Monthly Archives: February 2011

MythWeb and Flash streaming

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For a while I’ve heard of this mythical flash streaming that is now supposedly built into MythWeb. However I have yet to see it anywhere in the website. What gives? So I decided to get to the bottom of this. There’s a wiki article on MythTV web which describe how it’s done, but it’s said to be outdated and pointed to MythWeb’s wiki page, which only mentions it’s been rewritten to enable Flash streaming. So how do I enable it?

After digging in source and finding various shenanigans with the WebFLV_on variable, the answer revealed itself in the preference pages of MythWeb (Settings > MythWeb > Video Playback). There is a tick box to “Enable Video playback”. However, it says it requires ffmpeg with mp3 support.

I’m using gentoo so installing ffmpeg was just a matter of emerge -av ffmpeg. After installing, I can finally tick the “Enable Video Playback”. However arriving at the preference pages and tried playing on the flash player, a new stumbling block appeared. It says that the pl/stream/bla/bla.flv is not found. Navigating to it manually revealed a 500 Internal Server Error.

Since I’m using Lighttpd, I discovered that it has a deficiency logging CGI errors. The error.log was useless and I ended up running Lighttpd in non-daemon mode (/usr/sbin/lighttpd -D -f /etc/lighttpd/lighttpd.conf) and looked at the errors spat out on to the console. Turns out that it requires Math::Round which I haven’t installed.

The story is actually a bit more cumbersome as before everything I needed to enable CGI on lighttpd for perl to work and to get around a streaming path issue, I modified $stream_url in /includes/defines.php to not double slash on my root, but I know everyone just wants to see what I wanted to see when I embarked on this journey – a screenshot of it in action:

It’s by no means perfect with video being low quality, lack of seeking and some high CPU usage – but it works!

Match any character including new line in Javascript Regexp

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It seems like the dot character in Javascript’s regular expressions matches any character except new line and no number of modifiers could change that. Sometimes you just want to match everything and there’s a couple of ways to do that.

You can pick an obscure character and apply a don’t match character range with it ie [^`]+. This is not true match any character though. Or you can try [.\r\n]+ which doesn’t seem to work at all. (?:\r|\n|.)+ works fine, but as you’ll find out soon, it is notoriously slow as each time you use it, you are creating a new 3 way branching point because of the brackets.

The perfect way I’ve found is actually a nicer variation of the first idea:
[^]+
Which means ‘don’t match no characters’, a double negative that can re-read as ‘match any character’. Hacky, but works perfectly.

Avahi, setrlimit NPROC and lxc

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Over the weekend I installed Avahi (the open source bonjour equivalent) and bumped into a strange error while trying to restart the service. /var/log/message says chroot.c: fork() failed: Resource temporarily unavailable. Searching the interwebs revealed it is an issue with LXC and setrlimit.

The setrlimit call can limit set cetain limitations on processes. One such limitation is NPROC, the number of processes that can have the same UID. Using setrlimit NPROC can enhance security by preventing unexpected forking, like when an attacker is trying spawn a new process. However, the server I am running on uses LXC, and avahi is installed on the host. In LXC, the containers themselves have isolation between one another, but the host sees all processes. The PIDs of container processes are remapped but their UIDs stay the same. Thus, you will get UID collisions where user 102 of container can refer to say ntp, while 102 of host can refer to avahi. Because the host sees and accounts for all processes, setrlimit on avahi (102) of say 3 processes will also count existing processes in containers with UID 102 (such as ntp) and thus breach the limit and unable to spawn.

The only way to solve this is to edit avahi.conf and set rlimit-nproc or just disable rlimits altogether using the --no-rlimits switch.

I guess as LXC and control groups becomes more common, developers will need to adjust their assumptions about users and processes.