What is the origin of ZLIB's "Zlib error -2 deflating data"
Some of my sites are getting, seemingly without reason, strange errors like these:
[Fri Nov 06 08:29:07.677505 2020] [deflate:error] [pid 23820:tid 140019129566976] [client XXX.XXX.XXX.XXX:XXXXX] AH01386: Zlib error -2 deflating data ((null)), referer: https://www.XXX.XXX/XXX
These errors happen on seemingly random pages, and can be fixed by updating the source code with a comment even. Every change seems to fix this, but the error keeps popping up.
What I've tried
After reading this post https://stackoverflow.com/questions/37952355/zlib-z-stream-error-deflating-data-in-apache-error-log, we have tried migrating the website to another server, updating the zlib library, using a newer server os (debian), using another php version (currently on 7.4). There is nothing in the code that is using (g)zip functionality. There is also nothing in the .htaccess except redirects.
Some info from the phpinfo:
Configure command:
'./configure' '--enable-embed' '--prefix=/usr/local/php74' '--program-suffix=74' '--enable-fpm' '--with-fpm-systemd' '--enable-litespeed' '--with-config-file-scan-dir=/usr/local/php74/lib/php.conf.d' '--with-curl' '--enable-gd' '--with-gettext' '--with-jpeg' '--with-freetype' '--with-kerberos' '--with-openssl' '--with-mhash' '--with-mysql-sock=/var/lib/mysql/mysql.sock' '--with-mysqli=mysqlnd' '--with-pdo-mysql=mysqlnd' '--with-pear' '--with-sodium=/usr/local' '--with-webp' '--with-xsl' '--with-zlib' '--with-zip' '--with-iconv=/usr/local' '--enable-bcmath' '--enable-calendar' '--enable-exif' '--enable-ftp' '--enable-sockets' '--enable-soap' '--enable-mbstring' '--enable-intl' 'PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/usr/local/icu/lib/pkgconfig:/usr/local/lib64/pkgconfig:/usr/local/lib/pkgconfig'
ZLIB:
ZLib Support enabled
Stream Wrapper compress.zlib://
Stream Filter zlib.inflate, zlib.deflate
Compiled Version 1.2.7
Linked Version 1.2.7
Directive | Local Value | Master Value
------------------------------------------------------------
zlib.output_compression | Off | Off
zlib.output_compression_level | -1 | -1
zlib.output_handler | no value | no value
When inspecting the response the site gets from the server then te Content-Encoding header is set as gzip, which I think is strange, since (using my limited DevOps knowledge) I think the info given above should indicate to not use compression.
I would share some code, but I can't pinpoint a part of the code that seems to be the problem, and it seems counter productive to show the entire code for a legacy custom CMS.
Our website had a similar issue. When viewing the page source retrieved from the server we noticed it was just cut off at some point. We fixed it by adding a flush() call around the cutoff point. Not sure why it was needed and why it worked to be honest.
For me this was caused when the size of the web page was too large and the php.ini value for output_buffering was exceeded. The buffer is flushed early and you only get a partial web page.
You can set a higher limit for output_buffering or turn output_buffering off to fix this.
https://www.php.net/manual/en/outcontrol.configuration.php#ini.output-buffering
Be sure to restart php afterwards. Restarting apache2 didn't load the new php.ini settings for me, needed to restart php separately.