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Fix ALTER SEQUENCE locking 

In 1753b1b027, the pg_sequence system
catalog was introduced.  This made sequence metadata changes
transactional, while the actual sequence values are still behaving
nontransactionally.  This requires some refinement in how ALTER
SEQUENCE, which operates on both, locks the sequence and the catalog.

The main problems were:

- Concurrent ALTER SEQUENCE causes "tuple concurrently updated" error,
  caused by updates to pg_sequence catalog.

- Sequence WAL writes and catalog updates are not protected by same
  lock, which could lead to inconsistent recovery order.

- nextval() disregarding uncommitted ALTER SEQUENCE changes.

To fix, nextval() and friends now lock the sequence using
RowExclusiveLock instead of AccessShareLock.  ALTER SEQUENCE locks the
sequence using ShareRowExclusiveLock.  This means that nextval() and
ALTER SEQUENCE block each other, and ALTER SEQUENCE on the same sequence
blocks itself.  (This was already the case previously for the OWNER TO,
RENAME, and SET SCHEMA variants.)  Also, rearrange some code so that the
entire AlterSequence is protected by the lock on the sequence.

As an exception, use reduced locking for ALTER SEQUENCE ... RESTART.
Since that is basically a setval(), it does not require the full locking
of other ALTER SEQUENCE actions.  So check whether we are only running a
RESTART and run with less locking if so.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
Reported-by: Jason Petersen <jason@citusdata.com>
Reported-by: Andres Freund <andres@anarazel.de>
This commit is contained in:
Peter Eisentraut 2017-05-09 23:35:31 -04:00
parent b1c45afb01
commit f8dc1985fd
5 changed files with 180 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
doc/src/sgml/ref/alter_sequence.sgml
View File

@ -304,6 +304,13 @@ ALTER SEQUENCE [ IF EXISTS ] <replaceable class="parameter">name</replaceable> S
8.3, it sometimes did.)
</para>
<para>
<command>ALTER SEQUENCE</command> blocks
concurrent <function>nextval</function>, <function>currval</function>,
<function>lastval</function>, and <command>setval</command> calls, except
if only the <literal>RESTART</literal> clause is used.
</para>
<para>
For historical reasons, <command>ALTER TABLE</command> can be used with
sequences too; but the only variants of <command>ALTER TABLE</command>

