PATCH: zpty polling

[Peter Stephenson <pws@xxxxxxx>]

"zpty -rt" is supposed to return if there's nothing to do, but it's half
implemented: it tests before the *first* read, but it can block if it's
read a character, needs another one, and fails to read it.  This happens
in particular if there was no newline, since unless it's matching a
pattern it will wait for one.  (I'm assuming throughout at that reads
are in blocking mode (the default), otherwise this behaviour is
irrelevant.)

I would guess nobody wants the current "poll but get stuck anyway"
behaviour, so this makes it always poll before reading.  I've borrowed
the code from inside poll_read() to make it a bit more efficient;
actually, it ought to be good enough just to do the non-blocking read
bit without the select or FIONREAD before.  I didn't dare do this when
the current system seems to be working.

-t will now interact differently with a pattern argument: it will no
longer block if there was something available but not what you wanted,
and then no more was available.  This is already the behaviour when the
command exits.  I don't think the previous behaviour was satisfactory
since there's a race: if no input at all was there, it would return 1
immediately, otherwise it would consume that input and block waiting for
the pattern.

I discovered this when thinking about writing a zsh front-end to
smbclient, which is a bit clumsy.  Unfortunately a suitably recent
version of smbclient seems to be the only way to get at DFS filesystems
at the moment; mounting a CIFS share where the files are actually
provided by DFS doesn't yet work.  If anyone happens to know of a better
way of doing this I'm all ears, otherwise if I come up with something
I'll post it.  (Possibly I should be using non-blocking mode for this.)

Index: Doc/Zsh/mod_zpty.yo
===================================================================
RCS file: /cvsroot/zsh/zsh/Doc/Zsh/mod_zpty.yo,v
retrieving revision 1.6
diff -u -r1.6 mod_zpty.yo
--- Doc/Zsh/mod_zpty.yo	1 Apr 2005 12:04:22 -0000	1.6
+++ Doc/Zsh/mod_zpty.yo	24 Jan 2008 18:31:49 -0000
@@ -64,7 +64,10 @@
 
 If the tt(-r) option is combined with the tt(-t) option, tt(zpty) tests
 whether output is available before trying to read.  If no output is
-available, tt(zpty) immediately returns the status tt(1).
+available, tt(zpty) immediately returns the status tt(1).  When used
+with a var(pattern), the behaviour on a failed poll is similar to
+when the command has exited:  the return value is zero if at least
+one character could still be read even if the pattern failed to match.
 )
 item(tt(zpty) tt(-t) var(name))(
 The tt(-t) option without the tt(-r) option can be used to test
Index: Src/utils.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/utils.c,v
retrieving revision 1.175
diff -u -r1.175 utils.c
--- Src/utils.c	17 Dec 2007 18:34:25 -0000	1.175
+++ Src/utils.c	24 Jan 2008 18:31:53 -0000
@@ -1886,7 +1886,7 @@
 }
 
 /**/
-int
+mod_export int
 setblock_fd(int turnonblocking, int fd, long *modep)
 {
 #ifdef O_NDELAY
Index: Src/Modules/zpty.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/Modules/zpty.c,v
retrieving revision 1.36
diff -u -r1.36 zpty.c
--- Src/Modules/zpty.c	6 Jul 2007 21:52:40 -0000	1.36
+++ Src/Modules/zpty.c	24 Jan 2008 18:31:53 -0000
@@ -468,7 +468,7 @@
 }
 
 static int
-ptyread(char *nam, Ptycmd cmd, char **args)
+ptyread(char *nam, Ptycmd cmd, char **args, int noblock)
 {
     int blen, used, seen = 0, ret = 0;
     char *buf;
@@ -509,6 +509,45 @@
 	cmd->read = -1;
     }
     do {
+	if (noblock && cmd->read == -1) {
+	    int pollret;
+	    /*
+	     * Check there is data available.  Borrowed from
+	     * poll_read() in utils.c and simplified.
+	     */
+#ifdef HAVE_SELECT
+	    fd_set foofd;
+	    struct timeval expire_tv;
+	    expire_tv.tv_sec = 0;
+	    expire_tv.tv_usec = 0;
+	    FD_ZERO(&foofd);
+	    FD_SET(cmd->fd, &foofd);
+	    pollret = select(cmd->fd+1,
+			 (SELECT_ARG_2_T) &foofd, NULL, NULL, &expire_tv);
+#else
+#ifdef FIONREAD
+	    if (ioctl(cmd->fd, FIONREAD, (char *) &val) == 0)
+		pollret = (val > 0);
+#endif
+#endif
+
+	    if (pollret < 0) {
+		/*
+		 * See read_poll() for this.
+		 * Last despairing effort to poll: attempt to
+		 * set nonblocking I/O and actually read the
+		 * character.  cmd->read stores the character read.
+		 */
+		long mode;
+
+		if (setblock_fd(0, cmd->fd, &mode))
+		    pollret = read(cmd->fd, &cmd->read, 1);
+		if (mode != -1)
+		    fcntl(cmd->fd, F_SETFL, mode);
+	    }
+	    if (pollret == 0)
+		break;
+	}
 	if (!ret) {
 	    checkptycmd(cmd);
 	    if (cmd->fin)
@@ -648,12 +687,9 @@
 	}
 	if (p->fin)
 	    return 2;
-	if (OPT_ISSET(ops,'t') && p->read == -1 &&
-	    !read_poll(p->fd, &p->read, 0, 0))
-	    return 1;
 
 	return (OPT_ISSET(ops,'r') ?
-		ptyread(nam, p, args + 1) :
+		ptyread(nam, p, args + 1, OPT_ISSET(ops,'t')) :
 		ptywrite(p, args + 1, OPT_ISSET(ops,'n')));
     } else if (OPT_ISSET(ops,'d')) {
 	Ptycmd p;


-- 
Peter Stephenson <pws@xxxxxxx>                  Software Engineer
CSR PLC, Churchill House, Cambridge Business Park, Cowley Road
Cambridge, CB4 0WZ, UK                          Tel: +44 (0)1223 692070