Avoid duplicate build status entries

This effectively makes adding build status entries idempotent to some degree.

guix-data-service/model/build-status.scm

@@ -85,6 +85,8 @@ VALUES "
                ")")))
            data
            build-ids)
-      ", ")))
+      ", ")
+     "
+ON CONFLICT DO NOTHING"))
 
   (exec-query conn query '()))

sqitch/deploy/build_status_add_unique_index.sql

@@ -0,0 +1,18 @@
+-- Deploy guix-data-service:build_status_add_unique_index to pg
+
+BEGIN;
+
+DELETE FROM build_status
+WHERE id NOT IN (
+  SELECT MIN(id) FROM build_status GROUP BY build_id, timestamp, status
+);
+
+ALTER TABLE build_status
+ADD CONSTRAINT build_status_build_id_timestamp_status_unique
+UNIQUE (build_id, timestamp, status);
+
+CREATE UNIQUE INDEX build_status_build_id_status_unique_idx
+  ON build_status (build_id, status)
+  WHERE timestamp IS NULL;
+
+COMMIT;

sqitch/revert/build_status_add_unique_index.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:build_status_add_unique_index from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;

sqitch/sqitch.plan

@@ -63,3 +63,4 @@ translations_for_lint_checker_descriptions 2020-05-22T19:49:37Z daniela <daniela
 translations_for_package_synopsis_and_descriptions 2020-06-09T12:42:54Z daniela <daniela@linux-ijv5> # Support translations for package synopsis and descriptions
 build_status_nullable_timestamp 2020-06-27T12:34:29Z Christopher Baines <mail@cbaines.net> # Make build_status.timestamp nullable
 build_add_build_server_build_id 2020-06-27T12:48:57Z Christopher Baines <mail@cbaines.net> # Add builds.build_server_build_id
+build_status_add_unique_index 2020-06-30T17:19:30Z Christopher Baines <mail@cbaines.net> # Add a unique index to the build_status table

sqitch/verify/build_status_add_unique_index.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:build_status_add_unique_index on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;