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Overdue code review for transaction-level advisory locks patch.

Browse Source 
Commit 62c7bd31c8 had assorted problems, most
visibly that it broke PREPARE TRANSACTION in the presence of session-level
advisory locks (which should be ignored by PREPARE), as per a recent
complaint from Stephen Rees.  More abstractly, the patch made the
LockMethodData.transactional flag not merely useless but outright
dangerous, because in point of fact that flag no longer tells you anything
at all about whether a lock is held transactionally.  This fix therefore
removes that flag altogether.  We now rely entirely on the convention
already in use in lock.c that transactional lock holds must be owned by
some ResourceOwner, while session holds are never so owned.  Setting the
locallock struct's owner link to NULL thus denotes a session hold, and
there is no redundant marker for that.

PREPARE TRANSACTION now works again when there are session-level advisory
locks, and it is also able to transfer transactional advisory locks to the
prepared transaction, but for implementation reasons it throws an error if
we hold both types of lock on a single lockable object.  Perhaps it will be
worth improving that someday.

Assorted other minor cleanup and documentation editing, as well.

Back-patch to 9.1, except that in the 9.1 branch I did not remove the
LockMethodData.transactional flag for fear of causing an ABI break for
any external code that might be examining those structs.
This commit is contained in:
Tom Lane
2012-05-04 17:43:27 -04:00
parent 1715ff1128
commit 71b9549d05
6 changed files with 266 additions and 208 deletions
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67
doc/src/sgml/func.sgml
View File

@ -15382,7 +15382,7 @@ SELECT (pg_stat_file('filename')).modification;
<literal><function>pg_advisory_xact_lock_shared(<parameter>key1</> <type>int</>, <parameter>key2</> <type>int</>)</function></literal>
</entry>
<entry><type>void</type></entry>
<entry>Obtain shared advisory lock for the current transaction</entry>
<entry>Obtain shared transaction level advisory lock</entry>
</row>
<row>
<entry>
@ -15451,11 +15451,10 @@ SELECT (pg_stat_file('filename')).modification;
<function>pg_advisory_lock</> locks an application-defined resource,
which can be identified either by a single 64-bit key value or two
32-bit key values (note that these two key spaces do not overlap).
The key type is specified in <literal>pg_locks.objid</>. If
another session already holds a lock on the same resource, the
function will wait until the resource becomes available. The lock
If another session already holds a lock on the same resource identifier,
this function will wait until the resource becomes available. The lock
is exclusive. Multiple lock requests stack, so that if the same resource
is locked three times it must be also unlocked three times to be
is locked three times it must then be unlocked three times to be
released for other sessions' use.
</para>
@ -15489,6 +15488,35 @@ SELECT (pg_stat_file('filename')).modification;
a shared rather than an exclusive lock.
</para>
<indexterm>
<primary>pg_advisory_unlock</primary>
</indexterm>
<para>
<function>pg_advisory_unlock</> will release a previously-acquired
exclusive session level advisory lock. It
returns <literal>true</> if the lock is successfully released.
If the lock was not held, it will return <literal>false</>,
and in addition, an SQL warning will be reported by the server.
</para>
<indexterm>
<primary>pg_advisory_unlock_shared</primary>
</indexterm>
<para>
<function>pg_advisory_unlock_shared</> works the same as
<function>pg_advisory_unlock</>,
except it releases a shared session level advisory lock.
</para>
<indexterm>
<primary>pg_advisory_unlock_all</primary>
</indexterm>
<para>
<function>pg_advisory_unlock_all</> will release all session level advisory
locks held by the current session. (This function is implicitly invoked
at session end, even if the client disconnects ungracefully.)
</para>
<indexterm>
<primary>pg_advisory_xact_lock</primary>
</indexterm>
@ -15527,35 +15555,6 @@ SELECT (pg_stat_file('filename')).modification;
cannot be released explicitly.
</para>
<indexterm>
<primary>pg_advisory_unlock</primary>
</indexterm>
<para>
<function>pg_advisory_unlock</> will release a previously-acquired
exclusive session level advisory lock. It
returns <literal>true</> if the lock is successfully released.
If the lock was not held, it will return <literal>false</>,
and in addition, an SQL warning will be raised by the server.
</para>
<indexterm>
<primary>pg_advisory_unlock_shared</primary>
</indexterm>
<para>
<function>pg_advisory_unlock_shared</> works the same as
<function>pg_advisory_unlock</>,
except it releases a shared session level advisory lock.
</para>
<indexterm>
<primary>pg_advisory_unlock_all</primary>
</indexterm>
<para>
<function>pg_advisory_unlock_all</> will release all session level advisory
locks held by the current session. (This function is implicitly invoked
at session end, even if the client disconnects ungracefully.)
</para>
</sect2>
</sect1>

