Abort endless loops during roff macro and string expansion.

For now, use the simplest conceivable approach, like groff does:
Just a fixed, ugly input stack limit.
"check it in" kristaps@

1 files changed, 6 insertions, 0 deletions

diff --git a/roff.7 b/roff.7
index 2f007718..a29d6a92 100644
--- a/roff.7
+++ b/roff.7
@@ -196,6 +196,12 @@ string interpolation syntax described below
 .Sx ds ,
 but this is rarely useful because every macro definition contains at least
 one explicit newline character.
+.Pp
+In order to prevent endless recursion, both groff and
+.Xr mandoc 1
+limit the stack depth for expanding macros and strings
+to a large, but finite number.
+Do not rely on the exact value of this limit.
 .Ss \&dei
 Define a user-defined
 .Nm