Don't duplicate queued storage diffs

- currently, if we don't recognize the same diff several times (e.g.
  if you restart the storage diff watcher pointed at the same file),
  we'll add the same row to the queue on each run.
- these changes assure we only queue an unrecognized diff once.

libraries/shared/storage/storage_queue.go

@@ -38,7 +38,7 @@ func NewStorageQueue(db *postgres.DB) StorageQueue {
 func (queue StorageQueue) Add(row utils.StorageDiffRow) error {
 	_, err := queue.db.Exec(`INSERT INTO public.queued_storage (contract,
 		block_hash, block_height, storage_key, storage_value) VALUES
-		($1, $2, $3, $4, $5)`, row.Contract.Bytes(), row.BlockHash.Bytes(),
+		($1, $2, $3, $4, $5) ON CONFLICT DO NOTHING`, row.Contract.Bytes(), row.BlockHash.Bytes(),
 		row.BlockHeight, row.StorageKey.Bytes(), row.StorageValue.Bytes())
 	return err
 }

libraries/shared/storage/storage_queue_test.go

@@ -49,11 +49,22 @@ var _ = Describe("Storage queue", func() {
 		Expect(addErr).NotTo(HaveOccurred())
 	})
 
-	It("adds a storage row to the db", func() {
-		var result utils.StorageDiffRow
-		getErr := db.Get(&result, `SELECT contract, block_hash, block_height, storage_key, storage_value FROM public.queued_storage`)
-		Expect(getErr).NotTo(HaveOccurred())
-		Expect(result).To(Equal(row))
+	Describe("Add", func() {
+		It("adds a storage row to the db", func() {
+			var result utils.StorageDiffRow
+			getErr := db.Get(&result, `SELECT contract, block_hash, block_height, storage_key, storage_value FROM public.queued_storage`)
+			Expect(getErr).NotTo(HaveOccurred())
+			Expect(result).To(Equal(row))
+		})
+
+		It("does not duplicate storage rows", func() {
+			addErr := queue.Add(row)
+			Expect(addErr).NotTo(HaveOccurred())
+			var count int
+			getErr := db.Get(&count, `SELECT count(*) FROM public.queued_storage`)
+			Expect(getErr).NotTo(HaveOccurred())
+			Expect(count).To(Equal(1))
+		})
 	})
 
 	It("deletes storage row from db", func() {