Legacy write detection

After the upgrade, cataloginventory_stock_item and cataloginventory_stock_status are frozen: MSI neither reads nor writes them. A third-party extension that sets a quantity by writing them directly appears to succeed and changes nothing.

Because that failure is silent, the upgrade installs four database triggers that count such writes into inventory_legacy_stock_write. An admin notification names the table and the number of ignored writes, and the same record is available from the console.

A direct write to a frozen table changes nothing MSI reads, so triggers count it and the admin and the console report it. The fix is to write through MSI.

Reading the record

bin/magento inventory:legacy-stock:writes            # report; exits non-zero while any write is on record
bin/magento inventory:legacy-stock:writes --clear    # acknowledge and forget them
bin/magento inventory:legacy-stock:writes --disable  # drop the triggers
bin/magento inventory:legacy-stock:writes --enable   # install them again

The report lists, per table and operation, the number of writes, a sample product id and when the first and last writes were seen. It also says when the triggers are not installed.

Fixing the writer

Point the code that made the writes at SourceItemsSaveInterface, or at StockRegistryInterface::updateStockItemBySku(), which MSI projects onto the default source item.

What the triggers can and cannot see

The report is a starting point, not an audit. A trigger fires per row, so it catches every INSERT and every UPDATE that matches at least one row. An UPDATE matching nothing is invisible to the database, and no trigger can see it.

That is a reason not to truncate the tables. While they still hold their pre-upgrade rows, an extension updating a product by id still hits a row and gets counted; on an empty table the same statement passes unnoticed.

The triggers need the TRIGGER privilege. Without it, setup:upgrade logs a warning and continues, and detection stays off until the privilege is granted and --enable is run.

Magento itself no longer writes there

CatalogInventory used to be the loudest writer: saving the inventory configuration section made it rewrite every cataloginventory_stock_item row through UpdateItemsStockUponConfigChangeObserver, 2464 discarded writes on a 2.5k product catalog. That observer is disabled, so what the detector reports is third-party code and not Magento talking to itself.