dictionary_client.h

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/* Copyright (c) 2015, 2025, Oracle and/or its affiliates.
 
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License, version 2.0,
   as published by the Free Software Foundation.
 
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   as designated in a particular file or component or in included license
   documentation.  The authors of MySQL hereby grant you an additional
   permission to link the program and your derivative works with the
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   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License, version 2.0, for more details.
 
   You should have received a copy of the GNU General Public License
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   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
#ifndef DD_CACHE__DICTIONARY_CLIENT_INCLUDED
#define DD_CACHE__DICTIONARY_CLIENT_INCLUDED
 
#include <assert.h>
#include <stddef.h>
#include <functional>
#include <memory>
#include <string>
#include <vector>
 
#include "my_compiler.h"
 
#include "object_registry.h"  // Object_registry
#include "sql/dd/object_id.h"
#include "sql/dd/string_type.h"
#include "sql/mdl.h"
 
class THD;
struct LEX_USER;
 
namespace dd {
class Schema;
class Table;
class Entity_object;
}  // namespace dd
 
namespace dd {
namespace cache {
 
class SPI_lru_cache;
 
/**
  A smart-pointer for managing an SPI_lru_cache even when it is only
  forward declared. Automatically allocated cache with new, and assigns
  m_spi_lru_cache to it, when dereferenced using non-const
  operator->(). Destructor deletes the object pointed to by
  m_spi_lru_cache.
*/
class SPI_lru_cache_owner_ptr {
  SPI_lru_cache *m_spi_lru_cache = nullptr;
 
 public:
  /** Calls delete on m_spi_lru_cache unless nullptr. */
  ~SPI_lru_cache_owner_ptr();
 
  /**
    Creates cache on demand if m_spi_lru_cache is nullptr.
    @return pointer to cache.
  */
  SPI_lru_cache *operator->();
 
  /**
    Const overload which does not create cache on demand, but merely
    returns the pointer.
    @return pointer to cache (may be nullptr)
  */
  const SPI_lru_cache *operator->() const { return m_spi_lru_cache; }
 
  /*
    Predicate for nullptr.
    @return true if points to valid cache, false otherwise.
   */
  bool is_nullptr() const { return (m_spi_lru_cache == nullptr); }
};
 
/**
  Implementation of a dictionary client.
 
  The dictionary client provides a unified interface to accessing dictionary
  objects. The client is a member of the THD, and is typically used in
  server code to access the dictionary. When we refer to "the user" below,
  we mean the server code using the dictionary client.
 
  The main task of the client is to access a shared cache to retrieve
  dictionary objects. The shared cache, in its turn, will access the
  dictionary tables if there is a cache miss.
 
  To support cache eviction, the shared cache must keep track of which
  clients that have acquired an object. When a client acquires an object
  from the shared cache for the first time, it is added to a client local
  object registry. Further acquisition of the same object from the client
  will get the object from the client's registry. Thus, the usage tracking
  in the shared cache only keep track of the number of clients currently
  using the object, and hence, there must be an operation that complements
  acquisition, to inform the shared cache that the object is not used
  anymore. This complementing operation is called releasing the object.
 
  To manage releasing objects, the Auto_releaser class provides some
  support. When an auto releaser is instantiated, it will keep track of
  the objects that are acquired from the shared cache in its lifetime.
  Auto releasers may be nested or stacked, and the current releaser is
  the one at the top of the stack. The auto releaser stack is associated
  with a dictionary client instance. When the auto releaser goes out
  of scope, it will release all objects that have been acquired from the
  shared cache in its lifetime. Objects retrieved earlier than that will
  be automatically released by a releaser further down the auto releaser
  stack. For more coarse grained control, there is a release method that
  will release all objects acquired by the client.
 
  In addition to the auto releasers, the client has an object registry.
  The registry holds pointers to all currently acquired objects. Thus,
  the object registry is the union of the registers in the stack of
  auto releasers. The client's object registry is used for looking up
  objects, while the registers in the auto releasers are used for
  releasing objects.
 
  The client also has a second registry of objects with uncommitted changes.
  These are objects acquired by acquire_for_modification() or registered
  with register_uncommitted_object(). These objects are only present in
  the local registry and not in the shared cache. Once registered, the
  objects can also be retrieved with normal acquire(). This means that
  a given client has a view which includes uncommitted changes made
  using the same client, while other clients do not see these changes.
 
  @note We must handle situations where an object is actually acquired from
        the shared cache, while the dynamic cast to a subtype fails. We use
        the auto release mechanism to achieve that.
 
  @note When a dictionary client method returns true, indicating that an
        error has occurred, the error has been reported, either by the
        client itself, or by the dictionary subsystem.
*/
 
template <typename T>
class Cache_element;
 
#ifndef NDEBUG
/** Data needed to check if an MDL is held. */
struct MDL_descriptor {
  MDL_key m_mdl_key;
  enum_mdl_type m_mdl_type = MDL_TYPE_END;
};
 
/** Tracks MDLs for shared DD object. The object pointer
makes it easier to move the descriptor to a different AR or
remove it when the corresponding object is moved/removed. */
struct Release_lock_descriptor {
  MDL_descriptor m_mdld;
  const Entity_object *m_object = nullptr;
};
#endif /* not NDEBUG */
 
class Dictionary_client {
 public:
  /**
    Class to help releasing and deleting objects.
 
    This class keeps a register of shared objects that are automatically
    released when the instance goes out of scope. When a new instance
    is created, the encompassing dictionary client's current auto releaser
    is replaced by this one, keeping a link to the old one. When the
    auto releaser is deleted, it links the old releaser back in as the
    client's current releaser.
 
