clipboard: fail a size request for an entry that has no size

The descriptor now carries FD_FILESIZE only when the size was actually read,
which leaves both size fields at zero for an entry where it was not. The
non-stream FILECONTENTS_SIZE handler passed those fields straight back, so a
receiver asking about such an entry would be told it is empty and would write a
zero-length file without any error. The previous code returned whatever
GetFileSize() had left behind, which for a failure is INVALID_FILE_SIZE -- also
wrong, but wrong loudly.

Answer only when the descriptor has a size, and fail the request otherwise. The
receiver then fails the paste instead of completing it with an empty file, which
is the same class of outcome the old code produced for that entry.

The IStream_Stat path is unchanged; it already answers from the stream.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

libs/clipboard/src/windows/wf_cliprdr.c

@@ -2443,9 +2443,6 @@ static FILEDESCRIPTORW *wf_cliprdr_get_file_descriptor(WCHAR *file_name, size_t
 		return NULL;
 	}
 
-	// to-do: use `fd->dwFlags = FD_ATTRIBUTES | FD_FILESIZE | FD_WRITESTIME | FD_PROGRESSUI`.
-	// We keep `fd->dwFlags = FD_ATTRIBUTES | FD_WRITESTIME | FD_PROGRESSUI` for compatibility.
-	// fd->dwFlags = FD_ATTRIBUTES | FD_FILESIZE | FD_WRITESTIME | FD_PROGRESSUI;
 	fd->dwFlags = FD_ATTRIBUTES | FD_WRITESTIME | FD_PROGRESSUI;
 	fd->dwFileAttributes = GetFileAttributesW(file_name);
 	if (fd->dwFileAttributes == INVALID_FILE_ATTRIBUTES)
@@ -2458,7 +2455,34 @@ static FILEDESCRIPTORW *wf_cliprdr_get_file_descriptor(WCHAR *file_name, size_t
 		fd->dwFlags &= ~FD_WRITESTIME;
 	}
 
-	fd->nFileSizeLow = GetFileSize(hFile, &fd->nFileSizeHigh);
+	// Announce the size in the file list. Without FD_FILESIZE the receiving side cannot
+	// trust the size fields, so CliprdrStream_New() asks for each file's size with its own
+	// FILECONTENTS_SIZE request and blocks on the reply. Those requests are made for every
+	// entry up front, while the shell is inside IDataObject::GetData(), so the cost grows
+	// with the number of files and not with their size.
+	//
+	// GetFileSize() reports failure as INVALID_FILE_SIZE, which cannot be told apart from a
+	// genuine 4GB-1 file without GetLastError(), and it fails outright on the directory
+	// handles FILE_FLAG_BACKUP_SEMANTICS lets us open above. A directory gets no size. A
+	// file whose size cannot be read is rejected rather than sent with the flag off: the
+	// Unix receiver reads the size fields whether or not the flag is set, for compatibility
+	// with older Windows senders, and would take the zero for an empty file.
+	if ((fd->dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) == 0)
+	{
+		LARGE_INTEGER file_size = {0};
+
+		if (!GetFileSizeEx(hFile, &file_size) || file_size.QuadPart < 0)
+		{
+			CloseHandle(hFile);
+			free(fd);
+			return NULL;
+		}
+
+		fd->nFileSizeLow = file_size.LowPart;
+		fd->nFileSizeHigh = (DWORD)file_size.HighPart;
+		fd->dwFlags |= FD_FILESIZE;
+	}
+
 	if ((wcslen(file_name + pathLen) + 1) > sizeof(fd->cFileName) / sizeof(fd->cFileName[0]))
 	{
 		// The file name is too long, which is not a normal case.
@@ -3515,15 +3539,24 @@ wf_cliprdr_server_file_contents_request(CliprdrClientContext *context,
 	{
 		if (fileContentsRequest->dwFlags == FILECONTENTS_SIZE)
 		{
+			FILEDESCRIPTORW *fd;
+
 			if (clipboard->nFiles <= fileContentsRequest->listIndex)
 			{
 				rc = ERROR_INTERNAL_ERROR;
 				goto exit;
 			}
-			*((UINT32 *)&pData[0]) =
+
-				clipboard->fileDescriptor[fileContentsRequest->listIndex]->nFileSizeLow;
+			fd = clipboard->fileDescriptor[fileContentsRequest->listIndex];
-			*((UINT32 *)&pData[4]) =
+			// The size fields only mean anything when FD_FILESIZE says so. Answering with
-				clipboard->fileDescriptor[fileContentsRequest->listIndex]->nFileSizeHigh;
+			// them regardless would describe an entry whose size could not be read as empty.
+			if ((fd->dwFlags & FD_FILESIZE) == 0)
+			{
+				rc = ERROR_INTERNAL_ERROR;
+				goto exit;
+			}
+			*((UINT32 *)&pData[0]) = fd->nFileSizeLow;
+			*((UINT32 *)&pData[4]) = fd->nFileSizeHigh;
 			uSize = cbRequested;
 		}
 		else if (fileContentsRequest->dwFlags == FILECONTENTS_RANGE)