69
src/backend/commands/sequence.c
View File

@ -93,11 +93,12 @@ static HTAB *seqhashtab = NULL; /* hash table for SeqTable items */
static SeqTableData *last_used_seq = NULL;
static void fill_seq_with_data(Relation rel, HeapTuple tuple);
static Relation open_share_lock(SeqTable seq);
static Relation lock_and_open_sequence(SeqTable seq);
static void create_seq_hashtable(void);
static void init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel);
static Form_pg_sequence_data read_seq_tuple(Relation rel,
Buffer *buf, HeapTuple seqdatatuple);
static LOCKMODE alter_sequence_get_lock_level(List *options);
static void init_params(ParseState *pstate, List *options, bool for_identity,
bool isInit,
Form_pg_sequence seqform,
@ -427,7 +428,9 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
HeapTuple tuple;
/* Open and lock sequence. */
relid = RangeVarGetRelid(stmt->sequence, AccessShareLock, stmt->missing_ok);
relid = RangeVarGetRelid(stmt->sequence,
alter_sequence_get_lock_level(stmt->options),
stmt->missing_ok);
if (relid == InvalidOid)
{
ereport(NOTICE,
@ -443,12 +446,6 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
aclcheck_error(ACLCHECK_NOT_OWNER, ACL_KIND_CLASS,
stmt->sequence->relname);
/* lock page' buffer and read tuple into new sequence structure */
seqdata = read_seq_tuple(seqrel, &buf, &seqdatatuple);
/* Copy old sequence data into workspace */
memcpy(&newseqdata, seqdata, sizeof(FormData_pg_sequence_data));
rel = heap_open(SequenceRelationId, RowExclusiveLock);
tuple = SearchSysCacheCopy1(SEQRELID,
ObjectIdGetDatum(relid));
@ -458,6 +455,12 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
seqform = (Form_pg_sequence) GETSTRUCT(tuple);
/* lock page's buffer and read tuple into new sequence structure */
seqdata = read_seq_tuple(seqrel, &buf, &seqdatatuple);
/* Copy old sequence data into workspace */
memcpy(&newseqdata, seqdata, sizeof(FormData_pg_sequence_data));
/* Check and set new values */
init_params(pstate, stmt->options, stmt->for_identity, false, seqform, &changed_seqform, &newseqdata, &owned_by);
@ -508,12 +511,12 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
ObjectAddressSet(address, RelationRelationId, relid);
relation_close(seqrel, NoLock);
if (changed_seqform)
CatalogTupleUpdate(rel, &tuple->t_self, tuple);
heap_close(rel, RowExclusiveLock);
relation_close(seqrel, NoLock);
return address;
}
@ -594,7 +597,7 @@ nextval_internal(Oid relid, bool check_permissions)
bool cycle;
bool logit = false;
/* open and AccessShareLock sequence */
/* open and lock sequence */
init_sequence(relid, &elm, &seqrel);
if (check_permissions &&
@ -829,7 +832,7 @@ currval_oid(PG_FUNCTION_ARGS)
SeqTable elm;
Relation seqrel;
/* open and AccessShareLock sequence */
/* open and lock sequence */
init_sequence(relid, &elm, &seqrel);
if (pg_class_aclcheck(elm->relid, GetUserId(),
@ -869,7 +872,7 @@ lastval(PG_FUNCTION_ARGS)
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("lastval is not yet defined in this session")));
seqrel = open_share_lock(last_used_seq);
seqrel = lock_and_open_sequence(last_used_seq);
/* nextval() must have already been called for this sequence */
Assert(last_used_seq->last_valid);
@ -913,7 +916,7 @@ do_setval(Oid relid, int64 next, bool iscalled)
int64 maxv,
minv;
/* open and AccessShareLock sequence */
/* open and lock sequence */
init_sequence(relid, &elm, &seqrel);
if (pg_class_aclcheck(elm->relid, GetUserId(), ACL_UPDATE) != ACLCHECK_OK)
@ -1042,15 +1045,15 @@ setval3_oid(PG_FUNCTION_ARGS)
/*
* Open the sequence and acquire AccessShareLock if needed
* Open the sequence and acquire lock if needed
*
* If we haven't touched the sequence already in this transaction,
* we need to acquire AccessShareLock. We arrange for the lock to
* we need to acquire a lock. We arrange for the lock to
* be owned by the top transaction, so that we don't need to do it
* more than once per xact.
*/
static Relation
open_share_lock(SeqTable seq)
lock_and_open_sequence(SeqTable seq)
{
LocalTransactionId thislxid = MyProc->lxid;
@ -1063,7 +1066,7 @@ open_share_lock(SeqTable seq)
PG_TRY();
{
CurrentResourceOwner = TopTransactionResourceOwner;
LockRelationOid(seq->relid, AccessShareLock);
LockRelationOid(seq->relid, RowExclusiveLock);
}
PG_CATCH();
{
@ -1078,7 +1081,7 @@ open_share_lock(SeqTable seq)
seq->lxid = thislxid;
}
/* We now know we have AccessShareLock, and can safely open the rel */
/* We now know we have the lock, and can safely open the rel */
return relation_open(seq->relid, NoLock);
}
@ -1135,7 +1138,7 @@ init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel)
/*
* Open the sequence relation.
*/
seqrel = open_share_lock(elm);
seqrel = lock_and_open_sequence(elm);
if (seqrel->rd_rel->relkind != RELKIND_SEQUENCE)
ereport(ERROR,
@ -1216,6 +1219,30 @@ read_seq_tuple(Relation rel, Buffer *buf, HeapTuple seqdatatuple)
return seq;
}
/*
* Check the sequence options list and return the appropriate lock level for
* ALTER SEQUENCE.
*
* Most sequence option changes require a self-exclusive lock and should block
* concurrent nextval() et al. But RESTART does not, because it's not
* transactional. Also take a lower lock if no option at all is present.
*/
static LOCKMODE
alter_sequence_get_lock_level(List *options)
{
ListCell *option;
foreach(option, options)
{
DefElem *defel = (DefElem *) lfirst(option);
if (strcmp(defel->defname, "restart") != 0)
return ShareRowExclusiveLock;
}
return RowExclusiveLock;
}
/*
* init_params: process the options list of CREATE or ALTER SEQUENCE, and
* store the values into appropriate fields of seqform, for changes that go
@ -1850,7 +1877,7 @@ pg_sequence_last_value(PG_FUNCTION_ARGS)
bool is_called;
int64 result;
/* open and AccessShareLock sequence */
/* open and lock sequence */
init_sequence(relid, &elm, &seqrel);
if (pg_class_aclcheck(relid, GetUserId(), ACL_SELECT | ACL_USAGE) != ACLCHECK_OK)

