provided by the platfrom_file
+// library.
+File *openfile(const char *path, const char *mode);
+
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_SUPPORT_OSUTIL_FILE_H

libc/src/__support/File/file.cpp

case 'r':
       flags |= static_cast<ModeFlags>(OpenMode::READ);
       ++main_mode_count;
       break;
     case 'w':
       flags |= static_cast<ModeFlags>(OpenMode::WRITE);
       ++main_mode_count;
       break;
     case '+':
-      flags |= (static_cast<ModeFlags>(OpenMode::WRITE) |
-                static_cast<ModeFlags>(OpenMode::READ));
+      flags |= static_cast<ModeFlags>(OpenMode::PLUS);
       break;
     case 'b':
       flags |= static_cast<ModeFlags>(ContentType::BINARY);
       break;
     case 'a':
       flags |= static_cast<ModeFlags>(OpenMode::APPEND);
       ++main_mode_count;
       break;