pg_dump lock timeout

Attached is a patch to add a commandline option to pg_dump to limit how long
pg_dump will wait for locks during startup.

The intent of this patch is to allow pg_dump to fail if a table lock cannot
be taken in a reasonable time. This allows the caller of pg_dump to retry or
otherwise correct the situation, without having locks held for long periods,
and without pg_dump having a long window during which catalog changes can
occur.

It works by setting statement_timeout to the user specified delay during
the startup phase where it is taking access share locks on all the tables.
Once all the locks are taken, it sets statement_timeout back to the default.
If a lock table statement times out, the dump fails with the statement timed
out error.

The orginal motivation was a client who runs heavy batch workloads and uses
truncate table and other DML in long transactions. This has created some
unhappy interaction scenarios with pg_dump:

- pg_dump ends up waiting hours on a DML table lock that is part of a long
transaction. Once the lock is released, pg_dump runs only to find
some table later in the list has been dropped. So pg_dump fails.

- pg_dump waits on a lock while holding access share locks on most of the
tables. Other processes that want to do DML wait on pg_dump. After a
while, large parts of the application are blocked while pg_dump waits
on locks. Eventually the operations staff notice that pg_dump is
blocking production and kill the dump.

Please have a look and consider it for merging.

Thanks

-dg

--
David Gould
If simplicity worked, the world would be overrun with insects.