Forking processes and Gtk2

I made a change recently on the gjay program. gjay is a gtk program that basically analyzes your music and makes playlists. There is a gui frontend and a analyzer back-end and they communicate through a pipe.

One really useful debugging option gtk has is to make warnings fatal, so when gtk finds one it crashes at that point and you can use gdb to trap it. The flag is –g-fatal-warnings.  I have been updating gjay and initially it didn’t have this option, so I needed to add the gtk options, which is a simple one-liner.
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PHP uniqid() not always a unique ID

When is a unique id not unique? How can a file exist and not exist? All this caused a mysterious bug in JFFNMS with the reachability interface which goes to show the limitations of the PHP function uniqid().

For quite some time modern versions of JFFNMS have had a problem. In large installations hosts would randomly appear as down with the reachability interface going red. All other interface types worked, just this one.

Reachability interfaces are odd, because they call fping or fping6 do to the work. The reason is because to run a ping program you need to have root access to a socket and to do that is far too difficult and scary in PHP which is what JFFNMS is written in.

To capture the output of fping, the program is executed and the output captured to a temporary file. For my tiny setup this worked fine, for a lot of small setups this was also fine. For larger setups, it was not fine at all. Random failed interfaces and, most bizzarely of all, even though a file disappearing. The program checked for a file to exist and then ran stat in a loop to see if data was there. The file exist check worked but the stat said file not found.

At first I thought it was some odd load related problem, perhaps the filesystem not being happy and having a file there but not really there. That was, until someone said “Are these numbers supposed to be the same?”

The numbers he was referring to was the filename id of the temporary file. They were most DEFINITELY not supposed to be the same. They were supposed to be unique. Why were they always unique for me and not for large setups?

The problem is with the uniqid() function. It is basically a hex representation of the time.  Large setups often have large numbers of child processes for polling devices. As the number of poller children increases, the chance that two child processes start the reachability poll at the same time and have the same uniqid increases. It’s why the problem happened, but not all the time.

The stat error was another symptom of this bug, what would happen was:

  • Child 1 starts the poll, temp filename abc123
  • Child 2 starts the poll in the same microsecond, temp filename is also abc123
  • Child 1 and 2 wait poller starts, sees that the temp file exists and goes into a loop of stat and wait until there is a result
  • Child 1 finishes, grabs the details, deletes the temporary file
  • Child 2 loops, tries to run stat but finds no file

Who finishes first is entirely dependent on how quickly the fping returns and that is dependent on how quicky the remote host responds to pings, so its kind of random.

A minor patch to use tempnam() instead of uniqid() and adding the interface ID in the mix for good measure (no two children will poll the same interface, the parent’s scheduler makes sure of that.) The initial responses is that it is looking good.


killing a process in TCL

How to kill a process in TCL with no binary or tclx

Suppose you had spawned a process in TCL and knew its PID and wanted to kill it? Sounds simple enough thing to do, right? This problem has plagued me for many months because some things you can assume on a normal system do not hold true on strange environments, such as build deaemons.

Seems simple enough, I started off with:

exec kill $pid

Except.. not every environment has the kill binary, and with that piece of code exec has to be a binary and not a shell builtin. The funny thing is that /bin/kill is in the procps package, which is the package having the buildd problems.

So next idea was to use command -v to check for the existence of kill and skip those tests that needed kill if not found. Good idea except, so I found out later, it also finds built-ins. That means we are back to problem #1.

There is a kill command in tcl, but it requires tclx. That seems excessive for just one little simple command. How can I run a shell out of TCL that runs the kill builtin? On the command line, something like below would do it.

/bin/sh -c 'kill 1234'

I was closer, but then hit TCL quoting hell. No matter what I (initially) did I’d either get the shell to complain or my variable to not be evaluated. In the end, I had to write it to a separate variable for the command line then apply that to the exec. Not perfect but at least it works now.

The resulting code (found in testsuite/config/unix.exp) looks like:

proc kill_process pid {
    set cmdline "kill $pid"
    if { [catch { exec /bin/sh -c $cmdline } msg]} {
        warning "Could not kill process: $msgn"

Perhaps there is a more elegant way, I’m certainly no star TCL programmer, but of all the combinations I saw this was the only that worked.

Sneak peek of top graphs

Jim has been busy as part of the procps-ng team that looks after top. Basically all the changes you find in top from around 2.7 or so are by him. Not satisfied enough with fixing top, making it faster and showing more fields, he has given us CPU and memory graphs.

He also thinks I don’t have enough colours (or as he would put it colors) on my top output so I’ve posted what the new top looks like for me so you can see the graphs and he can see my colours.

top, with colours
top, with colours


I think it is both colourful and useful addition. The colours have been available for a while now and the graphs will appear in the next upstream release of procps-ng.


A python utf gotcha

This one had me stumped for a while:

# -*- coding: utf-7 -*-
import datetime
from sqlalchemy import ForeignKey, Column
from sqlalchemy.types import Integer, Unicode, Boolean, DateTime

default_due_date = + datetime.timedelta(days=30)

Syntax error found on last line.

Hmm, bring up a python interpreter and type the last line in with the imports. Works fine.

It’s the first line that is the problem, I typoed it and made it utf-7 not utf-8. I suppose it means that it is case-insenstive. Still, it wasn’t too clear to me at least, what was going on.

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