Re: funny subshell effect

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

On Mar 10,  4:11pm, Mikael Magnusson wrote:
} Subject: Re: funny subshell effect
}
} On 10 March 2011 15:56, Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx> wrote:
} > On Mar 10, 12:18am, Mikael Magnusson wrote:
} > } Subject: funny subshell effect
} > }
} > } % repeat 3; do command echo .(e:REPLY=\$RANDOM:); done
} > } 5801
} > } 5801
} > } 5801
} >
} ... what confused me was that I wasn't starting
} any subshells. But it seems that globbing is performed after forking
} to run an external command, and of course zsh doesn't bother to fork
} if the command is builtin. Is there some reason I'm not thinking of
} why it's like that?

It bothered me that I couldn't come up with a reason for this.  The one
that comes to mind is that if a glob goes runaway (you use ***/ with
a path that contains a cyclic symlink, for example) with a builtin you
may not be able to interrupt it; whereas with an external command where
the glob is running post-fork, you are able to interrupt.  Or that may
be a former bug and now it would be OK ... I wasn't willing to try to
lock up my shell to find out.

In fact zsh globs the word in command position separately before the
fork, and then globs the rest of the list afterward.  This leads to
strange bits like this:

% touch noglob
% n*b echo x*
x*
%

It also leads to this:

% repeat 3 do command echo x*  
repeat> done
zsh: no match
zsh: no match
zsh: no match
% repeat 3 do builtin echo x*
done
zsh: no match
% 

With the fork, the globbing error applies only to the forked process,
before the exec has occurred.  I tried wholesale moving the globlist()
up to before any decision about forking is made (sample patch below),
and all the "make check" tests pass; the only obvious difference is
that the two examples shown above become identical -- both run the
loop exactly once.

Compare this to discussion "exec -a and parameter expansion" from the
zsh-workers archive back in January.  That sort of died out without a
resolution.  I suspect that the patch below has some similar obscure
problems.  For example certain files in /proc will point to different
places, or possibly not exist at all, if globbing precedes forking;
that could in turn affect redirection, etc.


--- 8< --- EXAMPLE ONLY - NOT FOR ACTUAL USE --- 8< ---
Index: Src/exec.c
===================================================================
RCS file: /extra/cvsroot/zsh/zsh-4.0/Src/exec.c,v
retrieving revision 1.40
diff -c -r1.40 exec.c
--- exec.c	21 Dec 2010 16:41:16 -0000	1.40
+++ exec.c	17 Mar 2011 03:10:42 -0000
@@ -2735,6 +2744,16 @@
 	}
     }
 
+    if ((esglob = !(cflags & BINF_NOGLOB)) && args && htok) {
+	LinkList oargs = args;
+	globlist(args, 0);
+	args = oargs;
+    }
+    if (errflag) {
+	lastval = 1;
+	goto err;
+    }
+
     /* This is nonzero if the command is a current shell procedure? */
     is_cursh = (is_builtin || is_shfunc || nullexec || type >= WC_CURSH);
 
@@ -2844,16 +2863,6 @@
 	is_exec = 1;
     }
 
-    if ((esglob = !(cflags & BINF_NOGLOB)) && args && htok) {
-	LinkList oargs = args;
-	globlist(args, 0);
-	args = oargs;
-    }
-    if (errflag) {
-	lastval = 1;
-	goto err;
-    }
-
     /* Make a copy of stderr for xtrace output before redirecting */
     fflush(xtrerr);
     if (isset(XTRACE) && xtrerr == stderr &&


--