PhosphorRendering::ShaderNodeLiveness Struct Reference

Shared liveness block linking a render node to the ShaderEffect that tracks it. More...

#include <phosphor-rendering/include/PhosphorRendering/ShaderNodeLiveness.h>

Public Attributes
std::mutex	mutex
ShaderNodeRhi *	node = nullptr

Detailed Description

Shared liveness block linking a render node to the ShaderEffect that tracks it.

The scene graph owns render nodes and deletes them on the render thread without telling the item. An item that tracked its node with a plain pointer therefore could not tell "already deleted" from "still live" and had to guess from proxies (does the item still have a window?). That guess failed open on the detach-then-destroy path: the node outlived the item, its back-pointer was never severed, and the next prepare() walked a freed QQuickItem.

Both sides hold this block by shared_ptr, so it outlives whichever dies first. The node nulls node under mutex via ShaderNodeRhi::retractLiveness(); ShaderEffect takes the same mutex across every use of the pointer. A node therefore cannot be destroyed mid-call, and a null read is positive proof the node is gone rather than a guess.

Retraction is per-destructor, not once at the base. C++ runs the most-derived destructor body (and its member teardown) BEFORE the base's, so a retract that lived only in ~ShaderNodeRhi would leave the block advertising a node whose derived half was already gone. Every ShaderNodeRhi subclass therefore calls retractLiveness() as the first statement of its own destructor; the call is idempotent, so the base's repeat costs one uncontended lock.

Lock ordering: ShaderNodeRhi::m_itemMutex is taken AFTER this one, never before, so the two cannot deadlock. ShaderEffect::m_renderNodeMutex is a third mutex that is never held at the same time as this one at all — see ShaderEffect::withTrackedNode, which copies the share out and releases it before locking here.

This lives in its own header so both ShaderEffect.h and ShaderNodeRhi.h can hold a std::shared_ptr<ShaderNodeLiveness> member with the type complete — an incomplete type there would make every translation unit that merely destroys a ShaderEffect subclass depend on transitively pulling in ShaderNodeRhi.h (and with it qrhi.h).

Member Data Documentation

mutex

std::mutex PhosphorRendering::ShaderNodeLiveness::mutex

node

ShaderNodeRhi* PhosphorRendering::ShaderNodeLiveness::node = nullptr

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The documentation for this struct was generated from the following file:

• phosphor-rendering/include/PhosphorRendering/ShaderNodeLiveness.h