Function BufFileLoadBuffer

src/backend/storage/file/buffile.c:598–635

* BufFileLoadBuffer * * Load some data into buffer, if possible, starting from curOffset. * At call, must have dirty = false, pos and nbytes = 0. * On exit, nbytes is number of bytes loaded. */

 * On exit, nbytes is number of bytes loaded.
 */
static void
BufFileLoadBuffer(BufFile *file)
{
	File		thisfile;

	/*
	 * Advance to next component file if necessary and possible.
	 */
	if (file->curOffset >= MAX_PHYSICAL_FILESIZE &&
		file->curFile + 1 < file->numFiles)
	{
		file->curFile++;
		file->curOffset = 0L;
	}

	/*
	 * Read whatever we can get, up to a full bufferload.
	 */
	thisfile = file->files[file->curFile];
	file->nbytes = FileRead(thisfile,
							file->buffer.data,
							BLCKSZ,
							file->curOffset,
							WAIT_EVENT_BUFFILE_READ);
	if (file->nbytes < 0)
	{
		file->nbytes = 0;
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not read file \"%s\": %m",
						FilePathName(thisfile))));
	}

	/* we choose not to advance curOffset here */

	if (file->nbytes > 0)
		pgBufferUsage.temp_blks_read++;
}

/*
 * BufFileDumpBuffer