rake gitlab:db:drop_tables doesn't drop pre-defined functions

In gitlab-org/charts/gitlab#3178 (closed) we found that rake gitlab:db:drop_tables doesn't drop a number of functions that are used for triggers:

git grep -i "CREATE FUNCTION" db/structure.sql
db/structure.sql:CREATE FUNCTION delete_associated_project_namespace() RETURNS trigger
db/structure.sql:CREATE FUNCTION insert_into_loose_foreign_keys_deleted_records() RETURNS trigger
db/structure.sql:CREATE FUNCTION insert_namespaces_sync_event() RETURNS trigger
db/structure.sql:CREATE FUNCTION insert_or_update_vulnerability_reads() RETURNS trigger
db/structure.sql:CREATE FUNCTION insert_projects_sync_event() RETURNS trigger
db/structure.sql:CREATE FUNCTION integrations_set_type_new() RETURNS trigger
db/structure.sql:CREATE FUNCTION next_traversal_ids_sibling(traversal_ids integer[]) RETURNS integer[]
db/structure.sql:CREATE FUNCTION set_has_external_issue_tracker() RETURNS trigger
db/structure.sql:CREATE FUNCTION set_has_external_wiki() RETURNS trigger
db/structure.sql:CREATE FUNCTION set_has_issues_on_vulnerability_reads() RETURNS trigger
db/structure.sql:CREATE FUNCTION unset_has_issues_on_vulnerability_reads() RETURNS trigger
db/structure.sql:CREATE FUNCTION update_location_from_vulnerability_occurrences() RETURNS trigger
db/structure.sql:CREATE FUNCTION update_vulnerability_reads_from_vulnerability() RETURNS trigger

For example:

• delete_associated_project_namespace (introduced in 14.3 via !69200 (merged))
• insert_or_update_vulnerability_reads (introduced in 14.8 via !75230 (merged))

As a result, a restore from backup that only drops the tables (vs. dropping the entire database) fails because the delete_associated_project_namespace function already exists:

Performing migrations (this will initialized if needed)
psql:/srv/gitlab/db/structure.sql:23: ERROR:  function "delete_associated_project_namespace" already exists with same argument types
rake aborted!
failed to execute:
psql --set ON_ERROR_STOP=1 --quiet --no-psqlrc --file /srv/gitlab/db/structure.sql --single-transaction gitlabhq_production

Please check the output above for any errors and make sure that `psql` is installed in your PATH and has proper permissions

In psql --echo-hidden, \df+ shows the magic query to show all functions defined. I simplified it to this query:

SELECT p.proname FROM pg_catalog.pg_proc p
     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = p.pronamespace
     LEFT JOIN pg_catalog.pg_language l ON l.oid = p.prolang
WHERE pg_catalog.pg_function_is_visible(p.oid)
      AND n.nspname <> 'pg_catalog'
      AND n.nspname <> 'information_schema'
AND l.lanname = 'plpgsql';

                    proname
------------------------------------------------
 insert_namespaces_sync_event
 insert_or_update_vulnerability_reads
 insert_projects_sync_event
 next_traversal_ids_sibling
 set_has_issues_on_vulnerability_reads
 unset_has_issues_on_vulnerability_reads
 update_location_from_vulnerability_occurrences
 update_vulnerability_reads_from_vulnerability
 delete_associated_project_namespace
 insert_into_loose_foreign_keys_deleted_records
 integrations_set_type_new
 set_has_external_issue_tracker
 set_has_external_wiki
 trigger_91dc388a5fe6
(14 rows)

I think we'll need to drop these functions.