registry.h

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//
// Copyright 2024 Pixar
//
// Licensed under the terms set forth in the LICENSE.txt file available at
// https://openusd.org/license.
//
#ifndef PXR_USD_VALIDATION_USD_VALIDATION_REGISTRY_H
#define PXR_USD_VALIDATION_USD_VALIDATION_REGISTRY_H

#include "pxr/pxr.h"
#include "pxr/base/tf/singleton.h"
#include "pxr/usdValidation/usdValidation/api.h"
#include "pxr/usdValidation/usdValidation/validator.h"
#include "pxr/usdValidation/usdValidation/fixer.h"

#include <memory>
#include <shared_mutex>
#include <unordered_map>

/// \file

PXR_NAMESPACE_OPEN_SCOPE

/// \class UsdValidationRegistry
///
/// UsdValidationRegistry manages and provides access to UsdValidationValidator
/// / UsdValidationValidatorSuite for USD Validation.
///
/// UsdValidationRegistry is a singleton class, which serves as a central
/// registry to hold / own all validators and validatorSuites by their names.
/// UsdValidationRegistry is also immortal and its singleton instance is never
/// destroyed. This is to ensure that all validators and suites registered with
/// the registry are available throughout the lifetime of the application.
///
/// Both Core USD and client-provided validators are registered with the
/// registry. Validators can be registered and retrieved dynamically, supporting
/// complex validation scenarios across different modules or plugins.
///
/// Clients of USD can register validators either via plugin infrastructure,
/// which results in lazy loading of the validators, or explicitly register
/// validators in their code via appropriate APIs.
///
/// As discussed in UsdValidationValidator, validators are associated with
/// UsdValidateLayerTaskFn, UsdValidateStageTaskFn or UsdValidatePrimTaskFn,
/// which govern how a layer, stage or a prim needs to be validated.
/// UsdValidationValidator / UsdValidationValidatorSuite also have metadata,
/// which can either be provided in the plugInfo.json when registering the
/// validators via plugin mechanism, or by providing metadata field when
/// registering validators.
///
/// Example of registering a validator named "StageMetadataValidator" with
/// doc metadata using plufInfo.json:
///
/// \code
/// {
///     "Plugins": [
///     {
///         "Info": {
///             "Name": "usd"
///             "LibraryPath": "@PLUG_INFO_LIBRARY_PATH",
///             ....
///             ....
///             ....
///             "Validators": {
///                 "keywords" : ["UsdCoreValidators"],
///                 ...
///                 "StageMetadataValidator": {
///                     "doc": "Validates stage metadata."
///                 },
///                 ...
///                 ...
///                 ...
///             }
///         }
///     } ]
/// }
/// \endcode
///
/// The above example can then be registered in the plugin:
///
/// ```cpp
/// TF_REGISTRY_FUNCTION(UsdValidationRegistry)
/// {
///     UsdValidationRegistry& registry = UsdValidationRegistry::GetInstance();
///     const TfToken validatorName("usdValidation:StageMetadataValidator");
///     const UsdValidateStageTaskFn stageTaskFn =
///         [](const UsdStagePtr &usdStage,
///            const UsdValidationTimeRange &timeRange) {
///             UsdValidationErrorVector errors;
///             if (!usdStage->GetDefaultPrim()) {
///                 errors.emplace_back(UsdValidationErrorType::Error,
///                     {UsdValidationErrorSite(usdStage, SdfPath("/"))},
///                     "Stage has missing or invalid defaultPrim.");
///             }
///             if (!usdStage->HasAuthoredMetadata(
///                     UsdGeomTokens->metersPerUnit)) {
///                 errors.emplace_back(UsdValidationErrorType::Error,
///                     {UsdValidationErrorSite(usdStage, SdfPath("/"))},
///                     "Stage does not specify its linear scale in "
///                     "metersPerUnit.");
///             }
///             if (!usdStage->HasAuthoredMetadata(
///                     UsdGeomTokens->upAxis)) {
///                 errors.emplace_back(UsdValidationErrorType::Error,
///                     {UsdValidationErrorSite(usdStage, SdfPath("/"))},
///                     "Stage does not specify an upAxis.");
///             }
///             return errors;
///         };
///     registry.RegisterPluginValidator(validatorName, stageTaskFn);
/// }
/// ```
///
/// Clients can also register validators by explicitly providing
/// UsdValidationValidatorMetadata, instead of relying on plugInfo.json for the
/// same. Though it's recommended to use appropriate APIs when validator metadata
/// is being provided in the plugInfo.json.
///
/// Example of validator registration by explicitly providing metadata, when it's
/// not available in the plugInfo.json:
///
/// ```cpp
/// {
///     UsdValidationRegistry& registry = UsdValidationRegistry::GetInstance();
///     const UsdValidationValidatorMetadata &metadata =
///         GetMetadataToBeRegistered();
///     const UsdValidateLayerTaskFn &layerTask =
///         GetLayerTaskForValidator();
///     registry.RegisterValidator(metadata, layerTask);
/// }
/// ```
///
/// Usage:
///
/// As shown above, UsdValidationValidator or UsdValidationValidatorSuite can be
/// registered using specific metadata or names, and retrieved by their name.
/// The registry also provides functionality to check the existence of a
/// validator / suite, load validators / suites dynamically if they are not in
/// the registry.
///
/// Clients can also retrieve metadata for validators associated with a
/// specific plugin, keywords or schemaTypes, this can help clients filter out
/// relevant validators they need to validate their context / scene.
///
/// Note that this class is designed to be thread-safe:
/// Querying of validator metadata, registering new validator (hence mutating
/// the registry) or retrieving previously registered validator are designed to
/// be thread-safe.
///
/// Validators may also have a number of fixers associated with them, which
/// can provide potential fixes for various validation errors associated with a
/// validation task. Fixers can be retrieved from the UsdValidationValidator or
/// the UsdValidationError itself. Note that UsdValidationRegistry does not
/// manage fixers directly, and these are held by respective
/// UsdValidationValidator(s). It's the responsibility of the client to retrieve
/// appropriate fixers for a given error and apply them, on a provided
/// UsdEditTarget. UsdValidationErrorSite(s) associated with a validation error
/// provide the context of the error, which may be used while applying a fix on
/// a UsdEditTarget, or a stronger layer can be used as an edit target to apply
/// the fix.
///
/// \sa UsdValidationValidator
/// \sa UsdValidationValidatorSuite
/// \sa UsdValidationFixer
class UsdValidationRegistry
{
    UsdValidationRegistry(const UsdValidationRegistry &) = delete;
    UsdValidationRegistry &operator=(const UsdValidationRegistry &) = delete;

