tools/libxs: Stop playing with SIGPIPE

It's very rude for a library to play with signals behind the back of the
application, no matter ones views on the default behaviour of SIGPIPE under
POSIX.  Even if the application doesn't care about the xenstored socket, it my
care about others.

This logic has existed since xenstore/xenstored was originally added in commit
29c9e570b1 ("Add xenstore daemon and library") in 2005.

It's also unnecessary.  Pass MSG_NOSIGNAL when talking to xenstored over a
pipe (to avoid sucumbing to SIGPIPE if xenstored has crashed), and forgo any
playing with the signal disposition.

This has a side benefit of saving 2 syscalls per xenstore request.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>

tools/libs/store/xs.c

@@ -604,7 +604,7 @@ static bool sendmsg_exact(int fd, struct iovec *iov, unsigned int nr)
 	};
 
 	while (hdr.msg_iovlen) {
-		ssize_t res = sendmsg(fd, &hdr, 0);
+		ssize_t res = sendmsg(fd, &hdr, MSG_NOSIGNAL);
 
 		if (res < 0 && errno == EINTR)
 			continue;
@@ -664,7 +664,6 @@ static void *xs_talkv(struct xs_handle *h,
 	void *ret = NULL;
 	int saved_errno;
 	unsigned int i, msg_len;
-	struct sigaction ignorepipe, oldact;
 
 	/* Element 0 must be xsd_sockmsg */
 	assert(num_vecs >= 1);
@@ -681,11 +680,6 @@ static void *xs_talkv(struct xs_handle *h,
 
 	msg->len = msg_len;
 
-	ignorepipe.sa_handler = SIG_IGN;
-	sigemptyset(&ignorepipe.sa_mask);
-	ignorepipe.sa_flags = 0;
-	sigaction(SIGPIPE, &ignorepipe, &oldact);
-
 	mutex_lock(&h->request_mutex);
 
 	if (!write_request(h, iovec, num_vecs))
@@ -697,7 +691,6 @@ static void *xs_talkv(struct xs_handle *h,
 
 	mutex_unlock(&h->request_mutex);
 
-	sigaction(SIGPIPE, &oldact, NULL);
 	if (reply_type == XS_ERROR) {
 		saved_errno = get_error(ret);
 		free(ret);
@@ -716,7 +709,6 @@ fail:
 	/* We're in a bad state, so close fd. */
 	saved_errno = errno;
 	mutex_unlock(&h->request_mutex);
-	sigaction(SIGPIPE, &oldact, NULL);
 close_fd:
 	close(h->fd);
 	h->fd = -1;