[patch] dup_ent does not set dst when src is NULL

[Brian Dessent <brian at dessent dot net>]

In net.cc, there are several cases where dup_ent() is used as follows:

dup_ent (servent_buf, getservbyname (name, proto), t_servent);
syscall_printf ("%p = getservbyname (%s, %s)",
    _my_tls.locals.servent_buf, name, proto);
return _my_tls.locals.servent_buf;

This presents a problem if getservbyname() returns NULL, because
dup_ent just returns NULL, it does not modify 'dst'.  This results in
the function returning the previous successful value if the
get_foo_by_bar() function returned NULL.  This seems to be applicable to
getservbyname(), getservbyport(), gethostbyaddr(), and gethostbyname().  

In the case of gethostbyname() there's also another bug in that there
will be a spurious debug_printf() about dup_ent failing if the address
simply didn't resolve.  That should probably be fixed too but I wanted
to be sure the patch stayed "trivial".

A simple testcase that demonstrates the problem:

#include <stdio.h>
#include <string.h>
#include <netdb.h>
#include <sys/socket.h>
#include <netinet/in.h>

void mygetservbyname(char *serv, char *proto)
{
  struct servent *p;

  if((p = getservbyname(serv, proto)))
    printf("getservbyname(\"%s\", \"%s\") success, port = %u\n", 
       serv, proto, (unsigned int)ntohs (p->s_port));
  else
    printf("getservbyname(\"%s\", \"%s\") returns NULL\n", serv, proto);
}  

int main(int argc, char **argv)
{
  mygetservbyname("25", "tcp");
  mygetservbyname("auth", "tcp");
  mygetservbyname("25", "tcp");
  return 0;
}

$ ./getservbyname 
getservbyname("25", "tcp") returns NULL
getservbyname("auth", "tcp") success, port = 113
getservbyname("25", "tcp") success, port = 113


Brian

===================================================================
2005-04-05  Brian Dessent  <brian@dessent.net>

	* net.cc (__dup_ent): Make dst point to NULL if src is NULL.
	Free dst if it was previously allocated to not leak memory.

Index: net.cc
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/net.cc,v
retrieving revision 1.186
diff -u -p -r1.186 net.cc
--- net.cc	24 Mar 2005 14:04:06 -0000	1.186
+++ net.cc	6 Apr 2005 05:17:50 -0000
@@ -387,6 +387,9 @@ __dup_ent (unionent *&dst, unionent *src
   if (!src)
     {
       set_winsock_errno ();
+      if(dst)
+        free(dst);
+      dst = NULL;
       return NULL;
     }