Fix unsafe reference into relcache in constructed CommentStmt.

The CommentStmt made by RebuildConstraintComment() has to pstrdup the
relation name, else it will contain a dangling pointer after that
relcache entry is flushed.  (I'm less sure that pstrdup'ing conname
is necessary, but let's be safe.)  Failure to do this leads to weird
errors or crashes, as reported by Marko Elezovic.

Bug introduced by commit e42375fc8, so back-patch to 9.5 as that was.

Fix by David Rowley, regression test by Michael Paquier

Discussion: https://postgr.es/m/DB6PR03MB30775D58E732D4EB0C13725B9AE00@DB6PR03MB3077.eurprd03.prod.outlook.com

src/backend/commands/tablecmds.c

@@ -9764,8 +9764,8 @@ RebuildConstraintComment(AlteredTableInfo *tab, int pass, Oid objid,
 	cmd = makeNode(CommentStmt);
 	cmd->objtype = OBJECT_TABCONSTRAINT;
 	cmd->object = (Node *) list_make3(makeString(get_namespace_name(RelationGetNamespace(rel))),
-									  makeString(RelationGetRelationName(rel)),
-									  makeString(conname));
+						   makeString(pstrdup(RelationGetRelationName(rel))),
+									  makeString(pstrdup(conname)));
 	cmd->comment = comment_str;
 
 	/* Append it to list of commands */

src/test/regress/expected/alter_table.out

@@ -2788,6 +2788,39 @@ SELECT conname as constraint, obj_description(oid, 'pg_constraint') as comment F
  comment_test_positive_col_check | CHECK constraint on comment_test.positive_col
 (2 rows)
 
+-- Check compatibility for foreign keys and comments. This is done
+-- separately as rebuilding the column type of the parent leads
+-- to an error and would reduce the test scope.
+CREATE TABLE comment_test_child (
+  id text CONSTRAINT comment_test_child_fk REFERENCES comment_test);
+CREATE INDEX comment_test_child_fk ON comment_test_child(id);
+COMMENT ON COLUMN comment_test_child.id IS 'Column ''id'' on comment_test_child';
+COMMENT ON INDEX comment_test_child_fk IS 'Index backing the FOREIGN KEY of comment_test_child';
+COMMENT ON CONSTRAINT comment_test_child_fk ON comment_test_child IS 'FOREIGN KEY constraint of comment_test_child';
+-- Change column type of parent
+ALTER TABLE comment_test ALTER COLUMN id SET DATA TYPE text;
+ALTER TABLE comment_test ALTER COLUMN id SET DATA TYPE int USING id::integer;
+ERROR:  foreign key constraint "comment_test_child_fk" cannot be implemented
+DETAIL:  Key columns "id" and "id" are of incompatible types: text and integer.
+-- Comments should be intact
+SELECT col_description('comment_test_child'::regclass, 1) as comment;
+              comment              
+-----------------------------------
+ Column 'id' on comment_test_child
+(1 row)
+
+SELECT indexrelid::regclass::text as index, obj_description(indexrelid, 'pg_class') as comment FROM pg_index where indrelid = 'comment_test_child'::regclass ORDER BY 1, 2;
+         index         |                       comment                       
+-----------------------+-----------------------------------------------------
+ comment_test_child_fk | Index backing the FOREIGN KEY of comment_test_child
+(1 row)
+
+SELECT conname as constraint, obj_description(oid, 'pg_constraint') as comment FROM pg_constraint where conrelid = 'comment_test_child'::regclass ORDER BY 1, 2;
+      constraint       |                   comment                    
+-----------------------+----------------------------------------------
+ comment_test_child_fk | FOREIGN KEY constraint of comment_test_child
+(1 row)
+
 -- Check that we map relation oids to filenodes and back correctly.  Only
 -- display bad mappings so the test output doesn't change all the time.  A
 -- filenode function call can return NULL for a relation dropped concurrently

src/test/regress/sql/alter_table.sql

@@ -1786,6 +1786,24 @@ SELECT col_description('comment_test'::regclass, 1) as comment;
 SELECT indexrelid::regclass::text as index, obj_description(indexrelid, 'pg_class') as comment FROM pg_index where indrelid = 'comment_test'::regclass ORDER BY 1, 2;
 SELECT conname as constraint, obj_description(oid, 'pg_constraint') as comment FROM pg_constraint where conrelid = 'comment_test'::regclass ORDER BY 1, 2;
 
+-- Check compatibility for foreign keys and comments. This is done
+-- separately as rebuilding the column type of the parent leads
+-- to an error and would reduce the test scope.
+CREATE TABLE comment_test_child (
+  id text CONSTRAINT comment_test_child_fk REFERENCES comment_test);
+CREATE INDEX comment_test_child_fk ON comment_test_child(id);
+COMMENT ON COLUMN comment_test_child.id IS 'Column ''id'' on comment_test_child';
+COMMENT ON INDEX comment_test_child_fk IS 'Index backing the FOREIGN KEY of comment_test_child';
+COMMENT ON CONSTRAINT comment_test_child_fk ON comment_test_child IS 'FOREIGN KEY constraint of comment_test_child';
+
+-- Change column type of parent
+ALTER TABLE comment_test ALTER COLUMN id SET DATA TYPE text;
+ALTER TABLE comment_test ALTER COLUMN id SET DATA TYPE int USING id::integer;
+
+-- Comments should be intact
+SELECT col_description('comment_test_child'::regclass, 1) as comment;
+SELECT indexrelid::regclass::text as index, obj_description(indexrelid, 'pg_class') as comment FROM pg_index where indrelid = 'comment_test_child'::regclass ORDER BY 1, 2;
+SELECT conname as constraint, obj_description(oid, 'pg_constraint') as comment FROM pg_constraint where conrelid = 'comment_test_child'::regclass ORDER BY 1, 2;
 
 -- Check that we map relation oids to filenodes and back correctly.  Only
 -- display bad mappings so the test output doesn't change all the time.  A