Convert select() to poll() in sched.c

select() limits number of file descriptors that can be used by screen.
Migrate to poll() to avoid this limitation.

As can be seen in case of scheduler it requires quite some changes, care
must be taken to count poll() events properly.

Bug: 55697

Signed-off-by: Amadeusz Sławiński <amade@asmblr.net>

1 files changed, 67 insertions, 22 deletions

diff --git a/src/sched.c b/src/sched.c
index 9961a34..6ff92ae 100644
--- a/src/sched.c
+++ b/src/sched.c
@@ -30,6 +30,7 @@
 
 #include "sched.h"
 
+#include <poll.h>
 #include <stdint.h>
 #include <sys/types.h>
 #include <time.h>
@@ -41,11 +42,16 @@ static Event *evs;
 static Event *tevs;
 static Event *nextev;
 static int calctimeout;
+static struct pollfd *pfd;
+static int pfd_cnt;
+
 
 static Event *calctimo(void);
 
 void evenq(Event *ev)
 {
+	int i = 0;
+
 	Event *evp, **evpp;
 	if (ev->queued)
 		return;
@@ -54,12 +60,23 @@ void evenq(Event *ev)
 		calctimeout = 1;
 		evpp = &tevs;
 	}
+
 	for (; (evp = *evpp); evpp = &evp->next)
 		if (ev->priority > evp->priority)
 			break;
 	ev->next = evp;
 	*evpp = ev;
 	ev->queued = true;
+
+	/* check if we need more pollfd */
+	for (evp = evs; evp; evp = evp->next)
+		if (evp->type == EV_READ || evp->type == EV_WRITE)
+			i++;
+
+	if (i > pfd_cnt) {
+		pfd_cnt = i;
+		pfd = realloc(pfd, pfd_cnt * sizeof(struct pollfd));
+	}
 }
 
 void evdeq(Event *ev)
@@ -79,6 +96,11 @@ void evdeq(Event *ev)
 	ev->queued = false;
 	if (ev == nextev)
 		nextev = nextev->next;
+
+	/* mark fd to be skipped (see checks in sched()) */
+	for (int i = 0; i < pfd_cnt; i++)
+		if (pfd[i].fd == ev->fd)
+			pfd[i].fd = -pfd[i].fd;
 }
 
 static Event *calctimo(void)
@@ -103,10 +125,9 @@ static Event *calctimo(void)
 void sched(void)
 {
 	Event *ev;
-	fd_set r, w, *set;
 	Event *timeoutev = NULL;
 	struct timeval timeout;
-	int nsel;
+	int i, n;
 
 	for (;;) {
 		if (calctimeout)
@@ -126,42 +147,66 @@ void sched(void)
 			}
 		}
 
-		FD_ZERO(&r);
-		FD_ZERO(&w);
+		memset(pfd, 0, sizeof(struct pollfd) * pfd_cnt);
+		i = 0;
 		for (ev = evs; ev; ev = ev->next) {
-			if (ev->condpos && *ev->condpos <= (ev->condneg ? *ev->condneg : 0)) {
-				continue;
+			if (ev->condpos && *ev->condpos <= (ev->condneg ? *ev->condneg : 0))
+				goto skip;
+			if (ev->type == EV_READ) {
+				pfd[i].fd = ev->fd;
+				pfd[i].events = POLLIN;
+			} else if (ev->type == EV_WRITE) {
+				pfd[i].fd = ev->fd;
+				pfd[i].events = POLLOUT;
 			}
-			if (ev->type == EV_READ)
-				FD_SET(ev->fd, &r);
-			else if (ev->type == EV_WRITE)
-				FD_SET(ev->fd, &w);
+skip:
+			if (ev->type == EV_READ || ev->type == EV_WRITE)
+				i++;
 		}
 
-		nsel = select(FD_SETSIZE, &r, &w, (fd_set *) 0, timeoutev ? &timeout : NULL);
-		if (nsel < 0) {
+		n = poll(pfd, i, timeoutev ? (timeout.tv_sec * 1000 + timeout.tv_usec / 1000) : 0);
+		if (n < 0) {
 			if (errno != EINTR) {
-				Panic(errno, "select");
+				Panic(errno, "poll");
 			}
-			nsel = 0;
-		} else if (nsel == 0) {	/* timeout */
+			n = 0;
+		} else if (n == 0) {	/* timeout */
 			if (timeoutev) {
 				evdeq(timeoutev);
 				timeoutev->handler(timeoutev, timeoutev->data);
 			}
 		}
 
+		i = 0;
+
 		for (ev = evs; ev; ev = nextev) {
 			nextev = ev->next;
-			if (ev->type != EV_ALWAYS) {
-				set = ev->type == EV_READ ? &r : &w;
-				if (nsel == 0 || !FD_ISSET(ev->fd, set))
+			switch (ev->type) {
+			case EV_READ:
+			case EV_WRITE:
+				/* check if we parsed all events from poll()
+				 * if we did just continue, as we may still
+				 * need to run EV_ALWAYS event */
+				if (n == 0)
 					continue;
-				nsel--;
+				/* check if we have anything to do for EV_READ
+				 * or EV_WRITE, or if event is still queued,
+				 * if not skip to the next event */
+				if (!pfd[i].revents || pfd[i].fd < 0) {
+					i++;
+					continue;
+				}
+				/* this is one of events from poll(), decrease
+				 * counter */
+				n--;
+				/* advance pollfd pointer */
+				i++;
+				__attribute__ ((fallthrough));
+			default:
+				if (ev->condpos && *ev->condpos <= (ev->condneg ? *ev->condneg : 0))
+					continue;
+				ev->handler(ev, ev->data);
 			}
-			if (ev->condpos && *ev->condpos <= (ev->condneg ? *ev->condneg : 0))
-				continue;
-			ev->handler(ev, ev->data);
 		}
 	}
 }