[wpiutil] MemoryBuffer: Fix normal read and file type check (wpilibsu…

…ite#5875)

wpiutil/src/main/native/thirdparty/llvm/cpp/llvm/MemoryBuffer.cpp

@@ -230,30 +230,31 @@ static std::unique_ptr<WritableMemoryBuffer> GetMemoryBufferForStream(
     fs::file_t f, std::string_view bufferName, std::error_code& ec) {
   constexpr size_t ChunkSize = 4096 * 4;
   SmallVector<uint8_t, ChunkSize> buffer;
+  uint64_t size = 0;
 #ifdef _WIN32
   DWORD readBytes;
 #else
   ssize_t readBytes;
 #endif
   // Read into Buffer until we hit EOF.
   do {
-    size_t prevSize = buffer.size();
-    buffer.resize_for_overwrite(prevSize + ChunkSize);
+    buffer.resize_for_overwrite(size + ChunkSize);
 #ifdef _WIN32
-    if (!ReadFile(f, buffer.begin() + prevSize, ChunkSize, &readBytes,
-                  nullptr)) {
+    if (!ReadFile(f, buffer.begin() + size, ChunkSize, &readBytes, nullptr)) {
       ec = mapWindowsError(GetLastError());
       return nullptr;
     }
 #else
-    readBytes = sys::RetryAfterSignal(-1, ::read, f, buffer.begin() + prevSize,
+    readBytes = sys::RetryAfterSignal(-1, ::read, f, buffer.begin() + size,
                                       ChunkSize);
     if (readBytes == -1) {
       ec = std::error_code(errno, std::generic_category());
       return nullptr;
     }
 #endif
+    size += readBytes;
   } while (readBytes != 0);
+  buffer.truncate(size);
 
   return GetMemBufferCopyImpl(buffer, bufferName, ec);
 }
@@ -370,7 +371,7 @@ static std::unique_ptr<WriteThroughMemoryBuffer> GetReadWriteFile(
 
       // If this not a file or a block device (e.g. it's a named pipe
       // or character device), we can't mmap it, so error out.
-      if (status.st_mode != S_IFREG && status.st_mode != S_IFBLK) {
+      if (!S_ISREG(status.st_mode) && !S_ISBLK(status.st_mode)) {
         ec = make_error_code(errc::invalid_argument);
         return nullptr;
       }
@@ -433,7 +434,7 @@ static std::unique_ptr<MB> GetOpenFileImpl(fs::file_t f,
       // If this not a file or a block device (e.g. it's a named pipe
       // or character device), we can't trust the size. Create the memory
       // buffer by copying off the stream.
-      if (status.st_mode != S_IFREG && status.st_mode != S_IFBLK) {
+      if (!S_ISREG(status.st_mode) && !S_ISBLK(status.st_mode)) {
         return GetMemoryBufferForStream(f, filename, ec);
       }