script(1): work around slow reading child

If child is slow reading from its input, or even completely stops doing
the read, script(1) hangs in write(2) to the pts master waiting until
there is a space in the terminal discipline buffer.  This also stops
handling any outer io, as well as child output.

Work around the problem by making pts master fd non-blocking, and be
prepared for short writes to it.  The data to be written to master is
buffered in the tailq which is processed when select(2) detects that
master is ready for write.

PR:	260938
Reported by:	наб <nabijaczleweli@nabijaczleweli.xyz>
See also:	https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1003095
Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D33789

1 files changed, 51 insertions, 5 deletions

diff --git a/usr.bin/script/script.c b/usr.bin/script/script.c
index 4ecc2099926f..9c18dc73390f 100644
--- a/usr.bin/script/script.c
+++ b/usr.bin/script/script.c
@@ -45,6 +45,7 @@ static const char sccsid[] = "@(#)script.c	8.1 (Berkeley) 6/6/93";
 #include <sys/stat.h>
 #include <sys/ioctl.h>
 #include <sys/time.h>
+#include <sys/queue.h>
 #include <sys/uio.h>
 #include <sys/endian.h>
 #include <dev/filemon/filemon.h>
@@ -70,6 +71,13 @@ struct stamp {
 	uint32_t scr_direction; /* 'i', 'o', etc (also indicates endianness) */
 };
 
+struct buf_elm {
+	TAILQ_ENTRY(buf_elm) link;
+	int rpos;
+	int len;
+	char ibuf[];
+};
+
 static FILE *fscript;
 static int master, slave;
 static int child;
@@ -77,6 +85,7 @@ static const char *fname;
 static char *fmfname;
 static int fflg, qflg, ttyflg;
 static int usesleep, rawout, showexit;
+static TAILQ_HEAD(, buf_elm) obuf_list = TAILQ_HEAD_INITIALIZER(obuf_list);
 
 static struct termios tt;
 
@@ -98,8 +107,9 @@ main(int argc, char *argv[])
 	time_t tvec, start;
 	char obuf[BUFSIZ];
 	char ibuf[BUFSIZ];
-	fd_set rfd;
-	int aflg, Fflg, kflg, pflg, ch, k, n;
+	fd_set rfd, wfd;
+	struct buf_elm *be;
+	int aflg, Fflg, kflg, pflg, ch, k, n, fcm;
 	int flushtime, readstdin;
 	int fm_fd, fm_log;
 
@@ -189,6 +199,12 @@ main(int argc, char *argv[])
 			err(1, "openpty");
 		ttyflg = 1;
 	}
+	fcm = fcntl(master, F_GETFL);
+	if (fcm == -1)
+		err(1, "master F_GETFL");
+	fcm |= O_NONBLOCK;
+	if (fcntl(master, F_SETFL, fcm) == -1)
+		err(1, "master F_SETFL");
 
 	if (rawout)
 		record(fscript, NULL, 0, 's');
@@ -243,9 +259,12 @@ main(int argc, char *argv[])
 	readstdin = 1;
 	for (;;) {
 		FD_ZERO(&rfd);
+		FD_ZERO(&wfd);
 		FD_SET(master, &rfd);
 		if (readstdin)
 			FD_SET(STDIN_FILENO, &rfd);
+		if (!TAILQ_EMPTY(&obuf_list))
+			FD_SET(master, &wfd);
 		if (!readstdin && ttyflg) {
 			tv.tv_sec = 1;
 			tv.tv_usec = 0;
@@ -258,7 +277,7 @@ main(int argc, char *argv[])
 		} else {
 			tvp = NULL;
 		}
-		n = select(master + 1, &rfd, 0, 0, tvp);
+		n = select(master + 1, &rfd, &wfd, NULL, tvp);
 		if (n < 0 && errno != EINTR)
 			break;
 		if (n > 0 && FD_ISSET(STDIN_FILENO, &rfd)) {
@@ -275,10 +294,37 @@ main(int argc, char *argv[])
 			if (cc > 0) {
 				if (rawout)
 					record(fscript, ibuf, cc, 'i');
-				(void)write(master, ibuf, cc);
+				be = malloc(sizeof(*be) + cc);
+				be->rpos = 0;
+				be->len = cc;
+				memcpy(be->ibuf, ibuf, cc);
+				TAILQ_INSERT_TAIL(&obuf_list, be, link);
+			}
+		}
+		if (n > 0 && FD_ISSET(master, &wfd)) {
+			while ((be = TAILQ_FIRST(&obuf_list)) != NULL) {
+				cc = write(master, be->ibuf + be->rpos,
+				    be->len);
+				if (cc == -1) {
+					if (errno == EWOULDBLOCK ||
+					    errno == EINTR)
+						break;
+					warn("write master");
+					done(1);
+				}
+				if (cc == 0)
+					break;		/* retry later ? */
 				if (kflg && tcgetattr(master, &stt) >= 0 &&
 				    ((stt.c_lflag & ECHO) == 0)) {
-					(void)fwrite(ibuf, 1, cc, fscript);
+					(void)fwrite(be->ibuf + be->rpos,
+					    1, cc, fscript);
+				}
+				be->len -= cc;
+				if (be->len == 0) {
+					TAILQ_REMOVE(&obuf_list, be, link);
+					free(be);
+				} else {
+					be->rpos += cc;
 				}
 			}
 		}