When Boonweaver Giant dies, Mortuary and Pattern of Rebirth trigger.
Resolve the Mortuary trigger, putting Boonweaver Giant on the bottom of your library from your graveyard.
Resolve the Pattern of Rebirth trigger, putting Boonweaver Giant from your library onto the battlefield.
When Boonweaver Giant enters the battlefield, it triggers, returning Pattern of Rebirth from your graveyard to the battlefield attached to Boonweaver Giant.
Resolve the Yahenni ability from step 1, giving it indestructible until end of turn.