Back in 2004, the zlib maintainers fixed a bug in zlib involving the use of the (gzopen, gzgets, gzclose) functions on non-compressed empty files:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=258087

Per <http://www.zlib.net/ChangeLog.txt>, this bug was fixed upstream in zlib-1.2.1.2:

Changes in 1.2.1.2 (9 September 2004)
- Fix gzio.c to not return error on empty files [Brown]

However, sometime between 1.2.1.2 (exclusive) and 1.2.3 (inclusive), a regression occurred in zlib, and this bug was reintroduced.

Using this test program from the Debian bug:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <zlib.h>

#define BUFSIZE 80

int main() {
  int retval;
  char *input = "test", buf[BUFSIZE];
  FILE *fd;

  fd = gzopen(input, "rb");
  if (fd == NULL) {
    fprintf(stderr, "gzopen %s: %s\n", input, strerror(errno));
    exit(EXIT_FAILURE);
  }

  if(gzgets(fd, buf, BUFSIZE) == Z_NULL) {
    fprintf(stderr, "gzgets %s: %s\n", input, gzerror(fd, &retval));
  }

  retval = gzclose(fd);
  if (retval != 0) {
    if (retval != Z_ERRNO) {
      fprintf(stderr, "gzclose %s: %s\n", input, gzerror(fd, &retval));
    } else {
      perror("gzclose");
    }
    exit(EXIT_FAILURE);
  }

  return(0);
}

On RHEL4, zlib functions properly on a zero-length non-compressed file:

$ rpm -q redhat-release zlib
redhat-release-4AS-9
zlib-1.2.1.2-1.2
$ gcc -Os -Wall -pedantic -o gztest gztest.c -lz
$ rm -f test
$ touch test
$ ./gztest
gzgets test: test: stream end

But on RHEL5, it fails:

$ rpm -q redhat-release zlib
redhat-release-5Server-5.5.0.2
zlib-1.2.3-3
$ gcc -Os -Wall -pedantic -o gztest gztest.c -lz
$ rm -f test
$ touch test
$ ./gztest
gzgets test: test: buffer error
gzclose test: XX: buffer error

And on Fedora 12:

$ rpm -q fedora-release zlib
fedora-release-12-2.noarch
zlib-1.2.3-23.fc12.i686
zlib-1.2.3-23.fc12.x86_64
$ gcc -Os -Wall -pedantic -o gztest gztest.c -lz
$ rm -f test
$ touch test
$ ./gztest
gzgets test: test: buffer error
gzclose test: XX: buffer error

And on Fedora 13:

$ rpm -q fedora-release zlib
fedora-release-13-1.noarch
zlib-1.2.3-23.fc12.x86_64
zlib-1.2.3-23.fc12.i686
$ gcc -Os -Wall -pedantic -o gztest gztest.c -lz
$ rm -f test
$ touch test
$ ./gztest
gzgets test: test: buffer error
gzclose test: XXX: buffer error

However, if I compile zlib-1.2.5-1.fc14 from Rawhide, it works properly:

$ rpm -q fedora-release zlib
fedora-release-12-2.noarch
zlib-1.2.3-23.fc12.i686
zlib-1.2.5-1.fc12.x86_64
$ rm -f test
$ touch test
$ ./gztest
gzgets test: 

Per <http://www.zlib.net/>, version 1.2.4 of zlib had these improvements:

- Wholesale replacement of gz* functions with faster versions
- As part of that, added gzbuffer(), gzoffset(), gzclose_r(), and gzclose_w() functions 

So, I strongly suspect the regression was squashed (again) in version 1.2.4.

I encountered this regression the same way the Debian bug reporter encountered the original bug: I use fwlogwatch, and it is generates errors (which cron then emails to me) whenever it is run on a zero-length uncompressed file:

gzclose /var/log/messages.1: : buffer error

Given the massive dependencies on zlib, I'd be very hesitant to drop 1.2.5 as an update on F12/F13 (to say nothing of RHEL, where updating to 1.2.5 is -DNO_CHANCE). But if it is possible to fix the regression against 1.2.3 (which is what F12, F13, RHEL5, and RHEL6 all run), we should do so.