public:
    USDVALIDATION_API
    static UsdValidationRegistry &GetInstance()
    {
        return TfSingleton<UsdValidationRegistry>::GetInstance();
    }

    /// Register UsdValidationValidator defined in a plugin using \p
    /// validatorName and \p layerTaskFn with the UsdValidationRegistry.
    ///
    /// Here \p validatorName should include the name of the plugin the
    /// validator belongs to, delimited by ":".
    ///
    /// Note calling RegisterPluginValidator with a validatorName which is
    /// already registered will result in a coding error. HasValidator can be
    /// used to determine if a validator is already registered and associated
    /// with validatorName.
    ///
    /// Also note any other failure to register a validator results in a coding
    /// error.
    ///
    /// \p fixers can be provided to associate fixers with the validator.
    ///
    /// \sa HasValidator
    USDVALIDATION_API
    void RegisterPluginValidator(const TfToken &validatorName,
                                 const UsdValidateLayerTaskFn &layerTaskFn,
                                 std::vector<UsdValidationFixer> fixers = {});

    /// Register UsdValidationValidator defined in a plugin using \p
    /// validatorName and \p stageTaskFn with the UsdValidationRegistry.
    ///
    /// Here \p validatorName should include the name of the plugin the
    /// validator belongs to, delimited by ":".
    ///
    /// Note calling RegisterPluginValidator with a validatorName which is
    /// already registered will result in a coding error. HasValidator can be
    /// used to determine if a validator is already registered and associated
    /// with validatorName.
    ///
    /// Also note any other failure to register a validator results in a coding
    /// error.
    ///
    /// \p fixers can be provided to associate fixers with the validator.
    ///
    /// \sa HasValidator
    USDVALIDATION_API
    void RegisterPluginValidator(const TfToken &validatorName,
                                 const UsdValidateStageTaskFn &stageTaskFn,
                                 std::vector<UsdValidationFixer> fixers = {});

