pnstrdup considered armed and dangerous

Hi,

A colleage of me just wrote innocent looking code like
char *shardRelationName = pnstrdup(relationName, NAMEDATALEN);
which is at the moment wrong if relationName isn't preallocated to
NAMEDATALEN size.

/*
* pnstrdup
* Like pstrdup(), but append null byte to a
* not-necessarily-null-terminated input string.
*/
char *
pnstrdup(const char *in, Size len)
{
char *out = palloc(len + 1);

memcpy(out, in, len);
out[len] = '\0';
return out;
}

isn't that a somewhat weird behaviour / implementation? Not really like
strndup(), which one might believe to be analoguous...

Greetings,

Andres Freund