fix(compute_ctl): Properly escape identifiers inside PL/pgSQL blocks (#11045)

## Problem

In f37eeb56, I properly escaped the identifier, but I haven't noticed
that the resulting string is used in the `format('...')`, so it needs
additional escaping. Yet, after looking at it closer and with Heikki's
and Tristan's help, it appeared to be that it's a full can of worms and
we have problems all over the code in places where we use PL/pgSQL
blocks.

## Summary of changes

Add a new `pg_quote_dollar()` helper to deal with it, as dollar-quoting
of strings seems to be the only robust way to escape strings in dynamic
PL/pgSQL blocks. We mimic the Postgres' `pg_get_functiondef` logic here
[1].

While on it, I added more tests and caught a couple of more bugs with
string escaping:

1. `get_existing_dbs_async()` was wrapping `owner` in additional
double-quotes if it contained special characters
2. `construct_superuser_query()` was flawed in even more ways than the
rest of the code. It wasn't realistic to fix it quickly, but after
thinking about it more, I realized that we could drop most of it
altogether. IIUC, it was added as some sort of migration, probably back
when we haven't had migrations yet. So all the complicated code was
needed to properly update existing roles and DBs. In the current Neon,
this code only runs before we create the very first DB and role. When we
create roles and DBs, all `neon_superuser` grants are added in the
different places. So the worst thing that could happen is that there is
an ancient branch somewhere, so when users poke it, they will realize
that not all Neon features work as expected. Yet, the fix is simple and
self-serve -- just create a new role via UI or API, and it will get a
proper `neon_superuser` grant.

[1]:
8b49392b27/src/backend/utils/adt/ruleutils.c (L3153)

Closes neondatabase/cloud#25048

compute_tools/src/sql/create_neon_superuser.sql

@@ -0,0 +1,8 @@
+DO $$
+    BEGIN
+        IF NOT EXISTS (SELECT FROM pg_catalog.pg_roles WHERE rolname = 'neon_superuser')
+        THEN
+            CREATE ROLE neon_superuser CREATEDB CREATEROLE NOLOGIN REPLICATION BYPASSRLS IN ROLE pg_read_all_data, pg_write_all_data;
+        END IF;
+    END
+$$;

compute_tools/src/sql/drop_subscriptions.sql

@@ -1,4 +1,4 @@
-DO $$
+DO ${outer_tag}$
 DECLARE
     subname TEXT;
 BEGIN
@@ -9,4 +9,4 @@ BEGIN
         EXECUTE format('DROP SUBSCRIPTION %I;', subname);
     END LOOP;
 END;
-$$;
+${outer_tag}$;

compute_tools/src/sql/pre_drop_role_revoke_privileges.sql

@@ -1,6 +1,6 @@
 SET SESSION ROLE neon_superuser;
 
-DO $$
+DO ${outer_tag}$
 DECLARE
     schema TEXT;
     revoke_query TEXT;
@@ -16,13 +16,15 @@ BEGIN
         WHERE schema_name IN ('public')
     LOOP
         revoke_query := format(
-            'REVOKE ALL PRIVILEGES ON ALL TABLES IN SCHEMA %I FROM {role_name} GRANTED BY neon_superuser;',
-            schema
+            'REVOKE ALL PRIVILEGES ON ALL TABLES IN SCHEMA %I FROM %I GRANTED BY neon_superuser;',
+            schema,
+            -- N.B. this has to be properly dollar-escaped with `pg_quote_dollar()`
+            {role_name}
         );
 
         EXECUTE revoke_query;
     END LOOP;
 END;
-$$;
+${outer_tag}$;
 
 RESET ROLE;

compute_tools/src/sql/set_public_schema_owner.sql

@@ -1,5 +1,4 @@
-DO
-$$
+DO ${outer_tag}$
     DECLARE
         schema_owner TEXT;
     BEGIN
@@ -16,8 +15,8 @@ $$
 
             IF schema_owner = 'cloud_admin' OR schema_owner = 'zenith_admin'
             THEN
-                ALTER SCHEMA public OWNER TO {db_owner};
+                EXECUTE format('ALTER SCHEMA public OWNER TO %I', {db_owner});
             END IF;
         END IF;
     END
-$$;
+${outer_tag}$;

compute_tools/src/sql/unset_template_for_drop_dbs.sql

@@ -1,12 +1,12 @@
-DO $$
+DO ${outer_tag}$
     BEGIN
         IF EXISTS(
             SELECT 1
             FROM pg_catalog.pg_database
-            WHERE datname = {datname_str}
+            WHERE datname = {datname}
         )
         THEN
-            ALTER DATABASE {datname} is_template false;
+            EXECUTE format('ALTER DATABASE %I is_template false', {datname});
         END IF;
     END
-$$;
+${outer_tag}$;