    /// Register UsdValidationValidator defined in a plugin using \p
    /// validatorName and \p primTaskFn with the UsdValidationRegistry.
    ///
    /// Here \p validatorName should include the name of the plugin the
    /// validator belongs to, delimited by ":".
    ///
    /// Note calling RegisterPluginValidator with a validatorName which is
    /// already registered will result in a coding error. HasValidator can be
    /// used to determine if a validator is already registered and associated
    /// with validatorName.
    ///
    /// Also note any other failure to register a validator results in a coding
    /// error.
    ///
    /// \p fixers can be provided to associate fixers with the validator.
    ///
    /// \sa HasValidator
    USDVALIDATION_API
    void RegisterPluginValidator(const TfToken &validatorName,
                                 const UsdValidatePrimTaskFn &primTaskFn,
                                 std::vector<UsdValidationFixer> fixers = {});

    /// Register UsdValidationValidator using \p metadata and \p layerTaskFn
    /// with the UsdValidationRegistry.
    ///
    /// Clients can explicitly provide validator metadata, which is then used to
    /// register a validator and associate it with name metadata. The metadata
    /// here is not specified in a plugInfo.
    ///
    /// Note calling RegisterValidator with a validator name which is already
    /// registered will result in a coding error. HasValidator can be used to
    /// determine if a validator is already registered and associated with
    /// validatorName.
    ///
    /// Also note any other failure to register a validator results in a coding
    /// error.
    ///
    /// \p fixers can be provided to associate fixers with the validator.
    ///
    /// \sa HasValidator
    USDVALIDATION_API
    void RegisterValidator(const UsdValidationValidatorMetadata &metadata,
                           const UsdValidateLayerTaskFn &layerTaskFn,
                           std::vector<UsdValidationFixer> fixers = {});

    /// Register UsdValidationValidator using \p metadata and \p stageTaskFn
    /// with the UsdValidationRegistry.
    ///
    /// Clients can explicitly provide validator metadata, which is then used to
    /// register a validator and associate it with name metadata. The metadata
    /// here is not specified in a plugInfo.
    ///
    /// Note calling RegisterValidator with a validator name which is already
    /// registered will result in a coding error. HasValidator can be used to
    /// determine if a validator is already registered and associated with
    /// validatorName.
    ///
    /// Also note any other failure to register a validator results in a coding
    /// error.
    ///
    /// \p fixers can be provided to associate fixers with the validator.
    ///
    /// \sa HasValidator
    USDVALIDATION_API
    void RegisterValidator(const UsdValidationValidatorMetadata &metadata,
                           const UsdValidateStageTaskFn &stageTaskFn,
                           std::vector<UsdValidationFixer> fixers = {});

    /// Register UsdValidationValidator using \p metadata and \p primTaskFn
    /// with the UsdValidationRegistry.
    ///
    /// Clients can explicitly provide validator metadata, which is then used to
    /// register a validator and associate it with name metadata. The metadata
    /// here is not specified in a plugInfo.
    ///
    /// Note calling RegisterValidator with a validator name which is already
    /// registered will result in a coding error. HasValidator can be used to
    /// determine if a validator is already registered and associated with
    /// validatorName.
    ///
    /// Also note any other failure to register a validator results in a coding
    /// error.
    ///
    /// \p fixers can be provided to associate fixers with the validator.
    ///
    /// \sa HasValidator
    USDVALIDATION_API
    void RegisterValidator(const UsdValidationValidatorMetadata &metadata,
                           const UsdValidatePrimTaskFn &primTaskFn,
                           std::vector<UsdValidationFixer> fixers = {});

    /// Register UsdValidationValidatorSuite defined in a plugin using
    /// \p validatorSuiteName and \p containedValidators with the
    /// UsdValidationRegistry.
    ///
    /// Here \p validatorSuiteName should include the name of the plugin the
    /// validator belongs to, delimited by ":".
    ///
    /// Note UsdValidationValidatorMetadata::isSuite must be set to true in the
    /// plugInfo, else the validatorSuite will not be registered.
    ///
    /// Note calling RegisterPluginValidatorSuite with a validatorSuiteName
    /// which is already registered will result in a coding error.
    /// HasValidatorSuite can be used to determine if a validator is already
    /// registered and associated with validatorName.
    ///
    /// Also note any other failure to register a validator results in a coding
    /// error.
    ///
    /// \sa HasValidatorSuite
    USDVALIDATION_API
    void RegisterPluginValidatorSuite(
        const TfToken &validatorSuiteName,
        const std::vector<const UsdValidationValidator *> &containedValidators);