85
src/test/isolation/expected/sequence-ddl.out Normal file
View File

@ -0,0 +1,85 @@
Parsed test spec with 2 sessions
starting permutation: s1alter s1commit s2nv
step s1alter: ALTER SEQUENCE seq1 MAXVALUE 10;
step s1commit: COMMIT;
step s2nv: SELECT nextval('seq1') FROM generate_series(1, 15);
ERROR: nextval: reached maximum value of sequence "seq1" (10)
starting permutation: s1alter s2nv s1commit
step s1alter: ALTER SEQUENCE seq1 MAXVALUE 10;
step s2nv: SELECT nextval('seq1') FROM generate_series(1, 15); <waiting ...>
step s1commit: COMMIT;
step s2nv: <... completed>
error in steps s1commit s2nv: ERROR: nextval: reached maximum value of sequence "seq1" (10)
starting permutation: s2begin s2nv s1alter2 s2commit s1commit
step s2begin: BEGIN;
step s2nv: SELECT nextval('seq1') FROM generate_series(1, 15);
nextval
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
step s1alter2: ALTER SEQUENCE seq1 MAXVALUE 20; <waiting ...>
step s2commit: COMMIT;
step s1alter2: <... completed>
step s1commit: COMMIT;
starting permutation: s1restart s2nv s1commit
step s1restart: ALTER SEQUENCE seq1 RESTART WITH 5;
step s2nv: SELECT nextval('seq1') FROM generate_series(1, 15);
nextval
5
6
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8
9
10
11
12
13
14
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step s1commit: COMMIT;
starting permutation: s2begin s2nv s1restart s2commit s1commit
step s2begin: BEGIN;
step s2nv: SELECT nextval('seq1') FROM generate_series(1, 15);
nextval
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
step s1restart: ALTER SEQUENCE seq1 RESTART WITH 5;
step s2commit: COMMIT;
step s1commit: COMMIT;

1
src/test/isolation/isolation_schedule
View File

@ -58,6 +58,7 @@ test: alter-table-1
test: alter-table-2
test: alter-table-3
test: create-trigger
test: sequence-ddl
test: async-notify
test: vacuum-reltuples
test: timeouts

39
src/test/isolation/specs/sequence-ddl.spec Normal file
View File

@ -0,0 +1,39 @@
# Test sequence usage and concurrent sequence DDL
setup
{
CREATE SEQUENCE seq1;
}
teardown
{
DROP SEQUENCE seq1;
}
session "s1"
setup { BEGIN; }
step "s1alter" { ALTER SEQUENCE seq1 MAXVALUE 10; }
step "s1alter2" { ALTER SEQUENCE seq1 MAXVALUE 20; }
step "s1restart" { ALTER SEQUENCE seq1 RESTART WITH 5; }
step "s1commit" { COMMIT; }
session "s2"
step "s2begin" { BEGIN; }
step "s2nv" { SELECT nextval('seq1') FROM generate_series(1, 15); }
step "s2commit" { COMMIT; }
permutation "s1alter" "s1commit" "s2nv"
# Prior to PG10, the s2nv would see the uncommitted s1alter change,
# but now it waits.
permutation "s1alter" "s2nv" "s1commit"
# nextval doesn't release lock until transaction end, so s1alter2 has
# to wait for s2commit.
permutation "s2begin" "s2nv" "s1alter2" "s2commit" "s1commit"
# RESTART is nontransactional, so s2nv sees it right away
permutation "s1restart" "s2nv" "s1commit"
# RESTART does not wait
permutation "s2begin" "s2nv" "s1restart" "s2commit" "s1commit"