Comment 0 for bug 1282925

db_plugin.delete_ports() can lead to long transaction if plugin.deleete_port talks with external system.
it is observed first in nec plugin (bug 1282922), but it affects multiple plugins/drivers.

The detail is described in bug 1282922. Quoted from the original bug report.
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The case I observed is that delete-port from dhcp-agent (release_dhcp_port RPC call) and delete-port from delete-network API request are run in parallel. plugin.delete-port in nec plugin calls REST API call to an external controller in addition to operates on neutron database.

After my investigation and testing, db_plugin.delete_ports() calls plugin.delete_port() under a transaction.
https://github.com/openstack/neutron/blob/master/neutron/db/db_base_plugin_v2.py#L1367
This means the transaction continues over API calls to external controller and it leads to a long transaction.
When plugin.delete_ports() and plugin.delete_port() are run at the same time, even if plugin.delete_port() avoid long transaction, db operations in plugin.delete_port() is blocked and they can fail with timeout.
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