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Merged
jstone-lucasfilm merged 4 commits into from
Jan 5, 2026
Merged

Improvements to thread safety #2736

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88cfd14
Improvements to thread safety
jstone-lucasfilm Dec 26, 2025
2a288e3
Switch to std::shared_mutex
jstone-lucasfilm Jan 5, 2026
4ca58aa
Retore original comments
jstone-lucasfilm Jan 5, 2026
a7765b6
Include <mutex> for GCC and Clang
jstone-lucasfilm Jan 5, 2026
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8 changes: 8 additions & 0 deletions CONTRIBUTING.md
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Original file line number Diff line number Diff line change
Expand Up @@ -242,6 +242,14 @@ be cheap to copy. When returning shared pointers, they should be returned by
value rather than by reference. Methods should be marked as `const` whenever
they do not modify the object's state.

#### Thread Safety

MaterialX classes support multiple concurrent readers, but not concurrent
reads and writes, following the pattern of standard C++ containers. This
design enables efficient parallel processing in read-heavy workloads such
as shader generation and scene traversal, while keeping the implementation
simple and avoiding the overhead of fine-grained locking.

#### Exception Handling

Exceptions should be used for exceptional conditions rather than for normal
Expand Down
188 changes: 103 additions & 85 deletions source/MaterialXCore/Document.cpp
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Expand Up @@ -5,6 +5,7 @@

#include <MaterialXCore/Document.h>

#include <atomic>
#include <mutex>

MATERIALX_NAMESPACE_BEGIN
Expand All @@ -29,82 +30,120 @@ class Document::Cache
{
public:
Cache() :
valid(false)
_valid(false)
{
}
~Cache() = default;

void setDocument(weak_ptr<Document> document)
{
_doc = document;
invalidate();
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@JGamache-autodesk JGamache-autodesk Jan 5, 2026

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What happens if another thread tries to fetch cached data in the miniscule moment after _doc is set, but before invalidate() has had the time to set _valid to false?
I would recommend using a C++17 std::shared_mutex instead for robustness. It allows multiple-reader/single-writer semantics.

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@jstone-lucasfilm jstone-lucasfilm Jan 5, 2026

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That's a really good suggestion, @JGamache-autodesk, and I've updated the PR to use this approach.

}

void invalidate()
{
_valid.store(false, std::memory_order_relaxed);
}

vector<PortElementPtr> getMatchingPorts(const string& nodeName)
{
refresh();
auto it = _portElementMap.find(nodeName);
return (it != _portElementMap.end()) ? it->second : vector<PortElementPtr>();
}

vector<NodeDefPtr> getMatchingNodeDefs(const string& nodeName)
{
refresh();
auto it = _nodeDefMap.find(nodeName);
return (it != _nodeDefMap.end()) ? it->second : vector<NodeDefPtr>();
}

vector<InterfaceElementPtr> getMatchingImplementations(const string& nodeDef)
{
refresh();
auto it = _implementationMap.find(nodeDef);
return (it != _implementationMap.end()) ? it->second : vector<InterfaceElementPtr>();
}

private:
void refresh()
{
// Thread synchronization for multiple concurrent readers of a single document.
std::lock_guard<std::mutex> guard(mutex);
// Perform a lock-free read and return if the cache is valid.
if (_valid.load(std::memory_order_acquire))
{
return;
}

if (!valid)
// Acquire a lock and double-check the valid flag for robustness.
std::lock_guard<std::mutex> lock(_mutex);
if (_valid.load(std::memory_order_relaxed))
{
// Clear the existing cache.
portElementMap.clear();
nodeDefMap.clear();
implementationMap.clear();
return;
}

// Traverse the document to build a new cache.
for (ElementPtr elem : doc.lock()->traverseTree())
{
const string& nodeName = elem->getAttribute(PortElement::NODE_NAME_ATTRIBUTE);
const string& nodeGraphName = elem->getAttribute(PortElement::NODE_GRAPH_ATTRIBUTE);
const string& nodeString = elem->getAttribute(NodeDef::NODE_ATTRIBUTE);
const string& nodeDefString = elem->getAttribute(InterfaceElement::NODE_DEF_ATTRIBUTE);
// Verify that the document is still valid.
auto doc = _doc.lock();
if (!doc)
{
return;
}

if (!nodeName.empty())
{
PortElementPtr portElem = elem->asA<PortElement>();
if (portElem)
{
portElementMap[portElem->getQualifiedName(nodeName)].push_back(portElem);
}
}
else
// Clear the existing cache.
_portElementMap.clear();
_nodeDefMap.clear();
_implementationMap.clear();

