Fix potential constraint violation during thread merge.

app/src/main/java/org/thoughtcrime/securesms/database/MessageTable.kt

@@ -4931,11 +4931,43 @@ open class MessageTable(context: Context?, databaseHelper: SignalDatabase) : Dat
   }
 
   override fun remapThread(fromId: Long, toId: Long) {
-    writableDatabase
-      .update(TABLE_NAME)
-      .values(THREAD_ID to toId)
-      .where("$THREAD_ID = ?", fromId)
-      .run()
+    try {
+      writableDatabase
+        .update(TABLE_NAME)
+        .values(THREAD_ID to toId)
+        .where("$THREAD_ID = ?", fromId)
+        .run()
+    } catch (e: SQLiteException) {
+      Log.w(TAG, "Failed to remap threadId, likely causes a unique constraint violation. Fixing.", e)
+      // Looks at all of the messages where the fromRecipient is either fromId or toId, then
+      // finds all messages where the threadId and dateSent match, but the fromRecipients do not.
+      // Deletes the more recent of the messages to prevent duplicates.
+      val deleteCount = writableDatabase
+        .delete(TABLE_NAME)
+        .where(
+          """
+          $ID IN (
+            SELECT $ID FROM $TABLE_NAME AS m1
+            WHERE $THREAD_ID IN ($fromId, $toId)
+            AND EXISTS (
+              SELECT 1 FROM $TABLE_NAME AS m2
+              WHERE m1.$FROM_RECIPIENT_ID = m2.$FROM_RECIPIENT_ID
+              AND m1.$DATE_SENT = m2.$DATE_SENT
+              AN m1.$THREAD_ID != m2.$THREAD_ID
+              AND m1.$ID > m2.$ID
+            )
+          )
+          """
+        )
+        .run()
+
+      Log.w(TAG, "Deleted $deleteCount duplicates. Retrying the remap.", e)
+      writableDatabase
+        .update(TABLE_NAME)
+        .values(THREAD_ID to toId)
+        .where("$THREAD_ID = ?", fromId)
+        .run()
+    }
   }
 
   /**