feat(core-database-postgres): enforce chained blocks at database level (

#2753)

Make sure we don't end up with blocks having previous_block point to a
nonexistent block, even if bugs exist at the application level that
would attempt to do that.

packages/core-database-postgres/src/migrations/20190626000000-enforce-chained-blocks.sql

@@ -0,0 +1,43 @@
+ALTER TABLE blocks DROP CONSTRAINT IF EXISTS "chained_blocks";
+
+DROP FUNCTION IF EXISTS check_previous_block;
+
+CREATE FUNCTION check_previous_block(
+  id_arg VARCHAR(64),
+  previous_block_arg VARCHAR(64),
+  height_arg INTEGER
+) RETURNS BOOLEAN
+AS
+$$
+DECLARE
+  block_id_at_height_minus1 VARCHAR(64);
+  previous_block_height INTEGER;
+  fail_reason TEXT;
+BEGIN
+  IF height_arg = 1 THEN
+    RETURN TRUE;
+  END IF;
+
+  SELECT id INTO block_id_at_height_minus1 FROM blocks WHERE height = height_arg - 1;
+
+  IF previous_block_arg = block_id_at_height_minus1 THEN
+    RETURN TRUE;
+  END IF;
+
+  SELECT height INTO previous_block_height FROM blocks WHERE id = previous_block_arg;
+
+  IF previous_block_height IS NULL THEN
+    fail_reason := 'a block with id "' || previous_block_arg || '" does not exist';
+  ELSE
+    fail_reason := 'a block with id "' || previous_block_arg || '" exists but at an unexpected height ' ||
+      previous_block_height || ' instead of ' || height_arg - 1;
+  END IF;
+
+  RAISE 'Cannot insert new block (id="%", height=%, previous_block="%") because %',
+    id_arg, height_arg, previous_block_arg, fail_reason;
+END;
+$$
+LANGUAGE PLPGSQL
+VOLATILE;
+
+ALTER TABLE blocks ADD CONSTRAINT "chained_blocks" CHECK (check_previous_block(id, previous_block, height));

packages/core-database-postgres/src/migrations/index.ts

@@ -15,4 +15,5 @@ export const migrations = [
     loadQueryFile(__dirname, "./20190313000000-add-asset-column-to-transactions-table.sql"),
     loadQueryFile(__dirname, "./20190606000000-add-block-id-foreign-key-on-transactions.sql"),
     loadQueryFile(__dirname, "./20190619000000-drop-id-column-from-rounds-table.sql"),
+    loadQueryFile(__dirname, "./20190626000000-enforce-chained-blocks.sql"),
 ];