Fix two issues with .po (page offset) formatting:

1. Truncate excessive offsets to a width reasonable in the context
of manual pages instead of printing excessively long lines
and sometimes causing assertion failures;
found in an afl run performed by Jan Schreiber <jes at posteo dot de>.
2. Remember both the requested and the applied page offset; otherwise,
subtracting an excessive width, then adding it again, would end up
with an incorrectly large offset.
While here, simplify the code by reverting the previous offset up front,
and also add some comments to make the general ideas easier to understand.

3 files changed, 53 insertions, 2 deletions

diff --git a/regress/usr.bin/mandoc/roff/po/Makefile b/regress/usr.bin/mandoc/roff/po/Makefile
index 76a76915745..fa2f5774ca5 100644
--- a/regress/usr.bin/mandoc/roff/po/Makefile
+++ b/regress/usr.bin/mandoc/roff/po/Makefile
@@ -1,5 +1,13 @@
-# $OpenBSD: Makefile,v 1.1 2017/06/14 22:50:37 schwarze Exp $
+# $OpenBSD: Makefile,v 1.2 2020/09/03 20:33:20 schwarze Exp $
 
-REGRESS_TARGETS	= basic-mdoc
+REGRESS_TARGETS	= basic-mdoc range
+
+# groff-1.22.4 defect:
+# - Negative page objects sometimes result in output lines starting
+#   with backspace characters.
+# - Excessive page offsets cause excessive line lengths
+#   instead of being truncated to resonable widths.
+
+SKIP_GROFF	 = range
 
 .include <bsd.regress.mk>
diff --git a/regress/usr.bin/mandoc/roff/po/range.in b/regress/usr.bin/mandoc/roff/po/range.in
new file mode 100644
index 00000000000..e90d0d48d62
--- /dev/null
+++ b/regress/usr.bin/mandoc/roff/po/range.in
@@ -0,0 +1,24 @@
+.\" $OpenBSD: range.in,v 1.1 2020/09/03 20:33:20 schwarze Exp $
+.Dd $Mdocdate: September 3 2020 $
+.Dt PO-RANGE 1
+.Os
+.Sh NAME
+.Nm po-range
+.Nd page offsets out of range
+.Sh DESCRIPTION
+initial text
+.Pp
+.po -10n
+too small
+.Pp
+.po +10n
+back in range
+.Pp
+.po 80n
+too large
+.Pp
+.po -70n
+back in range
+.Pp
+.po 0
+final text
diff --git a/regress/usr.bin/mandoc/roff/po/range.out_ascii b/regress/usr.bin/mandoc/roff/po/range.out_ascii
new file mode 100644
index 00000000000..a90da5db7af
--- /dev/null
+++ b/regress/usr.bin/mandoc/roff/po/range.out_ascii
@@ -0,0 +1,19 @@
+PO-RANGE(1)                 General Commands Manual                PO-RANGE(1)
+
+NNAAMMEE
+     ppoo--rraannggee - page offsets out of range
+
+DDEESSCCRRIIPPTTIIOONN
+     initial text
+
+too small
+
+     back in range
+
+                                                                 too large
+
+               back in range
+
+     final text
+
+OpenBSD                          July 4, 2017                          OpenBSD