[augeas-devel] Lens Help: disambiguating 1] regex difference 2] overlapping lenses in union.put
Raphaël Pinson
raphael.pinson at camptocamp.com
Mon Jan 11 08:17:54 UTC 2016
Yes Xavier, this is a good (and useful) explanation.
It would be great if that could end up on a wiki page actually.
Raphaël
On Fri, Jan 8, 2016 at 2:24 PM, Mol, Xavier (SCC) <xavier.mol at kit.edu>
wrote:
> Hi Yclept,
>
>
>
> this is a late reply and you possibly don't need it anymore, but I think I
> have finally understood the issue regarding "overlapping lenses in
> union.put", so I want to share my experience with you.
>
>
>
> As you and David explained, Augeas has trouble when putting a tree into a
> file that (potentially) has nodes, which can be matched by more than one
> lens. This problem can be avoided in get direction, because parent nodes
> might be distinguishable in some way. We can make use of that key
> difference, by giving alternate node options to Augeas (as opposed to
> alternate expressions).
>
>
>
> Say we have a tree
>
>
>
> /a
>
> /a/color = "blue"
>
> /b
>
> /b/color = "red"
>
>
>
> Depending on the lens definitions for nodes "a" and "b", the "color"
> subnode might have to be read/written differently. Yet just from looking at
> the tree, Augeas cannot distinguish between either "color" node definition.
> That is what David said in his mail: "when Augeas sees a lens construct
> like 'l1|l2' it only looks at the labels of the tree node it's currently at
> to decide whether to use l1 or l2". In this situation, the label is
> identical for both nodes, hence the conflict.
>
>
>
> How can we avoid this problem with actually applying ugly hacks? We change
> the construct such that Augeas has to consider the depth of the tree.
> Without concrete examples, this will be difficult to understand, so lets
> expand on the above example with these lens definitions:
>
>
>
> let a = key "a" . [ label "color" . store /red|blue|green|yellow/ ]
>
> let b = key "b" . [ label "color" . del /dark-/ "dark-" . store
> /red|blue|green|yellow/ ]
>
> let lns = [ a | b ]
>
>
>
> So for "b"-nodes color strings are prefixed with "dark-". When parsing a
> source file, Augeas can clearly distinguish between when to apply lenses
> "a" or "b". But because in either case the resulting "color"-node is
> identical, saving the tree to a file fails. We make Augeas aware of the
> difference on node-level, by alternating between nodes:
>
>
>
> let lns = [ a ]|[ b ]
>
>
>
>
>
> I hope this was somewhat understandable and helpful to you (or others
> finding this mail with Google J ).
>
>
>
> Ciao,
>
> Xavier.
>
>
>
>
>
>
>
> *From:* augeas-devel-bounces at redhat.com [mailto:
> augeas-devel-bounces at redhat.com] *On Behalf Of *Yclept Nemo
> *Sent:* Friday, October 23, 2015 7:07 AM
> *To:* augeas-devel
> *Subject:* [augeas-devel] Lens Help: disambiguating 1] regex difference
> 2] overlapping lenses in union.put
>
>
>
> Hi,
>
> I'm working on a lens that has two errors:
>
> >> 1 (*
> >> 2 # example configuration follows:
> >> 3 # default = value
> >> 4 key = value
> >> 5
> >> 6 # default = value # inline comment
> >> 7 key = value # inline comment
> >> 8 *)
> >> 9
> >> 10 (*
> >> 11 # key/value formats:
> >> 12
> >> 13 keys
> >> 14 [a-z_]
> >> 15
> >> 16 values
> >> 17 boolean: true/false
> >> 18 int 0, 100
> >> 19 real 3.0
> >> 20 hex int ffeedd
> >> 21 string "..."
> >> 22 unquoted string sdl, f, f2
> >> 23 *)
> >> 24
> >> 25 (*
> >> 26 # quoted string regex scratchpad
> >> 27 quoted string regex =
> >> 28 good:
> >> 29 "asdf\"qwer"
> >> 30 "asdf\\qwer"
> >> 31 "asdf\\\\\"qwer"
> >> 32 bad:
> >> 33 "asdf\\"qwer"
> >> 34 "asdf\\\\\\"qwer"
> >> 35
> >> 36 /.*/ - /.*(\\)*[^\]".*/
> >> 37
> >> 38 /([^"](\")?)*/
> >> 39 *)
> >> 40
> >> 41 module ConfigObj =
> >> 42 let _ = print_regexp /"/
> >> 43 let _ = print_string "\n"
> >> 44 let var0 = "\""
> >> 45 let _ = print_string (var0 . "\n")
> >> 46 let _ = print_string "\n"
> >> 47
> >> 48 let var1 = "\"asdf\\\\\\\\"qwer\""
> >> 49 let _ = print_string (var1 . "\n")
> >> 50 let _ = print_string ((regexp_match (/\".*\"/ -
> /\"(.*[^\])?([\]{2})*".*\"/) var1) . "\n")
> >> 51
> >> 52 let option = key /[a-z]([a-z_]*[a-z])?/
> >> 53 let value_bool = store /(true|false)/ . [ label "type" .
