pgsql: Improve ilist.h's support for deletion of slist elements during

Improve ilist.h's support for deletion of slist elements during iteration.

Previously one had to use slist_delete(), implying an additional scan of
the list, making this infrastructure considerably less efficient than
traditional Lists when deletion of element(s) in a long list is needed.
Modify the slist_foreach_modify() macro to support deleting the current
element in O(1) time, by keeping a "prev" pointer in addition to "cur"
and "next".  Although this makes iteration with this macro a bit slower,
no real harm is done, since in any scenario where you're not going to
delete the current list element you might as well just use slist_foreach
instead.  Improve the comments about when to use each macro.

Back-patch to 9.3 so that we'll have consistent semantics in all branches
that provide ilist.h.  Note this is an ABI break for callers of
slist_foreach_modify().

Andres Freund and Tom Lane

Branch
------
REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/cf707aa70da6729a00593c49d5eb8a35ab27dd11

Modified Files
--------------
src/backend/lib/ilist.c         |    2 +-
src/backend/postmaster/pgstat.c |    7 +++++
src/include/lib/ilist.h         |   62 ++++++++++++++++++++++++++++++++-------
3 files changed, 59 insertions(+), 12 deletions(-)