When calling an empty macro, do not clobber existing arguments.

Fixing a bug found with the groffer(1) version 1.19 manual page
following a report from Jan Stary.

4 files changed, 37 insertions, 5 deletions

diff --git a/regress/usr.bin/mandoc/roff/de/Makefile b/regress/usr.bin/mandoc/roff/de/Makefile
index 6760dc6e21a..86ea8c8c56c 100644
--- a/regress/usr.bin/mandoc/roff/de/Makefile
+++ b/regress/usr.bin/mandoc/roff/de/Makefile
@@ -1,7 +1,7 @@
-# $OpenBSD: Makefile,v 1.12 2019/02/06 20:54:28 schwarze Exp $
+# $OpenBSD: Makefile,v 1.13 2019/04/21 23:45:50 schwarze Exp $
 
-REGRESS_TARGETS	 = append cond escname factorial indir infinite startde tab
-REGRESS_TARGETS	+= TH Dd
+REGRESS_TARGETS	 = append cond empty escname factorial
+REGRESS_TARGETS	+= indir infinite startde tab TH Dd
 LINT_TARGETS	 = escname indir infinite
 
 # groff-1.22.4 defect:
diff --git a/regress/usr.bin/mandoc/roff/de/empty.in b/regress/usr.bin/mandoc/roff/de/empty.in
new file mode 100644
index 00000000000..decfcb51077
--- /dev/null
+++ b/regress/usr.bin/mandoc/roff/de/empty.in
@@ -0,0 +1,18 @@
+.\" $OpenBSD: empty.in,v 1.1 2019/04/21 23:45:50 schwarze Exp $
+.Dd $Mdocdate: April 21 2019 $
+.Dt DE-EMPTY 1
+.Os
+.Sh NAME
+.Nm de-empty
+.Nd empty user-defined macro with arguments
+.Sh DESCRIPTION
+initial text
+.de empty
+..
+.de real
+arg=\\$1
+.empty wrong
+arg=\\$1
+..
+.real right
+final text
diff --git a/regress/usr.bin/mandoc/roff/de/empty.out_ascii b/regress/usr.bin/mandoc/roff/de/empty.out_ascii
new file mode 100644
index 00000000000..1883fd3e98e
--- /dev/null
+++ b/regress/usr.bin/mandoc/roff/de/empty.out_ascii
@@ -0,0 +1,9 @@
+DE-EMPTY(1)                 General Commands Manual                DE-EMPTY(1)
+
+NNAAMMEE
+     ddee--eemmppttyy - empty user-defined macro with arguments
+
+DDEESSCCRRIIPPTTIIOONN
+     initial text arg=right arg=right final text
+
+OpenBSD                         April 21, 2019                         OpenBSD
diff --git a/usr.bin/mandoc/roff.c b/usr.bin/mandoc/roff.c
index 27ef98143f5..cfa591d16e7 100644
--- a/usr.bin/mandoc/roff.c
+++ b/usr.bin/mandoc/roff.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: roff.c,v 1.236 2019/04/21 22:43:00 schwarze Exp $ */
+/*	$OpenBSD: roff.c,v 1.237 2019/04/21 23:45:50 schwarze Exp $ */
 /*
  * Copyright (c) 2008-2012, 2014 Kristaps Dzonsons <kristaps@bsd.lv>
  * Copyright (c) 2010-2015, 2017-2019 Ingo Schwarze <schwarze@openbsd.org>
@@ -3839,6 +3839,11 @@ roff_userdef(ROFF_ARGS)
 	char		 *arg, *ap, *dst, *src;
 	size_t		  sz;
 
+	/* If the macro is empty, ignore it altogether. */
+
+	if (*r->current_string == '\0')
+		return ROFF_IGN;
+
 	/* Initialize a new macro stack context. */
 
 	if (++r->mstackpos == r->mstacksz) {
@@ -3886,7 +3891,7 @@ roff_userdef(ROFF_ARGS)
 	buf->sz = strlen(buf->buf) + 1;
 	*offs = 0;
 
-	return buf->sz > 1 && buf->buf[buf->sz - 2] == '\n' ?
+	return buf->buf[buf->sz - 2] == '\n' ?
 	    ROFF_REPARSE | ROFF_USERCALL : ROFF_IGN | ROFF_APPEND;
 }