PATCH: skip command from debug trap

[Peter Stephenson <pws@csr.com>]

I believe this is the fairly simple code needed to skip a command from a
debug trap.

The only difficult bit is the way of triggering it, since it's long
established that any non-zero number returned from a trap causes the
enclosing function to return.  I was inclined to pick a negative number,
but Rocky wants that for something else.  So how about 255, which is
fairly special?

If we're going to do this kind of stuff there seems little point in
trying to retain the old default behaviour of debug traps, where it
won't work.

Index: README
===================================================================
RCS file: /cvsroot/zsh/zsh/README,v
retrieving revision 1.53
diff -u -r1.53 README
--- README	1 Jun 2008 18:35:50 -0000	1.53
+++ README	5 Aug 2008 14:17:32 -0000
@@ -56,6 +56,12 @@
 applies to expressions with forced splitting such as ${=1+"$@"}, but
 otherwise the case where SH_WORD_SPLIT is not set is unaffected.
 
+Debug traps (`trap ... DEBUG' or the function TRAPDEBUG) now run by default
+before the command to which they refer instead of after.  This is almost
+always the right behaviour for the intended purpose of debugging and is
+consistent with recent versions of other shells.  The option
+DEBUG_BEFORE_CMD can be unset to revert to the previous behaviour.
+
 In previous versions of the shell it was possible to use index 0 in an
 array or string subscript to refer to the same element as index 1 if the
 option KSH_ARRAYS was not in effect.  This was a limited approximation to
Index: Doc/Zsh/builtins.yo
===================================================================
RCS file: /cvsroot/zsh/zsh/Doc/Zsh/builtins.yo,v
retrieving revision 1.108
diff -u -r1.108 builtins.yo
--- Doc/Zsh/builtins.yo	10 Jun 2008 08:50:36 -0000	1.108
+++ Doc/Zsh/builtins.yo	5 Aug 2008 14:17:33 -0000
@@ -1301,8 +1301,13 @@
 after each command with a nonzero exit status.  tt(ERR) is an alias
 for tt(ZERR) on systems that have no tt(SIGERR) signal (this is the
 usual case).
+
 If var(sig) is tt(DEBUG) then var(arg) will be executed
-after each command.
+before each command if the option tt(DEBUG_BEFORE_CMD) is set
+(as it is by default), else after each command.  In the former case,
+executing a tt(return) with the value of 255 within the trap causes
+the command referred to to be skipped.
+
 If var(sig) is tt(0) or tt(EXIT)
 and the tt(trap) statement is executed inside the body of a function,
 then the command var(arg) is executed after the function completes.
Index: Doc/Zsh/func.yo
===================================================================
RCS file: /cvsroot/zsh/zsh/Doc/Zsh/func.yo,v
retrieving revision 1.20
diff -u -r1.20 func.yo
--- Doc/Zsh/func.yo	30 Jul 2008 19:46:20 -0000	1.20
+++ Doc/Zsh/func.yo	5 Aug 2008 14:17:33 -0000
@@ -308,8 +308,10 @@
 )
 findex(TRAPDEBUG)
 item(tt(TRAPDEBUG))(
-Executed after each command.  If the option tt(DEBUG_BEFORE_CMD)
-is set, executed before each command instead.
+If the option tt(DEBUG_BEFORE_CMD) is set (as it is by default), executed
+before each command; otherwise executed after each command.  In the
+former case, returning the value 255 from the trap function causes
+execution of the command to be skipped.
 )
 findex(TRAPEXIT)
 item(tt(TRAPEXIT))(
Index: Doc/Zsh/options.yo
===================================================================
RCS file: /cvsroot/zsh/zsh/Doc/Zsh/options.yo,v
retrieving revision 1.61
diff -u -r1.61 options.yo
--- Doc/Zsh/options.yo	12 Jun 2008 13:45:05 -0000	1.61
+++ Doc/Zsh/options.yo	5 Aug 2008 14:17:33 -0000
@@ -1046,7 +1046,7 @@
 ifzman(the section ARITHMETIC EVALUATION in zmanref(zshmisc))
 has an explicit list.
 )
-pindex(DEBUG_BEFORE_CMD)
+pindex(DEBUG_BEFORE_CMD <D>)
 cindex(traps, DEBUG, before or after command)
 cindex(DEBUG trap, before or after command)
 item(tt(DEBUG_BEFORE_CMD))(
Index: Src/exec.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/exec.c,v
retrieving revision 1.136
diff -u -r1.136 exec.c
--- Src/exec.c	1 Aug 2008 13:53:44 -0000	1.136
+++ Src/exec.c	5 Aug 2008 14:17:33 -0000
@@ -1061,6 +1061,9 @@
 	}
 
