rust/chg/src/sendfds.c

rust: translation of missingancestors

This is as direct as possible a translation of the ancestor.missingancestors
Python class in pure Rust. The goal for this changeset is to make it easy
to compare with the Python version.

We also add to Python tests the cases that helped us develop and debug
this implementation.

Some possible optimizations are marked along the way as TODO comments

Differential Revision: https://phab.mercurial-scm.org/D5416

/*
 * Utility to send fds via Unix domain socket
 *
 * Copyright 2011, 2018 Yuya Nishihara <[e-mail]>
 *
 * This software may be used and distributed according to the terms of the
 * GNU General Public License version 2 or any later version.
 */

#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>

#define MAX_FD_LEN 10

/*
 * Sends the given fds with 1-byte dummy payload.
 *
 * Returns the number of bytes sent on success, -1 on error and errno is set
 * appropriately.
 */
ssize_t sendfds(int sockfd, const int *fds, size_t fdlen)
{
	char dummy[1] = {0};
	struct iovec iov = {dummy, sizeof(dummy)};
	char fdbuf[CMSG_SPACE(sizeof(fds[0]) * MAX_FD_LEN)];
	struct msghdr msgh;
	struct cmsghdr *cmsg;

	/* just use a fixed-size buffer since we'll never send tons of fds */
	if (fdlen > MAX_FD_LEN) {
		errno = EINVAL;
		return -1;
	}

	memset(&msgh, 0, sizeof(msgh));
	msgh.msg_iov = &iov;
	msgh.msg_iovlen = 1;
	msgh.msg_control = fdbuf;
	msgh.msg_controllen = CMSG_SPACE(sizeof(fds[0]) * fdlen);

	cmsg = CMSG_FIRSTHDR(&msgh);
	cmsg->cmsg_level = SOL_SOCKET;
	cmsg->cmsg_type = SCM_RIGHTS;
	cmsg->cmsg_len = CMSG_LEN(sizeof(fds[0]) * fdlen);
	memcpy(CMSG_DATA(cmsg), fds, sizeof(fds[0]) * fdlen);
	msgh.msg_controllen = cmsg->cmsg_len;
	return sendmsg(sockfd, &msgh, 0);
}