Add exclusion constraint to prevent duplicate items within 10m (#1747)

* Add exclusion constraint to prevent duplicate items within 10m

* Fix missing extension

* More user-friendly error message

* Use MD5 for slightly better performance

* Always use MD5 for columns of type TEXT

* shift constraint into the future

---------

Co-authored-by: k00b <k00b@stacker.news>

api/paidAction/itemCreate.js

@@ -2,6 +2,7 @@ import { ANON_ITEM_SPAM_INTERVAL, ITEM_SPAM_INTERVAL, PAID_ACTION_PAYMENT_METHOD
 import { notifyItemMention, notifyItemParents, notifyMention, notifyTerritorySubscribers, notifyUserSubscribers } from '@/lib/webPush'
 import { getItemMentions, getMentions, performBotBehavior } from './lib/item'
 import { msatsToSats, satsToMsats } from '@/lib/format'
+import { GqlInputError } from '@/lib/error'
 
 export const anonable = true
 
@@ -137,7 +138,15 @@ export async function perform (args, context) {
       }
     })).bio
   } else {
-    item = await tx.item.create({ data: itemData })
+    try {
+      item = await tx.item.create({ data: itemData })
+    } catch (err) {
+      if (err.message.includes('violates exclusion constraint \\"Item_unique_time_constraint\\"')) {
+        const message = `you already submitted this ${itemData.title ? 'post' : 'comment'}`
+        throw new GqlInputError(message)
+      }
+      throw err
+    }
   }
 
   // store a reference to the item in the invoice

prisma/migrations/20241220000308_fix_duplicate_comments/migration.sql

@@ -0,0 +1,15 @@
+-- create a partial exclusion constraint that prevents insertion of duplicate items within 10 minutes
+CREATE EXTENSION IF NOT EXISTS btree_gist;
+ALTER TABLE "Item" ADD CONSTRAINT "Item_unique_time_constraint"
+  EXCLUDE USING gist (
+    "userId" WITH =,
+    -- we use COALESCE so NULL is considered equal to itself. we also use md5 hashes because columns
+    -- of type TEXT can make index row too large and columns of type CITEXT are not supported by GiST.
+    COALESCE("parentId", -1) WITH =,
+    md5(COALESCE("title", '')) WITH =,
+    md5(COALESCE("subName", '')) WITH =,
+    md5(COALESCE("text", '')) WITH =,
+    tsrange(created_at, created_at + INTERVAL '10 minutes') WITH &&
+  )
+  -- enforce constraint after this date
+  WHERE (created_at > '2024-12-30');