// Traverse the document to build a new cache.
for (ElementPtr elem : doc->traverseTree())
{
const string& nodeName = elem->getAttribute(PortElement::NODE_NAME_ATTRIBUTE);
const string& nodeGraphName = elem->getAttribute(PortElement::NODE_GRAPH_ATTRIBUTE);
const string& nodeString = elem->getAttribute(NodeDef::NODE_ATTRIBUTE);
const string& nodeDefString = elem->getAttribute(InterfaceElement::NODE_DEF_ATTRIBUTE);

const string& portKey = !nodeName.empty() ? nodeName : nodeGraphName;
if (!portKey.empty())
{
PortElementPtr portElem = elem->asA<PortElement>();
if (portElem)
{
if (!nodeGraphName.empty())
{
PortElementPtr portElem = elem->asA<PortElement>();
if (portElem)
{
portElementMap[portElem->getQualifiedName(nodeGraphName)].push_back(portElem);
}
}
_portElementMap[portElem->getQualifiedName(portKey)].push_back(portElem);
}
if (!nodeString.empty())
}
if (!nodeString.empty())
{
NodeDefPtr nodeDef = elem->asA<NodeDef>();
if (nodeDef)
{
NodeDefPtr nodeDef = elem->asA<NodeDef>();
if (nodeDef)
{
nodeDefMap[nodeDef->getQualifiedName(nodeString)].push_back(nodeDef);
}
_nodeDefMap[nodeDef->getQualifiedName(nodeString)].push_back(nodeDef);
}
if (!nodeDefString.empty())
}
if (!nodeDefString.empty())
{
InterfaceElementPtr interface = elem->asA<InterfaceElement>();
if (interface)
{
InterfaceElementPtr interface = elem->asA<InterfaceElement>();
if (interface)
if (interface->isA<Implementation>() || interface->isA<NodeGraph>())
{
if (interface->isA<Implementation>() || interface->isA<NodeGraph>())
{
implementationMap[interface->getQualifiedName(nodeDefString)].push_back(interface);
}
_implementationMap[interface->getQualifiedName(nodeDefString)].push_back(interface);
}
}
}

valid = true;
}

// Release semantics ensure all map writes are visible before valid becomes true.
_valid.store(true, std::memory_order_release);
}

public:
weak_ptr<Document> doc;
std::mutex mutex;
bool valid;
std::unordered_map<string, std::vector<PortElementPtr>> portElementMap;
std::unordered_map<string, std::vector<NodeDefPtr>> nodeDefMap;
std::unordered_map<string, std::vector<InterfaceElementPtr>> implementationMap;
private:
weak_ptr<Document> _doc;
std::mutex _mutex;
std::atomic<bool> _valid;
std::unordered_map<string, std::vector<PortElementPtr>> _portElementMap;
std::unordered_map<string, std::vector<NodeDefPtr>> _nodeDefMap;
std::unordered_map<string, std::vector<InterfaceElementPtr>> _implementationMap;
};

//
Expand All @@ -124,7 +163,7 @@ Document::~Document()
void Document::initialize()
{
_root = getSelf();
_cache->doc = getDocument();
_cache->setDocument(getDocument());

clearContent();
setVersionIntegers(MATERIALX_MAJOR_VERSION, MATERIALX_MINOR_VERSION);
Expand Down Expand Up @@ -284,18 +323,7 @@ std::pair<int, int> Document::getVersionIntegers() const

vector<PortElementPtr> Document::getMatchingPorts(const string& nodeName) const
{
// Refresh the cache.
_cache->refresh();

// Return all port elements matching the given node name.
if (_cache->portElementMap.count(nodeName))
{
return _cache->portElementMap.at(nodeName);
}
else
{
return vector<PortElementPtr>();
}
return _cache->getMatchingPorts(nodeName);
}

ValuePtr Document::getGeomPropValue(const string& geomPropName, const string& geom) const
Expand Down Expand Up @@ -342,19 +370,14 @@ vector<OutputPtr> Document::getMaterialOutputs() const
vector<NodeDefPtr> Document::getMatchingNodeDefs(const string& nodeName) const
{
// Recurse to data library if present.
vector<NodeDefPtr> matchingNodeDefs = hasDataLibrary() ?
vector<NodeDefPtr> matchingNodeDefs = hasDataLibrary() ?
getDataLibrary()->getMatchingNodeDefs(nodeName) :
vector<NodeDefPtr>();

// Refresh the cache.
_cache->refresh();
// Append all nodedefs matching the given node name.
vector<NodeDefPtr> localNodeDefs = _cache->getMatchingNodeDefs(nodeName);
matchingNodeDefs.insert(matchingNodeDefs.end(), localNodeDefs.begin(), localNodeDefs.end());

// Return all nodedefs matching the given node name.
if (_cache->nodeDefMap.count(nodeName))
{
matchingNodeDefs.insert(matchingNodeDefs.end(), _cache->nodeDefMap.at(nodeName).begin(), _cache->nodeDefMap.at(nodeName).end());
}

return matchingNodeDefs;
}

Expand All @@ -364,15 +387,10 @@ vector<InterfaceElementPtr> Document::getMatchingImplementations(const string& n
vector<InterfaceElementPtr> matchingImplementations = hasDataLibrary() ?
getDataLibrary()->getMatchingImplementations(nodeDef) :
vector<InterfaceElementPtr>();

// Refresh the cache.
_cache->refresh();

// Return all implementations matching the given nodedef string.
if (_cache->implementationMap.count(nodeDef))
{
matchingImplementations.insert(matchingImplementations.end(), _cache->implementationMap.at(nodeDef).begin(), _cache->implementationMap.at(nodeDef).end());
}
// Append all implementations matching the given nodedef string.
vector<InterfaceElementPtr> localImpls = _cache->getMatchingImplementations(nodeDef);
matchingImplementations.insert(matchingImplementations.end(), localImpls.begin(), localImpls.end());

return matchingImplementations;
}
Expand All @@ -388,7 +406,7 @@ bool Document::validate(string* message) const

void Document::invalidateCache()
{
_cache->valid = false;
_cache->invalidate();
}

//
Expand Down