patch for "bizarre read bug" in klibc dash

patch for "bizarre read bug" in klibc dash

[Joshua Hudson]

I had forked ash awhile back, patched up a few things to behave the
way I wanted them.
While I was at it, I fixed the "echo X | read X ; echo $X" bug that
echoed a blank line.
I tried awhile ago to raise who was responsible for the code and got nowhere.

I suppose you think the above code is a bizarre corner case. It is
not. I use very
complex shell expressions (think loops and subshells) in pipes, with
read in the end to
get the variable I was looking for. The work-around is particularly ugly.

I was quite surprised that this bug never got squashed in dash. So, I
backported it.

Well, here's the patch.

--- usr/dash/eval.c.orig        2006-06-14 19:48:47.000000000 -0700
+++ usr/dash/eval.c     2006-06-14 20:07:58.000000000 -0700
@@ -539,7 +539,17 @@
                                sh_error("Pipe call failed");
                        }
                }
-               if (forkshell(jp, lp->n, n->npipe.backgnd) == 0) {
+               if (!lp->next) {
+                       extern void tempredir0(int prevfd);
+                       /* Fix for "bizarre read bug" */
+                       if (prevfd > 0) {
+                               tempredir0(prevfd);
+                               close(prevfd);
+                       }
+                       evaltree(lp->n, 0);
+                       if (prevfd > 0)
+                               popredir(0);
+               } else if (forkshell(jp, lp->n, n->npipe.backgnd) == 0) {
                        INTON;
                        if (pip[1] >= 0) {
                                close(pip[0]);
--- usr/dash/redir.c.orig       2006-06-14 19:59:21.000000000 -0700
+++ usr/dash/redir.c    2006-06-14 20:03:38.000000000 -0700
@@ -310,6 +310,21 @@
 }


+void tempredir0(int fd)
+{
+       struct redirtab *sv = redirlist;
+       int i;
+       sv = ckmalloc(sizeof (struct redirtab));
+       for (i = 0 ; i < 10 ; i++)
+               sv->renamed[i] = EMPTY;
+       sv->next = redirlist;
+       redirlist = sv;
+       INTOFF;
+       sv->renamed[0] = copyfd(0, 10);
+       close(0);
+       copyfd(fd, 0);
+       INTON;
+}

 /*
  * Undo the effects of the last redirection.

Re: patch for "bizarre read bug" in klibc dash

[Alexander E. Patrakov]

Joshua Hudson wrote:
> I had forked ash awhile back, patched up a few things to behave the
> way I wanted them.
> While I was at it, I fixed the "echo X | read X ; echo $X" bug that
> echoed a blank line.
> I tried awhile ago to raise who was responsible for the code and got 
> nowhere.

Have you tested your line with bash? Here it prints an empty line. You really 
should use "echo X | ( read X ; echo $X )".

So I don't think that ash is broken, and thus cannot acknowledge the patch.

-- 
Alexander E. Patrakov

Re: patch for "bizarre read bug" in klibc dash

[Alexander E. Patrakov]

Joshua Hudson wrote:
> Oh, and the original code that found this long ago:
> grep MemFree /proc/meminfo | read J MEMFREE K
> case $MEMFREE in
> ...
> (MEMFREE used several times below)
> 
> Had to convert to:
> 
> MEMFREE=`grep MemFree /proc/meminfo | (READ J M K ; echo $M)`
> case $MEMFREE in
> ...

Pure-shell implementation:

while read J M K ; do
     case "$J" in
     MemFree:)
         MEMFREE="$M"
     esac
done </proc/meminfo
case $MEMFREE in
...

-- 
Alexander E. Patrakov