fix: replace old draft with a new one atomically

This prevents creation of multiple drafts per chat.

src/chat.rs

@@ -944,12 +944,18 @@ impl ChatId {
             }
         }
 
-        // insert new draft
-        self.maybe_delete_draft(context).await?;
         let row_id = context
             .sql
-            .insert(
-                "INSERT INTO msgs (
+            .transaction(|transaction| {
+                // Delete existing draft if it exists.
+                transaction.execute(
+                    "DELETE FROM msgs WHERE chat_id=? AND state=?",
+                    (self, MessageState::OutDraft),
+                )?;
+
+                // Insert new draft.
+                transaction.execute(
+                    "INSERT INTO msgs (
                  chat_id,
                  from_id,
                  timestamp,
@@ -961,19 +967,22 @@ impl ChatId {
                  hidden,
                  mime_in_reply_to)
          VALUES (?,?,?,?,?,?,?,?,?,?);",
-                (
-                    self,
-                    ContactId::SELF,
-                    time(),
-                    msg.viewtype,
-                    MessageState::OutDraft,
-                    &msg.text,
-                    message::normalize_text(&msg.text),
-                    msg.param.to_string(),
-                    1,
-                    msg.in_reply_to.as_deref().unwrap_or_default(),
-                ),
-            )
+                    (
+                        self,
+                        ContactId::SELF,
+                        time(),
+                        msg.viewtype,
+                        MessageState::OutDraft,
+                        &msg.text,
+                        message::normalize_text(&msg.text),
+                        msg.param.to_string(),
+                        1,
+                        msg.in_reply_to.as_deref().unwrap_or_default(),
+                    ),
+                )?;
+
+                Ok(transaction.last_insert_rowid())
+            })
             .await?;
         msg.id = MsgId::new(row_id.try_into()?);
         Ok(true)
@@ -4854,6 +4863,37 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_only_one_draft_per_chat() -> Result<()> {
+        let t = TestContext::new_alice().await;
+        let chat_id = create_group_chat(&t, ProtectionStatus::Unprotected, "abc").await?;
+
+        let msgs: Vec<message::Message> = (1..=1000)
+            .map(|i| {
+                let mut msg = Message::new(Viewtype::Text);
+                msg.set_text(i.to_string());
+                msg
+            })
+            .collect();
+        let mut tasks = Vec::new();
+        for mut msg in msgs {
+            let ctx = t.clone();
+            let task = tokio::spawn(async move {
+                let ctx = ctx;
+                chat_id.set_draft(&ctx, Some(&mut msg)).await
+            });
+            tasks.push(task);
+        }
+        futures::future::join_all(tasks.into_iter()).await;
+
+        assert!(chat_id.get_draft(&t).await?.is_some());
+
+        chat_id.set_draft(&t, None).await?;
+        assert!(chat_id.get_draft(&t).await?.is_none());
+
+        Ok(())
+    }
+
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn test_change_quotes_on_reused_message_object() -> Result<()> {
         let t = TestContext::new_alice().await;