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Commitdb01fc9

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Fix ALTER COLUMN TYPE to not open a relation without any lock.
If the column being modified is referenced by a foreign key constraintof another table, ALTER TABLE would open the other table (to re-parsethe constraint's definition) without having first obtained a lock on it.This was evidently intentional, but that doesn't mean it's really safe.It's especially not safe in 9.3, which pre-dates use of MVCC scans forcatalog reads, but even in current releases it doesn't seem like a goodidea.We know we'll need AccessExclusiveLock shortly to drop the obsoletedconstraint, so just get that a little sooner to close the hole.Per testing with a patch that complains if we open a relation withoutholding any lock on it. I don't plan to back-patch that patch, but weshould close the holes it identifies in all supported branches.Discussion:https://postgr.es/m/2038.1538335244@sss.pgh.pa.us
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‎src/backend/commands/tablecmds.c

Lines changed: 12 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -9513,8 +9513,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
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* appropriate work queue entries. We do this before dropping because in
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* the case of a FOREIGN KEY constraint, we might not yet have exclusive
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* lock on the table the constraint is attached to, and we need to get
9516-
* that before dropping. It's safe because the parser won't actually look
9517-
* at the catalogs to detect the existing entry.
9516+
* that before reparsing/dropping.
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*
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* We can't rely on the output of deparsing to tell us which relation to
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* operate on, because concurrent activity might have made the name
@@ -9530,6 +9529,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
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Form_pg_constraintcon;
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Oidrelid;
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Oidconfrelid;
9532+
charcontype;
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boolconislocal;
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tup=SearchSysCache1(CONSTROID,ObjectIdGetDatum(oldId));
@@ -9538,6 +9538,7 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
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con= (Form_pg_constraint)GETSTRUCT(tup);
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relid=con->conrelid;
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confrelid=con->confrelid;
9541+
contype=con->contype;
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conislocal=con->conislocal;
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ReleaseSysCache(tup);
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@@ -9550,6 +9551,15 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
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if (!conislocal)
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continue;
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9554+
/*
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* When rebuilding an FK constraint that references the table we're
9556+
* modifying, we might not yet have any lock on the FK's table, so get
9557+
* one now. We'll need AccessExclusiveLock for the DROP CONSTRAINT
9558+
* step, so there's no value in asking for anything weaker.
9559+
*/
9560+
if (relid!=tab->relid&&contype==CONSTRAINT_FOREIGN)
9561+
LockRelationOid(relid,AccessExclusiveLock);
9562+
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ATPostAlterTypeParse(oldId,relid,confrelid,
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(char*)lfirst(def_item),
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wqueue,lockmode,tab->rewrite);

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