Disallow changing an inherited column's type if not all parents changed.

If a table inherits from multiple unrelated parents, we must disallow
changing the type of a column inherited from multiple such parents, else
it would be out of step with the other parents.  However, it's possible
for the column to ultimately be inherited from just one common ancestor,
in which case a change starting from that ancestor should still be
allowed.  (I would not be excited about preserving that option, were
it not that we have regression test cases exercising it already ...)

It's slightly annoying that this patch looks different from the logic
with the same end goal in renameatt(), and more annoying that it
requires an extra syscache lookup to make the test.  However, the
recursion logic is quite different in the two functions, and a
back-patched bug fix is no place to be trying to unify them.

Per report from Manuel Rigger.  Back-patch to 9.5.  The bug exists in
9.4 too (and doubtless much further back); but the way the recursion
is done in 9.4 is a good bit different, so that substantial refactoring
would be needed to fix it in 9.4.  I'm disinclined to do that, or risk
introducing new bugs, for a bug that has escaped notice for this long.

Discussion: https://postgr.es/m/CA+u7OA4qogDv9rz1HAb-ADxttXYPqQdUdPY_yd4kCzywNxRQXA@mail.gmail.com

src/test/regress/expected/inherit.out

@@ -723,6 +723,16 @@ select * from d;
  32 | one | two | three
 (1 row)
 
+-- The above verified that we can change the type of a multiply-inherited
+-- column; but we should reject that if any definition was inherited from
+-- an unrelated parent.
+create temp table parent1(f1 int, f2 int);
+create temp table parent2(f1 int, f3 bigint);
+create temp table childtab(f4 int) inherits(parent1, parent2);
+NOTICE:  merging multiple inherited definitions of column "f1"
+alter table parent1 alter column f1 type bigint;  -- fail, conflict w/parent2
+ERROR:  cannot alter inherited column "f1" of relation "childtab"
+alter table parent1 alter column f2 type bigint;  -- ok
 -- Test non-inheritable parent constraints
 create table p1(ff1 int);
 alter table p1 add constraint p1chk check (ff1 > 0) no inherit;

src/test/regress/sql/inherit.sql

@@ -208,6 +208,15 @@ insert into d values('test','one','two','three');
 alter table a alter column aa type integer using bit_length(aa);
 select * from d;
 
+-- The above verified that we can change the type of a multiply-inherited
+-- column; but we should reject that if any definition was inherited from
+-- an unrelated parent.
+create temp table parent1(f1 int, f2 int);
+create temp table parent2(f1 int, f3 bigint);
+create temp table childtab(f4 int) inherits(parent1, parent2);
+alter table parent1 alter column f1 type bigint;  -- fail, conflict w/parent2
+alter table parent1 alter column f2 type bigint;  -- ok
+
 -- Test non-inheritable parent constraints
 create table p1(ff1 int);
 alter table p1 add constraint p1chk check (ff1 > 0) no inherit;