src/vs/platform/files/node/diskFileSystemProvider.ts/DiskFileSystemProvider/updatePos
class DiskFileSystemProvider:	private updatePos(fd: number, pos: number | null, bytesLength: number | null): void {
		const lastKnownPos = this.mapHandleToPos.get(fd);
		if (typeof lastKnownPos === 'number') {

			// pos !== null signals that previously a position was used that is
			// not null. node.js documentation explains, that in this case
			// the internal file pointer is not moving and as such we do not move
			// our position pointer.
			//
			// Docs: "If position is null, data will be read from the current file position,
			// and the file position will be updated. If position is an integer, the file position
			// will remain unchanged."
			if (typeof pos === 'number') {
				// do not modify the position
			}

			// bytesLength = number is a signal that the read/write operation was
			// successful and as such we need to advance the position in the Map
			//
			// Docs (http://man7.org/linux/man-pages/man2/read.2.html):
			// "On files that support seeking, the read operation commences at the
			// file offset, and the file offset is incremented by the number of
			// bytes read."
			//
			// Docs (http://man7.org/linux/man-pages/man2/write.2.html):
			// "For a seekable file (i.e., one to which lseek(2) may be applied, for
			// example, a regular file) writing takes place at the file offset, and
			// the file offset is incremented by the number of bytes actually
			// written."
			else if (typeof bytesLength === 'number') {
				this.mapHandleToPos.set(fd, lastKnownPos + bytesLength);
			}

			// bytesLength = null signals an error in the read/write operation
			// and as such we drop the handle from the Map because the position
			// is unspecificed at this point.
			else {
				this.mapHandleToPos.delete(fd);
			}
		}
	}