 	if (sigtrapped[SIGDEBUG] && isset(DEBUGBEFORECMD)) {
+	    int otrapskip = trapskip;
+	    trapskip = 0;
+
 	    exiting = donetrap;
 	    ret = lastval;
 	    dotrap(SIGDEBUG);
@@ -1071,7 +1074,8 @@
 	     * Only execute the trap once per sublist, even
 	     * if the DEBUGBEFORECMD option changes.
 	     */
-	    donedebug = 1;
+	    donedebug = trapskip ? 2 : 1;
+	    trapskip = otrapskip;
 	} else
 	    donedebug = 0;
 
@@ -1087,6 +1091,16 @@
 
 	/* Loop through code followed by &&, ||, or end of sublist. */
 	code = *state->pc++;
+	if (donedebug == 2) {
+	    /* Skip sublist. */
+	    while (wc_code(code) == WC_SUBLIST) {
+		state->pc = state->pc + WC_SUBLIST_SKIP(code);
+		code = *state->pc++;
+	    }
+	    donetrap = 1;
+	    /* yucky but consistent... */
+	    goto sublist_done;
+	}
 	while (wc_code(code) == WC_SUBLIST) {
 	    next = state->pc + WC_SUBLIST_SKIP(code);
 	    if (!oldnoerrexit)
Index: Src/options.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/options.c,v
retrieving revision 1.43
diff -u -r1.43 options.c
--- Src/options.c	31 Jul 2008 08:44:21 -0000	1.43
+++ Src/options.c	5 Aug 2008 14:17:33 -0000
@@ -112,7 +112,7 @@
 {{NULL, "cshjunkiequotes",    OPT_EMULATE|OPT_CSH},	 CSHJUNKIEQUOTES},
 {{NULL, "cshnullcmd",	      OPT_EMULATE|OPT_CSH},	 CSHNULLCMD},
 {{NULL, "cshnullglob",	      OPT_EMULATE|OPT_CSH},	 CSHNULLGLOB},
-{{NULL, "debugbeforecmd",     OPT_EMULATE},		 DEBUGBEFORECMD},
+{{NULL, "debugbeforecmd",     OPT_ALL},			 DEBUGBEFORECMD},
 {{NULL, "emacs",	      0},			 EMACSMODE},
 {{NULL, "equals",	      OPT_EMULATE|OPT_ZSH},	 EQUALS},
 {{NULL, "errexit",	      OPT_EMULATE},		 ERREXIT},
Index: Src/signals.c
===================================================================
RCS file: /cvsroot/zsh/zsh/Src/signals.c,v
retrieving revision 1.48
diff -u -r1.48 signals.c
--- Src/signals.c	1 Aug 2008 13:53:45 -0000	1.48
+++ Src/signals.c	5 Aug 2008 14:17:34 -0000
@@ -1076,6 +1076,11 @@
 /**/
 int trapisfunc;
 
+/* Signal to list code that we should skip the next statement. */
+
+/**/
+int trapskip;
+
 /**/
 void
 dotrapargs(int sig, int *sigtr, void *sigfn)
@@ -1188,7 +1193,9 @@
     execrestore();
     lexrestore();
 
-    if (trapret > 0) {
+    trapskip = (trapret == 255 && sig == SIGDEBUG);
+
+    if (trapret > 0 && !trapskip) {
 	if (isfunc) {
 	    breaks = loops;
 	    errflag = 1;



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