    /// Register UsdValidationValidatorSuite using \p metadata and
    /// \p containedValidators with the UsdValidationRegistry.
    ///
    /// Clients can explicitly provide validator metadata, which is then used to
    /// register a suite and associate it with name metadata. The metadata
    /// here is not specified in a plugInfo.
    ///
    /// Note UsdValidationValidatorMetadata::isSuite must be set to true in the
    /// plugInfo, else the validatorSuite will not be registered.
    ///
    /// Note calling RegisterPluginValidatorSuite with a validatorSuiteName
    /// which is already registered will result in a coding error.
    /// HasValidatorSuite can be used to determine if a validator is already
    /// registered and associated with validatorName.
    ///
    /// Also note any other failure to register a validator results in a coding
    /// error.
    ///
    /// \sa HasValidatorSuite
    USDVALIDATION_API
    void RegisterValidatorSuite(
        const UsdValidationValidatorMetadata &metadata,
        const std::vector<const UsdValidationValidator *> &containedValidators);

    /// Return true if a UsdValidationValidator is registered with the name \p
    /// validatorName; false otherwise.
    USDVALIDATION_API
    bool HasValidator(const TfToken &validatorName) const;

    /// Return true if a UsdValidationValidatorSuite is registered with the name
    /// \p validatorSuiteName; false otherwise.
    USDVALIDATION_API
    bool HasValidatorSuite(const TfToken &suiteName) const;

    /// Returns a vector of const pointer to UsdValidationValidator
    /// corresponding to all validators registered in the UsdValidationRegistry.
    ///
    /// If a validator is not found in the registry, this method will load
    /// appropriate plugins, if the validator is made available via a plugin.
    ///
    /// Note that this call will load in many plugins which provide a
    /// UsdValidationValidator, if not already loaded. Also note that returned
    /// validators will only include validators defined in plugins or any
    /// explicitly registered validators before this call.
    USDVALIDATION_API
    std::vector<const UsdValidationValidator *> GetOrLoadAllValidators();

    /// Returns a const pointer to UsdValidationValidator if \p validatorName is
    /// found in the registry.
    ///
    /// If a validator is not found in the registry, this method will load
    /// appropriate plugins, if the validator is made available via a plugin.
    ///
    /// Returns a nullptr if no validator is found.
    USDVALIDATION_API
    const UsdValidationValidator *
    GetOrLoadValidatorByName(const TfToken &validatorName);

    /// Returns a vector of const pointer to UsdValidationValidator
    /// corresponding to \p validatorNames found in the registry.
    ///
    /// If a validator is not found in the registry, this method will load
    /// appropriate plugins, if the validator is made available via a plugin.
    ///
    /// Size of returned vector might be less than the size of the input
    /// validatorNames, in case of missing validators.
    USDVALIDATION_API
    std::vector<const UsdValidationValidator *>
    GetOrLoadValidatorsByName(const TfTokenVector &validatorNames);

    /// Returns a vector of const pointer to UsdValidationValidatorSuite
    /// corresponding to all validator suites registered in the
    /// UsdValidationRegistry.
    ///
    /// If a suite is not found in the registry, this method will load
    /// appropriate plugins, if the suite is made available via a plugin.
    ///
    /// Note that this call might load in many plugins which provide a
    /// UsdValidationValidatorSuite, if not already loaded. Also note that
    /// returned suites will only include suites defined in plugins or any
    /// explicitly registered suites before this call.
    USDVALIDATION_API
    std::vector<const UsdValidationValidatorSuite *>
    GetOrLoadAllValidatorSuites();

    /// Returns a const pointer to UsdValidationValidatorSuite if \p suiteName
    /// is found in the registry.
    ///
    /// If a suite is not found in the registry, this method will load
    /// appropriate plugins, if the suite is made available via a plugin.
    ///
    /// Returns a nullptr if no validator is found.
    USDVALIDATION_API
    const UsdValidationValidatorSuite *
    GetOrLoadValidatorSuiteByName(const TfToken &suiteName);

    /// Returns a vector of const pointer to UsdValidationValidatorSuite
    /// corresponding to \p suiteNames found in the registry.
    ///
    /// If a suite is not found in the registry, this method will load
    /// appropriate plugins, if the suite is made available via a plugin.
    ///
    /// Size of returned vector might be less than the size of the input
    /// suiteNames, in case of missing validators.
    USDVALIDATION_API
    std::vector<const UsdValidationValidatorSuite *>
    GetOrLoadValidatorSuitesByName(const TfTokenVector &suiteNames);