80
doc/src/sgml/mvcc.sgml
View File

@ -1207,6 +1207,10 @@ UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 22222;
<sect2 id="advisory-locks">
<title>Advisory Locks</title>
<indexterm zone="advisory-locks">
<primary>advisory lock</primary>
</indexterm>
<indexterm zone="advisory-locks">
<primary>lock</primary>
<secondary>advisory</secondary>
@ -1218,35 +1222,51 @@ UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 22222;
called <firstterm>advisory locks</>, because the system does not
enforce their use &mdash; it is up to the application to use them
correctly. Advisory locks can be useful for locking strategies
that are an awkward fit for the MVCC model.</para>
<para>
There are two different types of advisory locks in
<productname>PostgreSQL</productname>: session level and transaction level.
Once acquired, a session level advisory lock is held until explicitly
released or the session ends. Unlike standard locks, session level
advisory locks do not honor transaction semantics: a lock acquired during
a transaction that is later rolled back will still be held following the
rollback, and likewise an unlock is effective even if the calling
transaction fails later. The same session level lock can be acquired
multiple times by its owning process: for each lock request there must be
a corresponding unlock request before the lock is actually released. (If a
session already holds a given lock, additional requests will always succeed,
even if other sessions are awaiting the lock.) Transaction level locks on
the other hand behave more like regular locks; they are automatically
released at the end of the transaction, and can not be explicitly unlocked.
Session and transaction level locks share the same lock space, which means
that a transaction level lock will prevent another session from obtaining
a session level lock on that same resource and vice versa.
Like all locks in <productname>PostgreSQL</productname>, a complete list of
advisory locks currently held by any session can be found in the
<link linkend="view-pg-locks"><structname>pg_locks</structname></link>
system view.
that are an awkward fit for the MVCC model.
For example, a common use of advisory locks is to emulate pessimistic
locking strategies typical of so called <quote>flat file</> data
management systems.
While a flag stored in a table could be used for the same purpose,
advisory locks are faster, avoid table bloat, and are automatically
cleaned up by the server at the end of the session.
</para>
<para>
Advisory locks are allocated out of a shared memory pool whose size
is defined by the configuration variables
There are two ways to acquire an advisory lock in
<productname>PostgreSQL</productname>: at session level or at
transaction level.
Once acquired at session level, an advisory lock is held until
explicitly released or the session ends. Unlike standard lock requests,
session-level advisory lock requests do not honor transaction semantics:
a lock acquired during a transaction that is later rolled back will still
be held following the rollback, and likewise an unlock is effective even
if the calling transaction fails later. A lock can be acquired multiple
times by its owning process; for each completed lock request there must
be a corresponding unlock request before the lock is actually released.
Transaction-level lock requests, on the other hand, behave more like
regular lock requests: they are automatically released at the end of the
transaction, and there is no explicit unlock operation. This behavior
is often more convenient than the session-level behavior for short-term
usage of an advisory lock.
Session-level and transaction-level lock requests for the same advisory
lock identifier will block each other in the expected way.
If a session already holds a given advisory lock, additional requests by
it will always succeed, even if other sessions are awaiting the lock; this
statement is true regardless of whether the existing lock hold and new
request are at session level or transaction level.
</para>
<para>
Like all locks in
<productname>PostgreSQL</productname>, a complete list of advisory locks
currently held by any session can be found in the <link
linkend="view-pg-locks"><structname>pg_locks</structname></link> system
view.
</para>
<para>
Both advisory locks and regular locks are stored in a shared memory
pool whose size is defined by the configuration variables
<xref linkend="guc-max-locks-per-transaction"> and
<xref linkend="guc-max-connections">.
Care must be taken not to exhaust this
@ -1257,13 +1277,7 @@ UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 22222;
</para>
<para>
A common use of advisory locks is to emulate pessimistic locking
strategies typical of so called <quote>flat file</> data management
systems.
While a flag stored in a table could be used for the same purpose,
advisory locks are faster, avoid MVCC bloat, and can be automatically
cleaned up by the server at the end of the session.
In certain cases using this advisory locking method, especially in queries
In certain cases using advisory locking methods, especially in queries
involving explicit ordering and <literal>LIMIT</> clauses, care must be
taken to control the locks acquired because of the order in which SQL
expressions are evaluated. For example: