BIND 10 #3002: extend datasrc::(Configurable)ClientList::find to return data source name

[BIND 10 Development]

#3002: extend datasrc::(Configurable)ClientList::find to return data source name
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  New
           Component:  data source   |  Tasks
            Keywords:                |                   Resolution:
           Sensitive:  0             |                 CVSS Scoring:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |  shared memory data source
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by jinmei):

 Replying to [comment:1 vorner]:
 > Isn't the name stored as a string directly in the client list somewhere?
 So if we returned just const reference to it, it should be safe to use
 that as long as we don't reconfigure the client list and it would be cheap
 operation, so we wouldn't need the boolean flag and could return it every
 time. It could be simpler interface (and backwards compatible).

 The data source name is stored in `DataSourceInfo`
 (ref.
 http://bind10.isc.org/attachment/wiki/DataSourceClasses/overview.png).

 I'm not sure what exactly you meant above, but from that I guess one
 other approach is to include `DataSourceInfo` in `ClientList::FindResult`
 as a private member, and provide accessor(s) to some selected members
 such as data_src_client_ or name_ (although I'd like to hide the
 existence of `DataSourceInfo` as much as possible even as a private
 member of some other class, and in that sense this approach is not
 very clean).  With this approach we'll probably need to maintain
 `DataSourceInfo` as a shared pointer in the vector, and this will
 eliminate the need for the `LifeKeeper` trick.

 Yet another approach is to simply include the data source name
 unconditionally.  It's a bit suboptimal in terms of efficiency
 especially because the data source name won't be needed in many cases
 (and especially not in performance sensitive cases).  But copying
 std::string should be reasonably cheap for its common implementations
 using a reference counter, and since `ClientList::find()` is called
 only once in the entire query processing, this may be a reasonable
 compromise.

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Ticket URL: <https://bind10.isc.org/ticket/3002#comment:2>
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