    /// Returns true if metadata is found in the _validatorNameToMetadata for
    /// a validator/suite name, false otherwise.
    ///
    /// \p metadata parameter is used as an out parameter here.
    USDVALIDATION_API
    bool GetValidatorMetadata(const TfToken &name,
                              UsdValidationValidatorMetadata *metadata) const;

    /// Return vector of all UsdValidationValidatorMetadata known to the
    /// registry
    USDVALIDATION_API
    UsdValidationValidatorMetadataVector GetAllValidatorMetadata() const;

    /// Returns vector of UsdValidationValidatorMetadata associated with the
    /// Validators which belong to the \p pluginName.
    ///
    /// This API can be used to curate a vector of validator metadata, that
    /// clients may want to load and use in their validation context.
    ///
    /// Note that this method does not result in any plugins to be loaded.
    USDVALIDATION_API
    UsdValidationValidatorMetadataVector
    GetValidatorMetadataForPlugin(const TfToken &pluginName) const;

    /// Returns vector of UsdValidationValidatorMetadata associated with the
    /// Validators which has the \p keyword.
    ///
    /// This API can be used to curate a vector of validator metadata, that
    /// clients may want to load and use in their validation context.
    ///
    /// Note that this method does not result in any plugins to be loaded.
    USDVALIDATION_API
    UsdValidationValidatorMetadataVector
    GetValidatorMetadataForKeyword(const TfToken &keyword) const;

    /// Returns vector of UsdValidationValidatorMetadata associated with the
    /// Validators which has the \p schemaType.
    ///
    /// This API can be used to curate a vector of validator metadata, that
    /// clients may want to load and use in their validation context.
    ///
    /// Note that this method does not result in any plugins to be loaded.
    USDVALIDATION_API
    UsdValidationValidatorMetadataVector
    GetValidatorMetadataForSchemaType(const TfToken &schemaType) const;

    /// Returns vector of UsdValidationValidatorMetadata associated with the
    /// Validators which belong to the \p pluginNames.
    ///
    /// The returned vector is a union of all UsdValidationValidatorMetadata
    /// associated with the plugins.
    ///
    /// This API can be used to curate a vector of validator metadata, that
    /// clients may want to load and use in their validation context.
    ///
    /// Note that this method does not result in any plugins to be loaded.
    USDVALIDATION_API
    UsdValidationValidatorMetadataVector
    GetValidatorMetadataForPlugins(const TfTokenVector &pluginNames) const;

    /// Returns vector of UsdValidationValidatorMetadata associated with the
    /// Validators which has at least one of the \p keywords.
    ///
    /// The returned vector is a union of all UsdValidationValidatorMetadata
    /// associated with the keywords.
    ///
    /// This API can be used to curate a vector of validator metadata, that
    /// clients may want to load and use in their validation context.
    ///
    /// Note that this method does not result in any plugins to be loaded.
    USDVALIDATION_API
    UsdValidationValidatorMetadataVector
    GetValidatorMetadataForKeywords(const TfTokenVector &keywords) const;

    /// Returns vector of UsdValidationValidatorMetadata associated with the
    /// Validators which has at least one of the \p schameTypes.
    ///
    /// The returned vector is a union of all UsdValidationValidatorMetadata
    /// associated with the schemaTypes.
    ///
    /// This API can be used to curate a vector of validator metadata, that
    /// clients may want to load and use in their validation context.
    ///
    /// Note that this method does not result in any plugins to be loaded.
    USDVALIDATION_API
    UsdValidationValidatorMetadataVector
    GetValidatorMetadataForSchemaTypes(const TfTokenVector &schemaTypes) const;

private:
    friend class TfSingleton<UsdValidationRegistry>;

    UsdValidationRegistry();

    // Initialize _validatorNameToMetadata, _keywordToValidatorNames and
    // _schemaTypeToValidatorNames by parsing all plugInfo.json, find all
    // Validators.
    void _PopulateMetadataFromPlugInfo();

    // Templated method to register validator, called by appropriate
    // RegisterValidator methods, providing UsdValidateLayerTaskFn,
    // UsdValidateStageTaskFn or UsdValidatePrimTaskFn.
    template <typename ValidateTaskFn>
    void _RegisterPluginValidator(const TfToken &validatorName,
                                  const ValidateTaskFn &taskFn,
                                  std::vector<UsdValidationFixer> fixers);

