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More tcp problems

There are a few more problems with the tcp module and its interaction with
zftp.  I was hoping to fix the first one before releasing a new development
version, but it seems it goes beyond that.

- Closing a zftp connection doesn't set the session pointer to NULL.  This
  results (in my case) in a segmentation violation when opening a second
  connection.  This is because the tcp_close() frees the session, so it
  can't be re-used or even tested again.  (Simply setting the pointer to
  NULL caused other problems I didn't understand, maybe related to the rest
  of this list.)
- Don't know if this is related, but I get
  BUG: attempt to free storage at invalid address
  when opening zftp connections, in particular the first (since I
  don't get as far as a second).
- In general, it seems a little bit difficult to tell whether tcp_close()
  has actually freed the session or not.  And if it hasn't, because it
  encountered an error with close(), it's hard to see how the session
  should be freed. I think another call to tcp_close() would do it --- but
  it's hard to know when you need that.  If you do it when the session has
  already been freed, you're in big trouble.
- There's a similiar problem with tests for (sess->fd == -1) in zftp.
  If they're true, the session is never freed; opening a new one will
  simply assign a different TCP session to the same pointer, so that
  the memory leaks.
- With a failed zfopen, I now get
    zfopen:42: connection close failed: bad file number
  (plus a segmentation violation which I guess is something to do with the
  previous stuff).  I don't get that message with 4.0.1.  The function tests
  at that point to see if $ZFTP_HOST is set, and if it is, attempts to
  close the file.  I *think* that all that's changed is the zfclose
  was silent before and isn't now, because of tcp_close().  This may be a
  knock-on effect of the things above, though, i.e. it goes away if
  the session pointers are handled properly.

I hope it's possible to make tcp only return a session pointer if the fd is
valid, and to free the session unconditionally on an attempt to close it.

Peter Stephenson <pws@xxxxxxx>                  Software Engineer
CSR Ltd., Science Park, Milton Road,
Cambridge, CB4 0WH, UK                          Tel: +44 (0)1223 392070

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