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

@@ -1679,7 +1679,7 @@ 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
@@ -1687,7 +1687,7 @@ 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;
 			}

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              

src/test/isolation/isolation_schedule

@@ -59,6 +59,7 @@ test: multiple-cic
 test: alter-table-1
 test: alter-table-2
 test: alter-table-3
+test: alter-table-4
 test: create-trigger
 test: sequence-ddl
 test: async-notify

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"