[Xapian-tickets] [Xapian] #788: Thread safe move semantics

[Xapian]

#788: Thread safe move semantics
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        Reporter:  Kronuz  |      Owner:  olly
            Type:  defect  |     Status:  new
        Priority:  high    |  Milestone:
       Component:  Other   |    Version:
        Severity:  normal  |   Keywords:
      Blocked By:          |   Blocking:
Operating System:  All     |
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 When using move semantics (using `std::move`), not all objects are really
 moved. For example, `Document` and `Database`; instead use
 `intrusive_ptr_nonnull`'s move constructor which essentially just
 increments the reference counter and copies the pointer `px`. This doesn't
 cut it when one needs to move objects from one thread to other; reason is
 this produces a race condition in the reference counter, in which both
 threads could be modifying the non-atomic counter simultaneously.

 What I did instead (in Xapiand) is I'm using a so called
 `internal_intrusive_ptr` which ensures an object with an "internal" type
 still remains valid (and with a valid, non-null `px`) after moving away
 it's original content. Implementation uses a `tmp`, from which it extracts
 the newly created "empty" internal `px` and can be seen here:
 https://github.com/Kronuz/Xapiand/commit/33ccddbdaa1e3a24a68700af24cf3c7562f43621
 #diff-bedbbf7159a3d537faadbb10befc15ceR375

 This `internal_intrusive_ptr` could be used everywhere where
 `intrusive_ptr_nonnull` is used ''if'' the object in question has a
 default constructor, such as:

 * `Database`
 * `Document`
 * `ESet`
 * `MSet`
 * `QueryParser`
 * `Registry`
 * `TermGenerator`

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Ticket URL: <https://trac.xapian.org/ticket/788>
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