`File.containsConfPair` property

A property to set key = value pairs under particular [sections] in config files. For example, in stock Debian Jessie /etc/lightdm/lightdm.conf contains the lines

[SeatDefaults]
#autologin-user=

With the property

"/etc/lightdm/lightdm.conf" `File.containsConfPair` ("SeatDefaults", "autologin-user", "swhitton")

this will get set to

[SeatDefaults]
autologin-user=swhitton

`LightDM.autoLogin` property

An application of File.containsConfPair to edit /etc/lightdm/lightdm.conf to enable autologin for a specified user: a property encapsulating the above example.

Patches

Please see the two commits in branch confpairs in the repo at git@github.com:spwhitton/propellor.git.

merged --Joey

RSS

comment 1

Thanks for submitting these patches!

Looking at containsConfPair, it assumes an ini-style file, so is a little misplaced in Property.File. which is otherwise about generic text files.

So, it would probably make sense to move it to a new Property.IniFile module.

However, parsing a config file recently pointed out that the tor config file has a similar need. It's not ini format, but shares the same basic idea of a "section" line which is followed by lines setting things specific to that section.

So, it would be great if containsConfPair could be generalized to also cover that tor config file use case. I think this would be pretty easy; just make it take one string containing the whole section line (including square brackets for ini file, or whatever for tor config file), and a second string containing the whole setting line.

Comment by joey — Tue Aug 4 14:23:33 2015

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comment 2

Thanks for the input!

I agree that generalising to lines under sections is a good idea, but I don't think it can be as simple as a property taking the full section header and the full settings line. That's because there is a need to update the values of keys under sections: in the example LightDM case, the line autologin-user=someone must replace any autologin-user=someone_else. So the function needs to know the key, not just the whole line.

So to generalise containsConfPair, it might take a section header, key, value and a specification of what kind of config file it is. That specification would be a type containing the comment character, the formatting of section headers and the use of spaces, colons or equals signs between keys and values. What do you think to this?

Comment by spwhitton — Wed Aug 5 21:29:04 2015

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comment 3

I'd suggest making it take some helper functions.

Something like these:

type SectionStart = String -> Bool -- ^ find the line that is the start of the wanted section (eg, == "<Foo>")
type SectionEnd = String -> Bool -- ^ find a line that is within the section, but that indicates the end of the section (eg == "</Foo>")
type SectionPast = String -> Bool -- ^ find a line that indicates we are past the section (eg, a new section header)
type AdjustSection = [String] -> [String] -- ^ run on all lines in the section, including the SectionStart line and any SectionEnd line; can add/delete/modify lines, or even delete entire section
type InsertSection = [String] -> [String] -- ^ if SectionStart does not find the section in the file, this is used to insert the section somewhere within it

adjustSection :: SectionStart -> SectionEnd -> AdjustSection -> InsertSection -> FilePath -> Property

Which seems sufficiently generic; it can even be used to delete entire sections!

Let's see..

iniHeader header = '[':header++"]"

adjustIniSection :: String -> AdjustSection -> InsertSection -> Property
adjustIniSection header = adjustSection
    (== iniHeader header)
    (const False)
    ("[" `isPrefixOf`)

containsConfPair header key value = adjustIniSection header
    go
    (++ [confheader, confline]) -- add missing section at end
  where
    confheader = iniHeader header
    confline = key ++ "=" ++ value
    go ls = undefined -- TODO find key= line and change it, or add confline

removeSection header = adjustIniSection header
    (const []) -- remove all lines of section
    id -- add no lines if section is missing

Comment by joey — Thu Aug 6 14:54:14 2015

Remove comment

comment 4

Thanks for the ideas. I've implemented them as a new commit to my confpairs branch. Please take a look.

Two points:

I dropped the SectionEnd helper function. My implementation of adjustSection didn't need it and I couldn't think up a case where it would be needed.
I'm using a tuple (section, key, value) as the second argument to ConfFile.containsIniPair, rather than just using four arguments as you suggested. If ConfFile.containsIniPair takes four arguments, then it cannot be used infix when attached to other properties with the & operator, without using extra brackets.

Comment by spwhitton — Mon Aug 17 00:57:54 2015

Remove comment

comment 5

And merged, thanks.

The SectionEnd would be useful for eg, bind-style or apache-style config files. However, those probably need a better parser than this one anyway.

Comment by joey — Thu Aug 20 14:37:43 2015

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Add a comment

File.containsConfPair property

LightDM.autoLogin property

Patches

`File.containsConfPair` property

`LightDM.autoLogin` property