    // Overloaded templated _RegisterValidator, where metadata is explicitly
    // provided.
    template <typename ValidateTaskFn>
    void _RegisterValidator(const UsdValidationValidatorMetadata &metadata,
                            const ValidateTaskFn &taskFn,
                            std::vector<UsdValidationFixer> fixers,
                            bool addMetadata = true);

    void _RegisterValidatorSuite(
        const UsdValidationValidatorMetadata &metadata,
        const std::vector<const UsdValidationValidator *> &containedValidators,
        bool addMetadata = true);

    // makes sure metadata provided is legal
    // checkForPrimTask parameter is used to determine if schemaTypes metadata
    // is provided and if the task being registered for the validator is
    // UsdValidatePrimTaskFn.
    // expectSuite parameter is used to determine if the isSuite metadata is
    // appropriately set (for UsdValidationValidatorSuite) or not (for
    // UsdValidationValidator).
    static bool _CheckMetadata(const UsdValidationValidatorMetadata &metadata,
                               bool checkForPrimTask, bool expectSuite = false);

    // Add validator metadata to _validatorNameToMetadata, also updates
    // _schemaTypeToValidatorNames and _keywordToValidatorNames, for easy access
    // to what validators are linked to specific schemaTypes or keywords.
    // _mutex must be acquired before calling this method.
    bool _AddValidatorMetadata(const UsdValidationValidatorMetadata &metadata);

    using _ValidatorNameToValidatorMap
        = std::unordered_map<TfToken, std::unique_ptr<UsdValidationValidator>,
                             TfToken::HashFunctor>;
    using _ValidatorSuiteNameToValidatorSuiteMap
        = std::unordered_map<TfToken,
                             std::unique_ptr<UsdValidationValidatorSuite>,
                             TfToken::HashFunctor>;
    using _ValidatorNameToMetadataMap
        = std::unordered_map<TfToken, UsdValidationValidatorMetadata,
                             TfToken::HashFunctor>;
    using _TokenToValidatorNamesMap
        = std::unordered_map<TfToken, TfTokenVector, TfToken::HashFunctor>;

    // Helper to query
    UsdValidationValidatorMetadataVector _GetValidatorMetadataForToken(
        const _TokenToValidatorNamesMap &tokenToValidatorNames,
        const TfTokenVector &tokens) const;

    // Helper to populate _keywordToValidatorNames and
    // _schemaTypeToValidatorNames
    // _mutex must be acquired before calling this method.
    static void
    _UpdateValidatorNamesMappings(_TokenToValidatorNamesMap &tokenMap,
                                  const TfToken &validatorName,
                                  const TfTokenVector &tokens);

    // Main datastructure which holds validatorName to
    // std::unique_ptr<UsdValidationValidator>
    _ValidatorNameToValidatorMap _validators;
    // Main datastructure which holds suiteName to
    // std::unique_ptr<UsdValidationValidatorSuite>
    _ValidatorSuiteNameToValidatorSuiteMap _validatorSuites;

    // ValidatorName to ValidatorMetadata map
    _ValidatorNameToMetadataMap _validatorNameToMetadata;

    // Following 3 are helper data structures to easy lookup for Validators,
    // when queried for keywords, schemaType or pluginName.

    // This map stores the mapping from keyword to validator names. It may get
    // updated as validators can be registered dynamically outside of the plugin
    // infrastructure.
    _TokenToValidatorNamesMap _keywordToValidatorNames;

    // This map stores the mapping from schemaTypes to validator names. It may
    // get updated as validators can be registered dynamically outside of the
    // plugin infrastructure.
    _TokenToValidatorNamesMap _schemaTypeToValidatorNames;

    // This map stores the mapping from plugin names to validator names.
    // It is populated during the initialization of UsdValidationRegistry
    // and remains constant thereafter.
    _TokenToValidatorNamesMap _pluginNameToValidatorNames;

    // Mutex to protect access to all data members.
    mutable std::shared_mutex _mutex;
};

// Specialize and delete the DeleteInstance function to prevent destruction.
// This will prevent the singleton instance for UsdValidationRegistry from
// being destroyed and hence making it immortal.
template <> void TfSingleton<UsdValidationRegistry>::DeleteInstance() = delete;

USDVALIDATION_API_TEMPLATE_CLASS(TfSingleton<UsdValidationRegistry>);

PXR_NAMESPACE_CLOSE_SCOPE

#endif // PXR_USD_VALIDATION_USD_VALIDATION_REGISTRY_H