fix xgethostname on OSX [patch]

[Neil Conway]

The attached one-liner fixes a bug in xgethostname on OSX. It appears 
that gethostname() on OSX will return ENOMEM via errno if the buffer 
that was passed is not sufficiently large to hold the hostname (although 
this isn't documented in the gethostname(3) OSX manpage). The 
implementation of xgethostname only expects ENAMETOOLONG or EINVAL to be 
returned in this case, so xgethostname bails out and returns NULL. Since 
CVS doesn't check the return value of xgethostname() (!), this can 
result in various badness later on (e.g. invoking strlen() on a NULL 
pointer, which was how I ran into this -- the hostname on my system 
happens to be > the initial xgethostname() buffer size).

The attached patch (against cvs 1.12.11) improves xgethostname() to 
check for ENOMEM as well as ENAMETOOLONG and EINVAL. I grant permission 
to distribute this patch under the terms of the GNU Public License. 
Please apply.

BTW, I think there is still room for improvement:
(a) this behavior is not well-standardized. POSIX claims the hostname 
will be truncated if it is larger than the passed-in buffer length (and 
it is undefined whether the return value will be NUL terminated), but 
AFAICS most systems return an error in this case. It seems glibc < 2.1 
will return EINVAL, glibc >= 2.1 and FreeBSD will return ENAMETOOLONG, 
and other systems may well return other errors. It is unnecessary to 
rely on this behavior, anyway: why not just make the buffer size 
HOST_NAME_MAX or MAXHOSTNAMELEN (etc.) to begin with? It seems silly to 
go through this kind of trouble to avoid the allocation of a few hundred 
bytes of memory.

(b) not checking the return value of xgethostname() is fragile.
-Neil
2005-02-21  Neil Conway  <[EMAIL PROTECTED]>
        * lib/xgethostname.c: Check for ENOMEM, which is returned by
        OSX/Darwin if the specified buffer is not large enough for the
        hostname.
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