pgpool: Fix disable_load_balance_on_write to not break query cache.

Fix disable_load_balance_on_write to not break query cache.

The disable_load_balance_on_write accepts four options:

transaction (the default)
trans_transaction
dml_adaptive
always

It appeared that except "transaction", all other options break query
cache feature. Sometimes a query result is cached even there's a write
query in a transaction, sometimes query is not cached even when it
should be.

The query cache relies on is_writing_transaction of session context to
judge whether cache can be safely used. However,
disable_load_balance_on_write overrides it to true when it should not,
and vice versa for its own purpose. To fix this new session context
variable "really_writing_transaction" is introduced. It is almost same
as existing writing_transaction, but it faithfully tracks whether a
writing query appears in an explicit transaction. The query cache uses
it instead of writing_transaction variable.

Add test cases for disable_load_balance_on_write =
"trans_transaction", "dml_adaptive" and "always" to 006.memqcahce
regression test. Note that "transaction" is the default and already
tested in "s" mode. So it's not necessary to add a test for the case.

Author: Tatsuo Ishii <ishii(at)postgresql(dot)org>
Discussion:
Backpatch-through: v4.3

Branch
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V4_6_STABLE

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=af256cadbebc228e6309e4a76f517ebbb0a6a410

Modified Files
--------------
src/context/pool_session_context.c | 22 ++++++++++++++++++++++
src/include/context/pool_session_context.h | 19 ++++++++++++++++++-
src/protocol/CommandComplete.c | 1 +
src/protocol/pool_process_query.c | 1 +
src/protocol/pool_proto_modules.c | 17 +++++++++++++----
src/test/regression/tests/006.memqcache/test.sh | 19 ++++++++++++++++++-
6 files changed, 73 insertions(+), 6 deletions(-)