pgsql: Add a crude facility for dealing with relative pointers.

Add a crude facility for dealing with relative pointers.

C doesn't have any sort of built-in understanding of a pointer
relative to some arbitrary base address, but dynamic shared memory
segments can be mapped at different addresses in different processes,
so any sort of shared data structure stored within a dynamic shared
memory segment can't use absolute pointers. We could use something
like Size to represent a relative pointer, but then the compiler
provides no type-checking. Use stupid macro tricks to get some
type-checking.

Patch originally by me. Concept suggested by Andres Freund. Recently
resubmitted as part of Thomas Munro's work on dynamic shared memory
allocation.

Discussion: 20131205144434(dot)GG12398(at)alap2(dot)anarazel(dot)de
Discussion: CAEepm=1z5WLuNoJ80PaCvz6EtG9dN0j-KuHcHtU6QEfcPP5-qA(at)mail(dot)gmail(dot)com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/fbc1c12a94a638cf4f577fef158175e22ab824a3

Modified Files
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src/include/utils/relptr.h | 74 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 74 insertions(+)