Re: Bug in [[ "" == "" ]] expressions?

[Peter Stephenson <pws@csr.com>]

Felix Rosencrantz wrote:
> It seems like zsh on 10-8 works like I'm use to, but a change since
> then has changed the behavior.    Not sure if Peter intended this.
> 
> Comparing an empty or unset string against an empty pattern returns
> false.  Here's the code:
> zsh -f
> % a=""
> % b="abc"
> % [[ "$a" == "" ]] ; echo $status
> 1
> % [[ "$b" == "abc" ]] ; echo $status
> 0
> % [[ "$c" == "" ]] ; echo $status
> 1

This is because of a feature I've never understood whereby zero-length
metafied strings are turned into a Nularg token plus the usual NULL.  I
didn't handle this properly.

This also makes sure that a zero-length string in a pattern is optimised
to be recognised as a pure string, for which memcmp rather than the full
pattern matcher can be used.  (Hmm, I hope memcmp'ing zero bytes is
portable.)

Index: Src/pattern.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/pattern.c,v
retrieving revision 1.22
diff -u -r1.22 pattern.c
--- Src/pattern.c	18 Oct 2004 11:56:17 -0000	1.22
+++ Src/pattern.c	22 Oct 2004 15:28:36 -0000
@@ -399,7 +399,7 @@
 	patendstr++;
 	patendseglen--;
 	patendstrlen--;
-	remnulargs(exp);
+	remnulargs(patparse);
 	patglobflags = 0;
     }
     /*
@@ -419,10 +419,20 @@
 	    || (!(patglobflags & ~GF_IGNCASE) && (patflags & PAT_FILE))
 #endif
 	    )
+	{
+	    /*
+	     * Waah!  I wish I understood this.
+	     * Empty metafied strings have an initial Nularg.
+	     * This never corresponds to a real character in
+	     * a glob pattern or string, so skip it.
+	     */
+	    if (*exp == Nularg)
+		exp++;
 	    for (strp = exp; *strp &&
 		     (!(patflags & PAT_FILE) || *strp != '/') && !itok(*strp);
 		 strp++)
 		;
+	}
 	if (!strp || (*strp && *strp != '/')) {
 	    /* No, do normal compilation. */
 	    strp = NULL;
@@ -1010,6 +1020,9 @@
 
 	/* Get length of string without metafication. */
 	nmeta = 0;
+	/* inherited from domatch, but why, exactly? */
+	if (*str0 == Nularg)
+	    str0++;
 	for (ptr = str0; ptr < patparse; ptr++) {
 	    if (*ptr == Meta) {
 		nmeta++;
Index: Test/D02glob.ztst
===================================================================
RCS file: /cvsroot/zsh/zsh/Test/D02glob.ztst,v
retrieving revision 1.6
diff -u -r1.6 D02glob.ztst
--- Test/D02glob.ztst	18 Oct 2004 11:56:18 -0000	1.6
+++ Test/D02glob.ztst	22 Oct 2004 15:28:36 -0000
@@ -308,3 +308,7 @@
  print glob.tmp/**/*~*/dir3(/*|(#e))(/)
 0:Exclusions with complicated path specifications
 >glob.tmp/dir1 glob.tmp/dir2 glob.tmp/dir4
+
+ [[ "" = "" ]] && echo OK
+0:Empty strings
+>OK

-- 
Peter Stephenson <pws@csr.com>                  Software Engineer
CSR Ltd., Science Park, Milton Road,
Cambridge, CB4 0WH, UK                          Tel: +44 (0)1223 692070


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