Re: bug: $? after empty command

[Peter Stephenson <pws@xxxxxxx>]

On Thu, 9 Jul 2009 13:12:20 +0000 (UTC)
Eric Blake <ebb9@xxxxxxx> wrote:
> POSIX requires that the execution of an empty command change $? to 0, as shown
> here with bash:
> 
> $ zsh -c 'foo=; false; $foo; echo $?'
> 1

I'm not aware that that's deliberate shell behaviour, and indeed I
wouldn't have expected it.  It *is* deliberate behaviour that the status is
not reset simply by hitting return at the interactive prompt, but that's
actually a different issue, not affected by the following.

The fix for this is quite subtle because of the case

  zsh -c 'false; $(exit 3); echo $?'

which should print 3, but I think I've finally found a way that handles
both.

Index: Src/exec.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/exec.c,v
retrieving revision 1.165
diff -u -r1.165 exec.c
--- Src/exec.c	15 Mar 2009 01:04:50 -0000	1.165
+++ Src/exec.c	9 Jul 2009 14:38:41 -0000
@@ -151,6 +151,15 @@
 /**/
 int cmdoutval;
 
+/*
+ * This is set by an exiting $(...) substitution to indicate we need
+ * to retain the status.  We initialize it to zero if we think we need
+ * to reset the status for a command.
+ */
+
+/**/
+int use_cmdoutval;
+
 /* The context in which a shell function is called, see SFC_* in zsh.h. */ 
 
 /**/
@@ -2262,6 +2271,14 @@
      */
     if (!args && varspc)
 	lastval = errflag ? errflag : cmdoutval;
+    /*
+     * If there are arguments, we should reset the status for the
+     * command before execution---unless we are using the result of a
+     * command substitution, which will be indicated by setting
+     * use_cmdoutval to 1.  We haven't kicked those off yet, so
+     * there's no race.
+     */
+    use_cmdoutval = !args;
 
     for (i = 0; i < 10; i++) {
 	save[i] = -2;
@@ -2478,7 +2495,12 @@
 		    lastval = 0;
 		    return;
 		} else {
-		    cmdoutval = lastval;
+		    /*
+		     * No arguments.  Reset the status if there were
+		     * arguments before and no command substitution
+		     * has provided a status.
+		     */
+		    cmdoutval = use_cmdoutval ? lastval : 0;
 		    if (varspc)
 			addvars(state, varspc, 0);
 		    if (errflag)
@@ -4674,6 +4696,7 @@
     es->badcshglob = badcshglob;
     es->cmdoutpid = cmdoutpid;
     es->cmdoutval = cmdoutval;
+    es->use_cmdoutval = use_cmdoutval;
     es->trap_return = trap_return;
     es->trap_state = trap_state;
     es->trapisfunc = trapisfunc;
@@ -4704,6 +4727,7 @@
     badcshglob = exstack->badcshglob;
     cmdoutpid = exstack->cmdoutpid;
     cmdoutval = exstack->cmdoutval;
+    use_cmdoutval = exstack->use_cmdoutval;
     trap_return = exstack->trap_return;
     trap_state = exstack->trap_state;
     trapisfunc = exstack->trapisfunc;
Index: Src/signals.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/signals.c,v
retrieving revision 1.54
diff -u -r1.54 signals.c
--- Src/signals.c	11 Feb 2009 20:42:16 -0000	1.54
+++ Src/signals.c	9 Jul 2009 14:38:41 -0000
@@ -494,6 +494,7 @@
 			*procsubval = (0200 | WTERMSIG(status));
 		    else
 			*procsubval = WEXITSTATUS(status);
+		    use_cmdoutval = 1;
 		    get_usage();
 		    cont = 1;
 		    break;
Index: Src/zsh.h
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/zsh.h,v
retrieving revision 1.158
diff -u -r1.158 zsh.h
--- Src/zsh.h	2 Jul 2009 13:48:36 -0000	1.158
+++ Src/zsh.h	9 Jul 2009 14:38:41 -0000
@@ -930,6 +930,7 @@
     int badcshglob;
     pid_t cmdoutpid;
     int cmdoutval;
+    int use_cmdoutval;
     int trap_return;
     int trap_state;
     int trapisfunc;
Index: Test/A01grammar.ztst
===================================================================
RCS file: /cvsroot/zsh/zsh/Test/A01grammar.ztst,v
retrieving revision 1.22
diff -u -r1.22 A01grammar.ztst
--- Test/A01grammar.ztst	6 Jul 2009 20:44:29 -0000	1.22
+++ Test/A01grammar.ztst	9 Jul 2009 14:38:41 -0000
@@ -23,6 +23,10 @@
   true | false
 1:Exit status of pipeline with builtins (false)
 
+  false
+  $nonexistent_variable
+0:Executing command that evaluates to empty resets status
+
   fn() { local foo; read foo; print $foo; }
   coproc fn
   print -p coproc test output


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