uci: unnecessary write accesses

[Felix Fietkau]

On 2021-09-21 13:19, Florian Eckert wrote:
> 
> I have written a small shell script, to track write access to the 
> '/etc/config' directory.
> For this task I am using the inotify-tool package [1].
> I am using the inotifywait tool to add the watchers [2] with a small 
> shell script and log them to the syslog.
> 
> If I change a 'uci' option with the  the 'uci' commands, then I get 
> create and write events for a temporary file in the '/etc/config/' 
> directory.
> I didn't expect it to be like this.
> 
> For example, if I now change a file using the 'uci' command, a tmp file 
> is created under '/etc/config'.
> Is this correct? Or do we have a problem there.
> I have always thought that for embedded systems with flashes it is 
> important to make as few write accesses as possible.
> 
> Shell commands to change uci config:
> 
> root at st-dev-07 ~ # uci set system.led_Power.trigger=none
> root at st-dev-07 ~ # uci commit system
> 
> Log output of my Script:
> 
> Tue Sep 21 12:50:38 2021 daemon.info loginotify[17375]: 
> {"directory":"/etc/config/","file":".system.uci-fEGgbp","event":"CREATE","time":"09/21/21 
> 12:50:38"}
> Tue Sep 21 12:50:38 2021 daemon.info loginotify[17375]: 
> {"directory":"/etc/config/","file":".system.uci-fEGgbp","event":"MODIFY","time":"09/21/21 
> 12:50:38"}
> Tue Sep 21 12:50:38 2021 daemon.info loginotify[17375]: 
> {"directory":"/etc/config/","file":".system.uci-fEGgbp","event":"MODIFY","time":"09/21/21 
> 12:50:38"}
> Tue Sep 21 12:50:38 2021 daemon.info loginotify[17375]: 
> {"directory":"/etc/config/","file":".system.uci-fEGgbp","event":"CLOSE_WRITE,CLOSE","time":"09/21/21 
> 12:50:38"}
> Tue Sep 21 12:50:38 2021 daemon.info loginotify[17375]: 
> {"directory":"/etc/config/","file":"system","event":"CLOSE_WRITE,CLOSE","time":"09/21/21 
> 12:50:38"}
> Tue Sep 21 12:50:38 2021 daemon.info loginotify[17375]: 
> {"directory":"/etc/config/","file":".system.uci-fEGgbp","event":"ATTRIB","time":"09/21/21 
> 12:50:38"}
> 
> I am bothered about the file '.system.uci-fEGgbp'?
The extra temporary file in /etc/config is intentional. uci is simply
using a common pattern in order to make rewriting config files atomic
and avoid leaving behind empty or incomplete files in case of a system
crash during commit.
The new file is written under a temporary name, and after a fsync, it is
renamed to the final filename. This only works if the temporary file and
the config file are on the same filesystem, that's why the temp file is
put in /etc/config as well.

- Felix