traceon(`traceon')

Back when I first started contributing to m4, prior to 1.4.4b, I broke a corner 
case of traceon while fixing a memory corruption bug [1].  The symptoms are 
that any attempt to turn on tracing of a macro currently being expanded will 
delete the current definition of that macro.  Or, for an example:

$ m4 <<\EOF
dumpdef(`traceon')dnl
traceon(`traceon')dnl
dumpdef(`traceon')dnl
traceon(`traceoff')
EOF
traceon:	<traceon>
m4:stdin:3: Warning: dumpdef: undefined macro `traceon'
traceon(traceoff)
$

[1] http://git.savannah.gnu.org/gitweb/?p=m4.git;a=commitdiff;h=f15fa#patch4

In the meantime, the workaround for 1.4.4b through 1.4.11 (ie. all m4 versions 
that I've ever released), is to use 'm4 -ttraceon' to trace traceon itself; and 
for all other macros, avoid using traceon inside argument collection of the 
macro that is intended to be traced.

I'll be hiding under a brown bag for a while.  And for the superstitious, the 
date at which I introduced the regression is rather ominous.  Fortunately, the 
master branch never suffered from this bug.


From c8a2c296df00c2e8b1b3b0372cea10d0d0a0b621 Mon Sep 17 00:00:00 2001
From: Eric Blake <[email protected]>
Date: Wed, 7 May 2008 10:55:33 -0600
Subject: [PATCH] Fix traceon regression introduced 2006-06-06.

* src/builtin.m4 (traceon): Only perform insertion if lookup
fails.
* doc/m4.texinfo (Trace): Test for the bug.
* NEWS: Document it.

Signed-off-by: Eric Blake <[email protected]>
---
 ChangeLog      |    8 ++++++++
 NEWS           |    4 ++++
 doc/m4.texinfo |   10 ++++++++++
 src/builtin.c  |    4 +++-
 4 files changed, 25 insertions(+), 1 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 91c1845..d5b23b6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2008-05-07  Eric Blake  <[email protected]>
+
+	Fix traceon regression introduced 2006-06-06.
+	* src/builtin.m4 (traceon): Only perform insertion if lookup
+	fails.
+	* doc/m4.texinfo (Trace): Test for the bug.
+	* NEWS: Document it.
+
 2008-05-05  Eric Blake  <[email protected]>
 
 	Stage 22: allow builtin token concatenation outside $@.
diff --git a/NEWS b/NEWS
index 052cbbc..9bde89c 100644
--- a/NEWS
+++ b/NEWS
@@ -5,6 +5,10 @@ Foundation, Inc.
 * Noteworthy changes in Version 1.6 (????-??-??) [stable]
   Released by ????, based on git version 1.4.10b.x-*
 
+** Fix regression introduced in 1.4.4b where using `traceon' could delete
+   a macro.  This was most noticeable with `traceon(`traceon')', but
+   would also happen in cases such as `foo(traceon(`foo'))'.
+
 ** Fix regression introduced in 1.4.10b (but not present in 1.4.11) where
    using `builtin' or `indir' to perform nested `shift' calls triggered an
    assertion failure.
diff --git a/doc/m4.texinfo b/doc/m4.texinfo
index fd04622..30db58a 100644
--- a/doc/m4.texinfo
+++ b/doc/m4.texinfo
@@ -3708,6 +3708,16 @@ does not transfer tracing status.
 
 @example
 $ @kbd{m4 -d}
+traceon(`traceon')
+@result{}
+traceon(`traceoff')
+@error{}m4trace: -1- traceon(`traceoff')
+@result{}
+traceoff(`traceoff')
+@error{}m4trace: -1- traceoff(`traceoff')
+@result{}
+traceoff(`traceon')
+@result{}
 traceon(`eval', `m4_divnum')
 @result{}
 define(`m4_eval', defn(`eval'))
diff --git a/src/builtin.c b/src/builtin.c
index 8ce6cf7..0549745 100644
--- a/src/builtin.c
+++ b/src/builtin.c
@@ -1672,7 +1672,9 @@ m4_traceon (struct obstack *obs, int argc, 
macro_arguments *argv)
 	    m4_warn (0, me, _("invalid macro name ignored"));
 	    continue;
 	  }
-	s = lookup_symbol (ARG (i), SYMBOL_INSERT);
+	s = lookup_symbol (ARG (i), SYMBOL_LOOKUP);
+	if (!s)
+	  s = lookup_symbol (ARG (i), SYMBOL_INSERT);
 	set_trace (s, obs);
       }
 }
-- 
1.5.5.1