Retrieving last modified node path (Was: [augeas-devel] First tests with aug mv)

[David Lutterkort]

On Fri, 2008-08-01 at 11:14 +0200, Raphaël Pinson wrote:


> After talking with David on IRC, I now understand that modifying
> tree_insert to return the path to the created node would mean to
> modify the API, which is not acceptable. David proposes another
> approach, which would be to store the path to this new node
> in /augeas/last_created_node.

Thanks for bringing this up on the list - definitely better to discuss
this here. The only change I would make to what we talked on IRC is to
put that info a little deeper in the tree, like put it
at /augeas/tree/last_created (mostly for cosmetic reasons)

> After thinking about this, I think a /augeas/last_modified_node would
> actually be more useful, although the most obvious use of it would be
> for insert.
>
> With insert, this is an obvious need :
> aug_ins("field", "after", path) does not give me any information on
> the node that was created, so I have no clue on what path to pass to
> aug_set or aug_rm if I want to set or remove this node.
> 
> On the other hand, 'aug_set("/files/etc/my/file/field[100]", "value")'
> or 'aug_set("/files/etc/my/file/field[last()]", "value")' do not give
> me any information either on the exact path to the node I just
> modified, but contrarily to aug_insert, I can still refer to this
> modified node as "/files/etc/my/file/field[100]" or
> "/files/etc/my/file/field[last()]".
> 
> I think it could be interesting to have a /augeas/last_modified_node
> where every function (whether insert, set, rm, mv, etc.) would write
> the name of the last modified node, so that the user can retrieve it
> to reuse it:

I like that idea a lot. Just thinking out loud, here's what a call to
aug_get(aug, "/augeas/tree/last_modified", &p) after each API call would
set teh variable p to:

   Previous call    p (the modfied path)
        aug_init     - NULL
        aug_get      - no influence on value
        of /augeas/tree/last_modified
        aug_set      - PATH passed to aug_set, fully qualified
        aug_insert   - fully qualified path to newly created node
        aug_rm       - PATH passed to aug_rm
        aug_mv       - PATH passed to aug_mv as DST argument
        aug_match    - no influence on value of /augeas/tree/last_modified
        aug_save     - either leave /augeas/tree/last_modified unchanged or set to NULL
        aug_print    - no influence on value of /augeas/tree/last_modified
        
'Fully qualified path' here means an absolute path like /a/b[3]/c[5]/d -
note that the node referenced by that may still change, for example if
you do 'ins b before /a/b[3]'. If we need a way to reference nodes by a
fixed path across such operations, we'll probably need a way to pass
pointers to such nodes through the API.

Some more notes:
      * you can use the return value of an aug_get on
        "/augeas/tree/last_modified" right after aug_init to see if that
        feature is supported in your version of Augeas - return of '0'
        means no, return of 1 means 'yes'
      * the path expression passed to aug_rm can actually specify more
        than one node (like 'rm /files/etc/hosts/*')
      * you still can't tell whether aug_set actually created a node or
        modified an existing node - that could be fixed by changing the
        return value of aug_set, e.g. return 0 when an already existing
        node was modified, and 1 when the node had to be created first
      * should aug_save clear /augeas/tree/last_modified or leave it
        alone ?
      * Looking at the table above, I don't think a last_modified entry
        would buy us much over a last_created entry; the last_created
        entry would only be changed by aug_set, aug_mv and aug_insert
        and would be guaranteed to always refer to exactly one node

David