[augeas-devel] How to write lens so that it can parse empty file.

[Raphaël Pinson]

On Thu, May 31, 2012 at 4:38 PM, Nathaniel Cook <nvcook42 at gmail.com> wrote:

> I have been working on writing a simplified lens for the ganglia
> gmetad.conf file. I have the lens working as I like but when I try to
> use augeas to add entries to the file I get errors on save if the file
> doesn't exist or is empty. It says it cannot parse the entire input:
>
> /augeas/files/etc/gmetad.conf/error = "parse_failed"
> /augeas/files/etc/gmetad.conf/error/pos = "0"
> /augeas/files/etc/gmetad.conf/error/line = "1"
> /augeas/files/etc/gmetad.conf/error/char = "0"
> /augeas/files/etc/gmetad.conf/error/lens =
> "/usr/share/augeas/lenses/gmetad.aug:22.12-.56:"
> /augeas/files/etc/gmetad.conf/error/message = "Get did not match entire
> input"
>
>
> Here is the lens:
>
>
> module Gmetad =
>    autoload xfm
>
>    let quote = Util.del_str "\""
>    let space = del /[ \t]+/ " "
>    let eol = Util.del_str "\n"
>    let empty =  del /[ \t]*/ "" . eol
>
>    let host = store /[a-zA-Z0-9._]+/
>    let hosts = ( space . [ seq "hosts" . host ] )+
>    let group = [ label "group" . quote . store /[a-zA-Z0-9._ ]+/ . quote ]
>    let data_source = [ counter "hosts" .  label "data_source" .
> Util.del_str "data_source" . space . group . [ label "hosts" . hosts ]
> ]
>
>    let gridname = [ label "gridname" . Util.del_str "gridname" .
> space . quote . store /[a-zA-Z0-9._]+/ . quote ]
>
>    let lns  = (gridname . empty)* . (data_source . empty)*
>
>    let filter = incl "/etc/gmetad.conf"
>    let xfm = transform lns filter
>
>
> Here is the test module:
>
> module Test_gmetad =
>    let conf = "gridname \"grid\"
> data_source \"group_name\" ds1 ds2 ds3.example.com
> data_source \"group_other\" www1 www2 www3.example.com
> "
>    test Gmetad.lns get  conf =
>    { "gridname" = "grid" }
>    { "data_source"
>            { "group" = "group_name" }
>            { "hosts"
>                { "1" = "ds1" }
>                { "2" = "ds2" }
>                { "3" = "ds3.example.com" }
>            }
>    }
>    { "data_source"
>        { "group" = "group_other" }
>        { "hosts"
>            { "1" = "www1" }
>            { "2" = "www2" }
>            { "3" = "www3.example.com" }
>        }
>    }
>
>    let empty = ""
>
>    test Gmetad.lns get empty = ?
>
>

An empty (or non existent) file is not an empty string, but a single
carriage return. The test should thus be:

test Gmetad.lns get "\n" = ?

which fails since the lens cannot parse a single carriage return. This is
the reason why it fails in augtool.


Doesn't this format not support empty lines? If it does, I'd advise to use
Util.eol in gridname and data_source, and write lns as:

let lns = (gridname | data_source | Util.empty)*

or does the order strictly matter?


Raphaël
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