pgsql: Fix possible "tuple concurrently updated" error in ALTER TABLE.

Fix possible "tuple concurrently updated" error in ALTER TABLE.

When adding an inheritance parent to a table, an AccessShareLock on the
parent isn't strong enough to prevent trouble, so take
ShareUpdateExclusiveLock instead.  Since this is a behavior change,
albeit a fairly unobtrusive one, and since we have only one report
from the field, no back-patch.

Report by Jon Nelson, analysis by Alvaro Herrera, fix by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/fbcf4b92aa64d4577bcf25925b055316b978744a

Modified Files
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src/backend/commands/tablecmds.c |   15 +++++++++++----
1 files changed, 11 insertions(+), 4 deletions(-)