Incron cannot create watch for system table

Today I started playing with incron. If you don't know it, it's a nifty tool for launching scripts on file system events. The configuration is very simple, just some "incrontab" files in /etc/incron.d. Each line in those files has 3 fields: directory to watch, a comma-separated list of inotify events, and the script and parameters to launch.

I was doing some tests when I started to get this kind of messages in the logs:

Mar  1 09:42:30 test-machine incrond[32310]: cannot create watch for system table foo: (2) No such file or directory

I checked and rechecked several times, but the directories I was watching existed, in the same machine. That is, no obvious mistake. Then I stopped incron and launched it in strace and the problem became evident:

inotify_add_watch(9, "#", 0)            = -1 ENOENT (No such file or directory)

It happens that incron does not support comments in the incrontab files, so it treats the # at the beginning of the line as the path to watch. Silly thing, if you ask me.

Actually, if you go to its BTS, you'll find bug reports about it¹² and some others about its incrontab file parsing³⁴. I'm pretty sure there must be more around.

So, for now, my "solution" is to count the lines that begin with #, count the lines that report this errors, double check, and if they coincide, just ignore them. Otherwise, as incron stupidly does not report which file it didn't find, just stop the service and run it under strace -ff -e file.

There are other caveats with incron. For instance, cron and at launch the commands with sh (or the shell you define), but incron doesn't, so you can't easily do any redirections, and any output is lost, even the stderr. I couldn't get it to do any kind of redirection, so I ended up doing a wrapper. Lastly, it can't watch more than once the same dir with the same set of flags.