BIND 10 #2974: Implement Hooks Framework (part 1) - all but library loading and unloading (was: Implement Hooks Framework)

[BIND 10 Development]

#2974: Implement Hooks Framework (part 1) - all but library loading and unloading
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            Reporter:  stephen       |                        Owner:
                Type:  task          |  UnAssigned
            Priority:  medium        |                       Status:
           Component:  Unclassified  |  reviewing
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-DHCP-20130606
         Sub-Project:  DHCP          |                   Resolution:
Estimated Difficulty:  0             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Changes (by stephen):

 * owner:  stephen => UnAssigned
 * status:  accepted => reviewing


Old description:

> Implement the hooks framework as per the design in
> http://bind10.isc.org/wiki/Bind10HooksFramework

New description:

 Implement the hooks framework as per the design in
 http://bind10.isc.org/wiki/Bind10HooksFramework.

 This ticket only covers the implementation of the data structures for
 argument passing, context and calling callouts: it does not cover the
 loading and unloading of the libraries.

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Comment:

 Now ready for review, as is the (updated) design on #2938.

 One design issue that I came across just before putting this in for review
 and on which I would like some feedback concerns the context_create and
 context_destroy hooks.  These are called to create and destroy per-packet
 context.  At the moment, the calls are made in the !CalloutHandle's
 constructor and destructor respectively.  As currently coded, a callout
 failure causes an exception to be thrown.

 The following issues need to be considered.

 1. The current configuration is good in that the server does not need to
 worry about calling the creation and destruction callouts.  This means
 that !CalloutHandles can be created and destroyed without worrying about
 not allocating or releasing context.  However, if an exception is thrown
 in the constructor or destructor, we may end up with a boost::shared_ptr
 pointing to a half-created or half-destroyed object.  This may be
 difficult to recover (as calling a reset() on the shared_ptr will cause it
 to try destroying the pointed-to object).

 2. We can just ignore failures in the context_create and context_destroy
 callouts.  This eliminates the problem above, but means that a failure is
 difficult to detect and may lead to obscure problems later on.

 3. We can call context_create and context_destroy outside the
 !CalloutHandle.  The former is not a problem - we can have a
 !CalloutHandle factory object create the !CalloutHandles in a single place
 and call the context_create hook right after creation.  The latter is more
 an issue - if the !CalloutHandle is destroyed in multiple places (possibly
 by the last shared_ptr to it being reset), the context_destroy callback
 may not get called and we have a resource leak.

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Ticket URL: <http://bind10.isc.org/ticket/2974#comment:2>
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