deflatePending not available in zlib on OpenBSD (undefined symbol)

Anatoli me at
Tue Jun 2 18:19:00 EDT 2020

Hi Ken,

According to the docs [1]: If deflate returns Z_OK and with zero
avail_out, it must be called again after making room in the buffer
because there might be more output pending.

On the other hand it says: Z_FINISH can be used in the first deflate
call after deflateInit if all the compression is to be done in a single
step. In order to complete in one call, avail_out must be at least the
value returned by deflateBound. Then deflate is guaranteed to return

But: Note that it is possible for the compressed size to be larger than
the value returned by deflateBound() if flush options other than
Z_FINISH or Z_NO_FLUSH are used.

At the beginning of zlib_compress() there's code that decides with which
flush option to call deflate().

/* Only flush for static content or on last (zero-length) chunk */

If it's possible to make flush to be always Z_FINISH, then I guess the

do { ... } while (!zstrm->avail_out);

loop could be simplified to just:

zstrm->next_out = (Bytef *) txn->zbuf.s + txn->zbuf.len;
zstrm->avail_out = txn->zbuf.alloc - txn->zbuf.len;

zr = deflate(zstrm, flush);
if (zr != Z_STREAM_END) {
	/* something went wrong */
	syslog(LOG_ERR, "zlib deflate error: %d %s", zr, zstrm->msg);
	return -1;

I changed the "if (zr)" condition as the only good value would be

But if the flush option can't be always Z_FINISH, then I believe the
loop should be kept with the checks for !zstrm->avail_out as it's
possible that there would be not enough buffer for deflate() to complete
in one call.



On 2/6/20 17:36, Ken Murchison wrote:
> Hi Anatoli,
> Thanks for the report.  I'm not sure that we even need the
> deflatePending() call, since we use deflateBound() to create an
> appropriately-sized buffer to hold the entire compressed response body. 
> Let me do some testing.
> On 6/2/20 3:48 AM, Anatoli wrote:
>> Cyrus developers,
>> Is it possible to somehow rework the code in imap/httpd.c lines 158-169
>> in order to NOT use deflatePending as this func is not available on
>> OpenBSD? The zlib version there is 1.2.3 and deflatePending appeared in
>> 1.2.5 so the code doesn't compile with --enable-http (undefined symbol:
>> deflatePending). The packaged version disables http for now.
>> Is it safe to reduce these lines:
>>   158         if (!zstrm->avail_out) {
>>   159             unsigned pending;
>>   160
>>   161             zr = deflatePending(zstrm, &pending, Z_NULL);
>>   162             if (zr != Z_OK) {
>>   163                 /* something went wrong */
>>   164                 syslog(LOG_ERR, "zlib deflate error: %d %s", zr,
>> zstrm->msg);
>>   165                 return -1;
>>   166             }
>>   167
>>   168             buf_ensure(&txn->zbuf, pending);
>>   169         }
>> to something like:
>>   158         if (!zstrm->avail_out) {
>>   159             buf_ensure(&txn->zbuf, 256 * 1024);
>>   160         }
>> If I understand it correctly, deflatePending in this case is only used
>> to get the needed buffer size and we could replace it with a hardcoded
>> buffer size (like in my example of 256K).
>> Thanks,
>> Anatoli

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