Re: suspend while loop.

Re: suspend while loop.

[Sven Wischnowsky]

Bart Schaefer wrote:

> On May 16, 12:02pm, Bart Schaefer wrote:
> } Subject: Re: suspend while loop.
> }
> } > Z(4):akr@serein% Src/zsh -f
> } > serein% cat |while read line; do echo $line; done
> } > <C-z>
> } > 
> } > `cat' is suspended.  But zsh wait something yet...
> } 
> } I think you can't break out of `read' with ^Z.  Try it without the cat|
> } and you'll find you can't suspend the loop.
> 
> Does anyone have any insights on this?  I was about to announce 3.0.8,
> but discovered that this loop is not suspendable there either.  I don't
> have much time for debugging it myself this week ...
> 
> If it doesn't appear to be easily fixed, I'm going to go ahead and put
> 3.0.8 out anyway.

I can't see how we could (really) make in suspendible. For that we
would need to be able to get out of the read, back to the place where
we can create the sub-shell for the loop. Then, on continuing, we
would need to be able to go into the same read and continue with
it. But the first thing can only be done by returing from the
read. Everyone can easily test this by adding child_unblock() before
the read() in zread() and child_unblock()s after it. This makes the
loop suspendible, but when it is continued, the loop exits immediately
because the read that was suspended returned non-zero.

Depending on personal taste it is either ugly that the cat gets
suspended or that we can't ^C out of the loop after the ^Z.

I've no idea how to solve this.

Bye
 Sven


--
Sven Wischnowsky                         wischnow@informatik.hu-berlin.de

PATCH (?): Re: suspend while loop.

[Bart Schaefer]

On May 18, 10:37am, Sven Wischnowsky wrote:
} Subject: Re: suspend while loop.
}
} Everyone can easily test this by adding child_unblock() before
} the read() in zread() and child_unblock()s after it. This makes the
                                s/un// ?
} loop suspendible, but when it is continued, the loop exits immediately
} because the read that was suspended returned non-zero.

That would be OK, but the overhead of blocking and unblocking signals
around every one-character read() makes me cringe.  

Furthermore, SIGCHLD will cause the read() to return -1 with EINTR,
which we can't have happening in normal operation.

} Depending on personal taste it is either ugly that the cat gets
} suspended or that we can't ^C out of the loop after the ^Z.

It's particularly ugly that you can't ^C out of the loop after the ^Z.
The shell should not hang uninterrupibly unless the user has really
wrapped a figurative rope around its figurative neck.

Does anyone see any problems with the following compromise patch?  This
makes `read' (the builtin, not read(2)) interruptible with ^C even when
it is at the right-hand-end of a pipeline.  For comparison, before and
after the patch try:

zsh% cat | read line ; echo $?
^C

In the before case, echo runs and prints 1.  In the after case, echo does
not run.  This is consistent with

zsh% read line ; echo $?
^C

which, in both before and after cases, does not run echo.

The thing that most worries me is that zread() can invoke zle, which can
invoke shell functions, which ....

Another question is, is there a more generic way to do this that covers all
builtin commands and not just `read'?  Or is `read' really the only one
that needs it?

Index: Src/builtin.c
===================================================================
@@ -3536,6 +3536,7 @@
     first = 1;
     bslash = 0;
     while (*args || (ops['A'] && !gotnl)) {
+	sigset_t s = child_unblock();
 	buf = bptr = (char *)zalloc(bsiz = 64);
 	/* get input, a character at a time */
 	while (!gotnl) {
@@ -3573,6 +3574,7 @@
 		bptr = buf + blen;
 	    }
 	}
+	signal_setmask(s);
 	if (c == '\n' || c == EOF)
 	    gotnl = 1;
 	*bptr = '\0';
@@ -3627,7 +3629,8 @@
     buf = bptr = (char *)zalloc(bsiz = 64);
     /* any remaining part of the line goes into one parameter */
     bslash = 0;
-    if (!gotnl)
+    if (!gotnl) {
+	sigset_t s = child_unblock();
 	for (;;) {
 	    c = zread(izle);
 	    /* \ at the end of a line introduces a continuation line, except in
@@ -3662,6 +3665,8 @@
 		bptr = buf + blen;
 	    }
 	}
+	signal_setmask(s);
+    }
     while (bptr > buf && iwsep(bptr[-1]))
 	bptr--;
     *bptr = '\0';
@@ -3718,8 +3723,8 @@
 	case 1:
 	    /* return the character read */
 	    return STOUC(cc);
-#if defined(EAGAIN) || defined(EWOULDBLOCK)
 	case -1:
+#if defined(EAGAIN) || defined(EWOULDBLOCK)
 	    if (!retry && readfd == 0 && (
 # ifdef EAGAIN
 		    errno == EAGAIN
@@ -3733,9 +3738,11 @@
 		) && setblock_stdin()) {
 		retry = 1;
 		continue;
-	    }
-	    break;
+	    } else
 #endif /* EAGAIN || EWOULDBLOCK */
+	    if (errno == EINTR && !(errflag || retflag || breaks || contflag))
+		continue;
+	    break;
 	}
 	return EOF;
     }

-- 
Bart Schaefer                                 Brass Lantern Enterprises
http://www.well.com/user/barts              http://www.brasslantern.com

Re: suspend while loop.

[Bart Schaefer]

On May 16, 12:02pm, Bart Schaefer wrote:
} Subject: Re: suspend while loop.
}
} > Z(4):akr@serein% Src/zsh -f
} > serein% cat |while read line; do echo $line; done
} > <C-z>
} > 
} > `cat' is suspended.  But zsh wait something yet...
} 
} I think you can't break out of `read' with ^Z.  Try it without the cat|
} and you'll find you can't suspend the loop.

Does anyone have any insights on this?  I was about to announce 3.0.8,
but discovered that this loop is not suspendable there either.  I don't
have much time for debugging it myself this week ...

If it doesn't appear to be easily fixed, I'm going to go ahead and put
3.0.8 out anyway.

-- 
Bart Schaefer                                 Brass Lantern Enterprises
http://www.well.com/user/barts              http://www.brasslantern.com

Re: suspend while loop.

[Bart Schaefer]

On May 17,  3:41am, Tanaka Akira wrote:
> Subject: suspend while loop.
> Z(4):akr@serein% Src/zsh -f
> serein% cat |while read line; do echo $line; done
> <C-z>
> 
> `cat' is suspended.  But zsh wait something yet...

I think you can't break out of `read' with ^Z.  Try it without the cat|
and you'll find you can't suspend the loop.

suspend while loop.

[Tanaka Akira]

Z(4):akr@serein% Src/zsh -f
serein% cat |while read line; do echo $line; done
<C-z>

This doesn't suspend the job.

Z(4):akr@serein% ps jax|sed -n -e 1p -e /2380/p
 PPID   PID  PGID   SID TTY      TPGID STAT   UID   TIME COMMAND
28769  2380  2380 28769 ttyp5     2381 S    23483   0:00 Src/zsh -f
 2380  2381  2381 28769 ttyp5     2381 T    23483   0:00 cat
28772  2393  2392 28772 ttyp9     2392 S    23483   0:00 sed -n -e 1p -e /2380/p

`cat' is suspended.  But zsh wait something yet...
-- 
Tanaka Akira