Fix pg_constraint oid duplication (#8166)

Multiple of our invested constraints were being given the same oid
which caused problems.

One specific problem was generating bogus pgdumps, which annoyingly
mostly only showed up in inplace-upgrade tests:

https://github.com/edgedb/edgedb/actions/runs/12442350513/job/34740389082?pr=8159

The problem was that pg_get_constraintdef was returning a constraint
definition for the "wrong" object; the defining query would return 3
rows, and postgres silently returns the first.

Fix this by adding some bits to the oid separate from the uuid of the
link, and test that the fix works by putting the body of
pg_get_constraintdef into a subquery.

This hopefully will unblock #8159.

edb/pgsql/metaschema.py

@@ -6389,12 +6389,14 @@ def make_wrapper_view(name: str) -> trampoline.VersionedView:
         name=('edgedbsql', 'uuid_to_oid'),
         args=(
             ('id', 'uuid'),
+            # extra is two extra bits to throw into the oid, for now
+            ('extra', 'int4', '0'),
         ),
         returns=('oid',),
         volatility='immutable',
         text="""
             SELECT (
-                ('x' || substring(id::text, 2, 7))::bit(28)::bigint
+                ('x' || substring(id::text, 2, 7))::bit(28)::bigint*4 + extra
                  + 40000)::oid;
         """
     )
@@ -7194,7 +7196,9 @@ def make_wrapper_view(name: str) -> trampoline.VersionedView:
 
         -- foreign keys for object tables
         SELECT
-          edgedbsql_VER.uuid_to_oid(sl.id) as oid,
+          -- uuid_to_oid needs "extra" arg to disambiguate from the link table
+          -- keys below
+          edgedbsql_VER.uuid_to_oid(sl.id, 0) as oid,
           vt.table_name || '_fk_' || sl.name AS conname,
           edgedbsql_VER.uuid_to_oid(vt.module_id) AS connamespace,
           'f'::"char" AS contype,
@@ -7240,7 +7244,9 @@ def make_wrapper_view(name: str) -> trampoline.VersionedView:
         -- - single link with link properties (source & target),
         -- these constraints do not actually exist, so we emulate it entierly
         SELECT
-            edgedbsql_VER.uuid_to_oid(sp.id) AS oid,
+            -- uuid_to_oid needs "extra" arg to disambiguate from other
+            -- constraints using this pointer
+            edgedbsql_VER.uuid_to_oid(sp.id, spec.attnum) AS oid,
             vt.table_name || '_fk_' || spec.name AS conname,
             edgedbsql_VER.uuid_to_oid(vt.module_id) AS connamespace,
             'f'::"char" AS contype,
@@ -7856,6 +7862,10 @@ def construct_pg_view(
             returns=('text',),
             volatility='stable',
             text=r"""
+                -- Wrap in a subquery SELECT so that we get a clear failure
+                -- if something is broken and this returns multiple rows.
+                -- (By default it would silently return the first.)
+                SELECT (
                 SELECT CASE
                     WHEN contype = 'p' THEN
                     'PRIMARY KEY(' || (
@@ -7868,7 +7878,6 @@ def construct_pg_view(
                         SELECT attname
                         FROM edgedbsql_VER.pg_attribute
                         WHERE attrelid = conrelid AND attnum = ANY(conkey)
-                        LIMIT 1
                     ) || '")' || ' REFERENCES "'
                     || pn.nspname || '"."' || pc.relname || '"(id)'
                     ELSE ''
@@ -7878,6 +7887,7 @@ def construct_pg_view(
                 LEFT JOIN edgedbsql_VER.pg_namespace pn
                   ON pc.relnamespace = pn.oid
                 WHERE con.oid = conid
+                )
             """
         ),
         trampoline.VersionedFunction(