> value "bool" ]
> >> 54 let value_int = store /(0|[1-9][0-9]*)/ . [ label "type"
> . value "int" ]
> >> 55 let value_decimal = store ((lens_ctype value_int) .
> /\.[0-9]+/) . [ label "type" . value "decimal" ]
> >> 56 let value_string = (del "\"" "\"") . (store (/.*/ -
> /(.*[^\]|)([\]{2})*".*/)) . (del "\"" "\"") . [ label "type" . value
> "string" ]
> >> 57 let value_all = value_bool | value_int | value_decimal
> >> 58 let sep_assign = del /[ \t]*=[ \t]*/ " = "
> >> 59 let sep_comment = del /[ \t]*#[ \t]*/ " # "
> >> 60 let record_comment = [ label "comment" . sep_comment . store
> /([^ \t\n].*)?/ ]
> >> 61 let record = option . sep_assign . value_all .
> record_comment?
> >> 62 let record_not = label "comment" . store (/([^ \t\n].*)?/
> - lens_ctype record)
> >> 63 let comment = del /#[ \t]*/ "# " . ( [ record . [ label
> "commented" ] ] | [ record_not ] )
> >> 64 let line = del /[ \t]*/ "" . ( comment | [ record ]
> ) . del "\n" "\n"
> >> 65 let lns = line *
>
> The first is a regex problem: I want to match double-quoted c-style
> strings, ie strings with escape sequences such as "\"", "\\\"" and so on.
> The regex I've crafted (line#50) evidently fits the bill - try modifying
> the input test values (line#48) to verify - so I don't understand why the
> derived lens (line#56) is failing, and thereby causing this "ambiguous
> concatenation" error:
>
> >> Syntax error in lens definition
> >> configobj.aug:61.4-.75:Failed to compile record
> >> configobj.aug:61.26-.75:exception: ambiguous concatenation
> >> First regexp: /([a-z]([a-z_]*[a-z])?)([ \t]*=[
> \t]*)((((true|false)))|(((0|[1-9][0-9]*)))|(((((0|[1-9][0-9]*)))(
> \\.[0-9]+)))|((")(([^\n"\\][^\n"\\]*\\\\|\\\\)([^\n\\][^\n"\\]*\\\\)*\\\\(([^\n"\\][^\n"\\]*\\\\|\\\\)([^\n\\][^\n"\\]*\\\\)*\\\\)*(([^\n"\\][^\n"\\]*\\\\|\\\\)([^\n\\][^\n"\\]*\\\\)*([^\n\\][^\n"\\]*|)|[^\n"\\][^\n"\\]*|)|([^\n"\\][^\n"\\]*\\\\|\\\\)([^\n\\][^\n"\\]*\\\\)*([^\n\\][^\n"\\]*|)|[^\n"\\][^\n"\\]*|)(")))/
> >> Second regexp: /(([ \t]*#[ \t]*)(([^ \t\n].*)?))?/
> >> 'a="\\"#"' can be split into
> >> 'a="\\"|=|#"'
> >>
> >> and
> >> 'a="\\"#"|=|' <----- not possible per the regex
> >>
> >> First lens: configobj.aug:61.26-.57:
> >> Second lens: configobj.aug:61.60-.75:
>
> The second problem - you'll see if the first is fixed - is a limitation of
> augeas' tree-text conversion, the inability to differentiate trees on
> depth. This generates the error "exception: overlapping lenses in tree
> union.put" and crops up in two locations:
>
> '[ record . [ label "commented" ] ]' (line#63) ----vs---- '[ record_not
> ]' (line#63)
> ::: The "record_not" label of "comment" is also a possible valid label
> of the "record" lens [line#52]. Hacky workaround is to change the label to
> an invalid key, ie "comment2", etc.
> '[ record . [ label "commented" ] ]' (line#63) ----vs---- '[ record ]'
> (line#64)
> ::: Unlike previous, no hacky workaround.
>
> Obviously this can be fixed by scanning the *depth* of the tree. In both
> cases, one branch is guaranteed to have a subnode with the label
> "commented" unlike the other. While augeas doesn't implement this, there
> was a suggestion on the mailing list regarding a lens-based workaround[1]
> that I didn't understand. I'd appreciate if someone could provide a
> concrete example for my case.
>
>
> Sincerely,
>
>
> [1] https://www.redhat.com/archives/augeas-devel/2009-June/msg00088.html
>
> _______________________________________________
> augeas-devel mailing list
> augeas-devel at redhat.com
> https://www.redhat.com/mailman/listinfo/augeas-devel
>
--
Raphaël Pinson
Infrastructure Developer & Training Leader
+33 458 482 013
Camptocamp France
Savoie Technolac
BP 352
48, avenue du Lac du Bourget
73372 Le Bourget du Lac, Cedex
www.camptocamp.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/augeas-devel/attachments/20160111/bc857d72/attachment.htm>
More information about the augeas-devel
mailing list