[OE-core] [pseudo] [RFC 1/2] pseudo_ipc.c: Use MSG_NOSIGNAL if available

[Rasmus Villemoes]

MSG_NOSIGNAL has been in Linux since 2.2, and has been standardized in
POSIX 2008. Using that when available avoids the overhead of the two
syscalls to set and restore the SIGPIPE handler. Moreover, we can
eliminate one write() call by making use of sendmsg() to do
scatter-gather I/O.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>
---
 pseudo_ipc.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 50 insertions(+), 13 deletions(-)

diff --git a/pseudo_ipc.c b/pseudo_ipc.c
index 82c6f70..7786880 100644
--- a/pseudo_ipc.c
+++ b/pseudo_ipc.c
@@ -26,13 +26,17 @@
 #include <ctype.h>
 #include <errno.h>
 #include <sys/stat.h>
+#include <sys/socket.h>
 
 #include "pseudo.h"
 #include "pseudo_ipc.h"
 
-/* Short reads or writes can cause a sigpipe, killing the program, so we
- * trap it and report that something happened.
+/* Short reads or writes can cause a sigpipe, killing the program, so
+ * we trap it and report that something happened. We can avoid the
+ * overhead of setting/restoring the SIGPIPE handler if MSG_NOSIGNAL
+ * is available.
  */
+#ifndef MSG_NOSIGNAL
 static sig_atomic_t pipe_error = 0;
 static void (*old_handler)(int) = SIG_DFL;
 
@@ -51,6 +55,9 @@ static void
 allow_sigpipe(void) {
 	signal(SIGPIPE, old_handler);
 }
+#else
+#define pipe_error 0
+#endif
 
 #if 0
 /* useful only when debugging crazy stuff */
@@ -66,6 +73,36 @@ display_msg_header(pseudo_msg_t *msg) {
 }
 #endif
 
+#ifdef MSG_NOSIGNAL
+static int
+do_send(int fd, struct iovec *iov, int iovlen)
+{
+	struct msghdr hdr;
+
+	memset(&hdr, 0, sizeof(hdr));
+	hdr.msg_iov = iov;
+	hdr.msg_iovlen = iovlen;
+
+	return sendmsg(fd, &hdr, MSG_NOSIGNAL);
+}
+#else
+static int
+do_send(int fd, struct iovec *iov, int iovlen)
+{
+	int r, s = 0, i;
+
+	ignore_sigpipe();
+	for (i = 0; i < iovlen; ++i) {
+		r = write(fd, iov[i].iov_base, iov[i].iov_len);
+		s += r;
+		if (r != (int)iov[i].iov_len)
+			break;
+	}
+	allow_sigpipe();
+	return s;
+}
+#endif
+
 /*
  * send message on fd
  * return:
@@ -75,6 +112,7 @@ display_msg_header(pseudo_msg_t *msg) {
  */
 int
 pseudo_msg_send(int fd, pseudo_msg_t *msg, size_t len, const char *path) {
+	struct iovec iov[2];
 	int r;
 
 	if (!msg)
@@ -89,14 +127,13 @@ pseudo_msg_send(int fd, pseudo_msg_t *msg, size_t len, const char *path) {
 		if (len == (size_t) -1)
 			len = strlen(path) + 1;
 		msg->pathlen = len;
-		ignore_sigpipe();
-		r = write(fd, msg, PSEUDO_HEADER_SIZE);
-		if (r == PSEUDO_HEADER_SIZE) {
-			r += write(fd, path, len);
-		}
-		allow_sigpipe();
+		iov[0].iov_base = msg;
+		iov[0].iov_len = PSEUDO_HEADER_SIZE;
+		iov[1].iov_base = (void*)path;
+		iov[1].iov_len = len;
+		r = do_send(fd, iov, 2);
 		pseudo_debug(PDBGF_IPC | PDBGF_VERBOSE, "wrote %d bytes\n", r);
-		if (pipe_error || (r == -1 && errno == EBADF))
+		if (pipe_error || (r == -1 && (errno == EBADF || errno == EPIPE)))
 			return -1;
 		return ((size_t) r != PSEUDO_HEADER_SIZE + len);
 	} else {
@@ -105,11 +142,11 @@ pseudo_msg_send(int fd, pseudo_msg_t *msg, size_t len, const char *path) {
 			msg->result, pseudo_res_name(msg->result),
 			msg->pathlen, msg->path, (int) msg->mode);
 		// display_msg_header(msg);
-		ignore_sigpipe();
-		r = write(fd, msg, PSEUDO_HEADER_SIZE + msg->pathlen);
-		allow_sigpipe();
+		iov[0].iov_base = msg;
+		iov[0].iov_len = PSEUDO_HEADER_SIZE + msg->pathlen;
+		r = do_send(fd, iov, 1);
 		pseudo_debug(PDBGF_IPC | PDBGF_VERBOSE, "wrote %d bytes\n", r);
-		if (pipe_error || (r == -1 && errno == EBADF))
+		if (pipe_error || (r == -1 && (errno == EBADF || errno == EPIPE)))
 			return -1;
 		return ((size_t) r != PSEUDO_HEADER_SIZE + msg->pathlen);
 	}
-- 
2.16.4