order of begin, prepare, and commit of three concurrent transactions that
have conflicts between them.
The test runs for a quite long time, and the expected output file is huge,
but this test caught some serious bugs during development, so seems
worthwhile to keep. The test uses prepared transactions, so it fails if the
server has max_prepared_transactions=0. Because of that, it's marked as
"ignore" in the schedule file.
Dan Ports
The previous value of 20ms is dangerously close to the time actually
spent just waiting for the deadlock to happen, so on occasion it causes
the test to fail simply because the other session didn't get to run
early enough, not managing to cause the deadlock that needs to be
detected. With this new value, it's expected that most machines on
normal load will be able to pass the test.
Author: Noah Misch
This enables us to test that blocking commands (such as foreign keys
checks that conflict with some other lock) act as intended. The set of
tests that this adds is pretty minimal, but can easily be extended by
adding new specs.
The intention is that this will serve as a basis for ensuring that
further tweaks of locking implementation preserve (or improve) existing
behavior.
Author: Noah Misch
On further analysis, it turns out that it is not needed to duplicate predicate
locks to the new row version at update, the lock on the version that the
transaction saw as visible is enough. However, there was a different bug in
the code that checks for dangerous structures when a new rw-conflict happens.
Fix that bug, and remove all the row-version chaining related code.
Kevin Grittner & Dan Ports, with some comment editorialization by me.
Until now, our Serializable mode has in fact been what's called Snapshot
Isolation, which allows some anomalies that could not occur in any
serialized ordering of the transactions. This patch fixes that using a
method called Serializable Snapshot Isolation, based on research papers by
Michael J. Cahill (see README-SSI for full references). In Serializable
Snapshot Isolation, transactions run like they do in Snapshot Isolation,
but a predicate lock manager observes the reads and writes performed and
aborts transactions if it detects that an anomaly might occur. This method
produces some false positives, ie. it sometimes aborts transactions even
though there is no anomaly.
To track reads we implement predicate locking, see storage/lmgr/predicate.c.
Whenever a tuple is read, a predicate lock is acquired on the tuple. Shared
memory is finite, so when a transaction takes many tuple-level locks on a
page, the locks are promoted to a single page-level lock, and further to a
single relation level lock if necessary. To lock key values with no matching
tuple, a sequential scan always takes a relation-level lock, and an index
scan acquires a page-level lock that covers the search key, whether or not
there are any matching keys at the moment.
A predicate lock doesn't conflict with any regular locks or with another
predicate locks in the normal sense. They're only used by the predicate lock
manager to detect the danger of anomalies. Only serializable transactions
participate in predicate locking, so there should be no extra overhead for
for other transactions.
Predicate locks can't be released at commit, but must be remembered until
all the transactions that overlapped with it have completed. That means that
we need to remember an unbounded amount of predicate locks, so we apply a
lossy but conservative method of tracking locks for committed transactions.
If we run short of shared memory, we overflow to a new "pg_serial" SLRU
pool.
We don't currently allow Serializable transactions in Hot Standby mode.
That would be hard, because even read-only transactions can cause anomalies
that wouldn't otherwise occur.
Serializable isolation mode now means the new fully serializable level.
Repeatable Read gives you the old Snapshot Isolation level that we have
always had.
Kevin Grittner and Dan Ports, reviewed by Jeff Davis, Heikki Linnakangas and
Anssi Kääriäinen
