bug in the proxy module ...

Poujol Christophe Christophe.Poujol at atosorigin.com
Tue Jul 29 03:54:07 EDT 2008

This is a proposal to solve the possible dead lock in proxy.

The problem was : if the length of the  last packet  received by the proxy from one back process is 4096 bytes, the proxy expects to receive data and the back process waits for commands.

I propose you a shortcut :
if the proxy receives 4096 bytes the proxy puts back the last byte into the buffer and it sends the first 4095 bytes through the outputstream
(the last byte will be read at the next loop).
if the proxy receives less than 4096 bytes it works like previously.

The initial code could be modified from  :


 if (pout) {
               const char *err;
               char buf[4096];
               int c;

               do {
                   c = prot_read(pin, buf, sizeof(buf));

                   if (c == 0 || c < 0) break;
                   prot_write(pout, buf, c);
               } while (c == sizeof(buf));

               if ((err = prot_error(pin)) != NULL) {



  if (pout) {
               const char *err;
               char buf[4096];
               int c;

               do {
                   c = prot_read(pin, buf, sizeof(buf));

                   if (c == 0 || c < 0) break;

                   if (c == sizeof(buf)) {
                       prot_ungetc(buf[sizeof(buf) - 1], pin);
                       prot_write(pout, buf, c - 1);
                   else {
                       prot_write(pout, buf, c);

               } while (c == sizeof(buf));

               if ((err = prot_error(pin)) != NULL) {


-----Message d'origine-----
De : cyrus-devel-bounces at lists.andrew.cmu.edu [mailto:cyrus-devel-bounces at lists.andrew.cmu.edu] De la part de Wesley Craig
Envoyé : mercredi 11 juin 2008 21:47
À : Ken Murchison
Cc : cyrus-devel at lists.andrew.cmu.edu
Objet : Re: bug in the proxy module ...

On 10 Jun 2008, at 10:07, Ken Murchison wrote:
> Any suggestions?  I'm off thinking about other things at the moment.

The comment associated with the change is:

        make sure we send all available data, not just one buffer full.
        this solves a pipelining problem where a response to a command run
on a proxy
        could be output in the middle of a response to a command run on a

Both versions call prot_select() once.  The old code The new code
(attempts) to copy input to output until end of input, but since it's
only called prot_select() once, that's a problem.  There are a couple
of possibilities, perhaps you're more familiar with prot and it's
byzantine usage, but here's my analysis:

        1) Instead of looping on the size of the read, we loop until
prot_read() returns == 0 or < 0.  This assumes that pin isn't set to
allow blocking.  I don't like this solution, since I'm not terribly
interested in an exhaustive analysis of every possible pin that
proxy_check_input() might get.  Maybe you know something I don't, tho.

        2) Introduce prot_select() into the read/write loop.  This will
allow you to know that there's still input available really without
blocking.  Of course, if it's a very large block of data, you might
not see the next block, return control to the calling function, and
get the same pipelining problem mentioned in the CVS log above.
Assuming you're not worried about that scenario, it's a good solution
because it introduces the idea that output from the backend server is
handled prior to input from the client.

        3) Continuing on the precedence idea above, split the loop handling
so that backend output is always handled first.  Also, always return
control to the caller if you ever have backend output.  This way,
you'll only ever take input from the client if the backend isn't
sending anything.  I doubt this solves the race mentioned in (2),
either tho.

        4) Restructure the routines calling proxy_check_input to know the
structure of the commands being sent and the corresponding
responses.  This is the surest way to fix the above problem, i.e.,
don't let the proxy server respond to a command until the response to
the command sent by the backend is done.  Of course, tho is a huge
pain, probably involving a ton of additional code.


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