From a99922f966f57fcccee3b6e02a9df62789493dbc Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 12 Jan 2018 15:46:37 -0500
Subject: [PATCH] Avoid unnecessary failure in SELECT concurrent with ALTER NO
INHERIT.
If a query against an inheritance tree runs concurrently with an ALTER
TABLE that's disinheriting one of the tree members, it's possible to get
a "could not find inherited attribute" error because after obtaining lock
on the removed member, make_inh_translation_list sees that its columns
have attinhcount=0 and decides they aren't the columns it's looking for.
An ideal fix, perhaps, would avoid including such a just-removed member
table in the query at all; but there seems no way to accomplish that
without adding expensive catalog rechecks or creating a likelihood of
deadlocks. Instead, let's just drop the check on attinhcount. In this
way, a query that's included a just-disinherited child will still
succeed, which is not a completely unreasonable behavior.
This problem has existed for a long time, so back-patch to all supported
branches. Also add an isolation test verifying related behaviors.
Patch by me; the new isolation test is based on Kyotaro Horiguchi's work.
Discussion: https://postgr.es/m/20170626.174612.23936762.horiguchi.kyotaro@lab.ntt.co.jp
---
src/backend/optimizer/prep/prepunion.c | 4 +-
src/test/isolation/expected/alter-table-4.out | 57 +++++++++++++++++++
src/test/isolation/isolation_schedule | 1 +
src/test/isolation/specs/alter-table-4.spec | 37 ++++++++++++
4 files changed, 97 insertions(+), 2 deletions(-)
create mode 100644 src/test/isolation/expected/alter-table-4.out
create mode 100644 src/test/isolation/specs/alter-table-4.spec
diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index e80ad7753de..0c2bb5d51f3 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -1498,7 +1498,7 @@ make_inh_translation_list(Relation oldrelation, Relation newrelation,
*/
if (old_attno < newnatts &&
(att = new_tupdesc->attrs[old_attno]) != NULL &&
- !att->attisdropped && att->attinhcount != 0 &&
+ !att->attisdropped &&
strcmp(attname, NameStr(att->attname)) == 0)
new_attno = old_attno;
else
@@ -1506,7 +1506,7 @@ make_inh_translation_list(Relation oldrelation, Relation newrelation,
for (new_attno = 0; new_attno < newnatts; new_attno++)
{
att = new_tupdesc->attrs[new_attno];
- if (!att->attisdropped && att->attinhcount != 0 &&
+ if (!att->attisdropped &&
strcmp(attname, NameStr(att->attname)) == 0)
break;
}
diff --git a/src/test/isolation/expected/alter-table-4.out b/src/test/isolation/expected/alter-table-4.out
new file mode 100644
index 00000000000..d2dac0be098
--- /dev/null
+++ b/src/test/isolation/expected/alter-table-4.out
@@ -0,0 +1,57 @@
+Parsed test spec with 2 sessions
+
+starting permutation: s1b s1delc1 s2sel s1c s2sel
+step s1b: BEGIN;
+step s1delc1: ALTER TABLE c1 NO INHERIT p;
+step s2sel: SELECT SUM(a) FROM p; <waiting ...>
+step s1c: COMMIT;
+step s2sel: <... completed>
+sum
+
+11
+step s2sel: SELECT SUM(a) FROM p;
+sum
+
+1
+
+starting permutation: s1b s1delc1 s1addc2 s2sel s1c s2sel
+step s1b: BEGIN;
+step s1delc1: ALTER TABLE c1 NO INHERIT p;
+step s1addc2: ALTER TABLE c2 INHERIT p;
+step s2sel: SELECT SUM(a) FROM p; <waiting ...>
+step s1c: COMMIT;
+step s2sel: <... completed>
+sum
+
+11
+step s2sel: SELECT SUM(a) FROM p;
+sum
+
+101
+
+starting permutation: s1b s1dropc1 s2sel s1c s2sel
+step s1b: BEGIN;
+step s1dropc1: DROP TABLE c1;
+step s2sel: SELECT SUM(a) FROM p; <waiting ...>
+step s1c: COMMIT;
+step s2sel: <... completed>
+sum
+
+1
+step s2sel: SELECT SUM(a) FROM p;
+sum
+
+1
+
+starting permutation: s1b s1delc1 s1modc1a s2sel s1c s2sel
+step s1b: BEGIN;
+step s1delc1: ALTER TABLE c1 NO INHERIT p;
+step s1modc1a: ALTER TABLE c1 ALTER COLUMN a TYPE float;
+step s2sel: SELECT SUM(a) FROM p; <waiting ...>
+step s1c: COMMIT;
+step s2sel: <... completed>
+error in steps s1c s2sel: ERROR: attribute "a" of relation "c1" does not match parent's type
+step s2sel: SELECT SUM(a) FROM p;
+sum
+
+1
diff --git a/src/test/isolation/isolation_schedule b/src/test/isolation/isolation_schedule
index 94db2bf07f3..4dd251b3c0e 100644
--- a/src/test/isolation/isolation_schedule
+++ b/src/test/isolation/isolation_schedule
@@ -50,5 +50,6 @@ test: multiple-cic
test: alter-table-1
test: alter-table-2
test: alter-table-3
+test: alter-table-4
test: create-trigger
test: timeouts
diff --git a/src/test/isolation/specs/alter-table-4.spec b/src/test/isolation/specs/alter-table-4.spec
new file mode 100644
index 00000000000..a9c1a937238
--- /dev/null
+++ b/src/test/isolation/specs/alter-table-4.spec
@@ -0,0 +1,37 @@
+# ALTER TABLE - Add and remove inheritance with concurrent reads
+
+setup
+{
+ CREATE TABLE p (a integer);
+ INSERT INTO p VALUES(1);
+ CREATE TABLE c1 () INHERITS (p);
+ INSERT INTO c1 VALUES(10);
+ CREATE TABLE c2 (a integer);
+ INSERT INTO c2 VALUES(100);
+}
+
+teardown
+{
+ DROP TABLE IF EXISTS c1, c2, p;
+}
+
+session "s1"
+step "s1b" { BEGIN; }
+step "s1delc1" { ALTER TABLE c1 NO INHERIT p; }
+step "s1modc1a" { ALTER TABLE c1 ALTER COLUMN a TYPE float; }
+step "s1addc2" { ALTER TABLE c2 INHERIT p; }
+step "s1dropc1" { DROP TABLE c1; }
+step "s1c" { COMMIT; }
+
+session "s2"
+step "s2sel" { SELECT SUM(a) FROM p; }
+
+# NO INHERIT will not be visible to concurrent select,
+# since we identify children before locking them
+permutation "s1b" "s1delc1" "s2sel" "s1c" "s2sel"
+# adding inheritance likewise is not seen if s1 commits after s2 locks p
+permutation "s1b" "s1delc1" "s1addc2" "s2sel" "s1c" "s2sel"
+# but we do cope with DROP on a child table
+permutation "s1b" "s1dropc1" "s2sel" "s1c" "s2sel"
+# this case currently results in an error; doesn't seem worth preventing
+permutation "s1b" "s1delc1" "s1modc1a" "s2sel" "s1c" "s2sel"
--
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