    Shared objects that are added to the auto releaser will be released when
    the releaser is deleted. Only the dictionary client is allowed to add
    objects to the auto releaser.
 
    The usage pattern is that objects that are retrieved from the shared
    dictionary cache are added to the current auto releaser. Objects that
    are retrieved from the client's local object register are not added to
    the auto releaser. Thus, when the releaser is deleted, it releases all
    objects that have been retrieved from the shared cache during the
    lifetime of the releaser.
 
    Similarly the auto releaser maintains a list of objects created
    by acquire_uncached(). These objects are owned by the Auto_releaser
    and are deleted when the auto releaser goes out of scope.
  */
 
  class Auto_releaser {
    friend class Dictionary_client;
 
   private:
    Dictionary_client *m_client;
    Object_registry m_release_registry;
    Auto_releaser *m_prev;
 
#ifndef NDEBUG
    /** Cache of MDL_keys which have been asserted to hold when acquiring,
     and which should be re-checked before this releaser goes out of scope. */
    std::vector<Release_lock_descriptor> m_release_locks;
#endif /* not NDEBUG */
 
    /**
      Register an object to be auto released.
 
      @tparam T        Dictionary object type.
      @param  element  Cache element to auto release.
    */
 
    template <typename T>
    void auto_release(Cache_element<T> *element) {
      // Catch situations where we do not use a non-default releaser.
      assert(m_prev != nullptr);
      m_release_registry.put(element);
    }
 
    /**
      Transfer an object from the current to the previous auto releaser.
 
      @tparam T        Dictionary object type.
      @param  object   Dictionary object to transfer.
    */
 
    template <typename T>
    void transfer_release(const T *object);
 
    /**
      Remove an element from some auto releaser down the chain.
 
      Return a pointer to the releaser where the element was found.
      Thus, the element may be re-inserted into the appropriate
      auto releaser after e.g. changing the keys.
 
      @tparam T        Dictionary object type.
      @param  element  Cache element to auto remove.
 
      @return Pointer to the auto releaser where the object was signed up.
     */
 
    template <typename T>
    Auto_releaser *remove(Cache_element<T> *element);
 
    // Create a new empty auto releaser. Used only by the Dictionary_client.
    Auto_releaser();
 
   public:
    /**
      Create a new auto releaser and link it into the dictionary client
      as the current releaser.
 
      @param  client  Dictionary client for which to install this auto
                      releaser.
    */
 
    explicit Auto_releaser(Dictionary_client *client);
 
    // Release all objects registered and restore previous releaser.
    ~Auto_releaser();
 
    // Debug dump to stderr.
    template <typename T>
    void dump() const;
  };
 
 private:
  std::vector<Entity_object *> m_uncached_objects;  // Objects to be deleted.
  Object_registry m_registry_committed;    // Registry of committed objects.
  Object_registry m_registry_uncommitted;  // Registry of uncommitted objects.
  Object_registry m_registry_dropped;      // Registry of dropped objects.
  THD *m_thd;                        // Thread context, needed for cache misses.
  Auto_releaser m_default_releaser;  // Default auto releaser.
  Auto_releaser *m_current_releaser;  // Current auto releaser.
 
  /**
    Se-private ids known not to exist in either TABLES or PARTITIONS
    or both.
  */
  SPI_lru_cache_owner_ptr m_no_table_spids;
 
 private:
  /**
    Get a dictionary object.
 
    The operation retrieves a dictionary object by one of its keys from the
    cache and returns it through the object parameter. If the object is
    already present in the client's local object registry, it is fetched
    from there. Otherwise, it is fetched from the shared cache (possibly
    involving a cache miss), and eventually added to the local object
    registry.
 
    If no object is found for the given key, NULL is returned. The shared
    cache owns the returned object, i.e., the caller must not delete it.
    After using the object(s), the user must release it using one of the
    release mechanisms described earlier.
 
    The reference counter for the object is incremented if the object is
    retrieved from the shared cache. If the object was present in the local
    registry, the reference counter stays the same. A cache miss is handled
    transparently by the shared cache.
 
    @note This function must be called with type T being the same as
          T::Cache_partition. Dynamic casting to the actual subtype
          must be done at an outer level.
 
    @tparam      K       Key type.
    @tparam      T       Dictionary object type.
    @param       key     Key to use for looking up the object.
    @param [out] object  Object pointer, if an object exists, otherwise NULL.
    @param [out] local_committed
                         Whether the object was read from the local
                         committed object registry.
    @param [out] local_uncommitted
                         Whether the object was read from the local
                         uncommitted registry.
 
    @retval      false   No error.
    @retval      true    Error (from handling a cache miss).
  */
 
  template <typename K, typename T>
  [[nodiscard]] bool acquire(const K &key, const T **object,
                             bool *local_committed, bool *local_uncommitted);
 
  /**
    Get an uncommitted dictionary object that can be modified safely.
 
    The difference between this method and acquire(), is that this method
    only looks in the local registry of uncommitted objects. That is, object
    created by acquire_for_modification() or registered with
    register_uncommitted_object(). It will not access the shared cache.
    Objects that have been dropped are returned as nullptr, but
    with the value of the parameter 'dropped' set to 'true'.
 
    @tparam      K       Key type.
    @tparam      T       Dictionary object type.
    @param       key     Key to use for looking up the object.
    @param [out] object  Object pointer, if an object exists, otherwise NULL.
    @param [out] dropped Object exists, but has been dropped and has not yet
                         committed. In this case, 'object' is set to nullptr.
  */
 
  template <typename K, typename T>
  void acquire_uncommitted(const K &key, T **object, bool *dropped);
 
  /**
    Mark all objects of a certain type as not being used by this client.
 
    This function is called with the client's own object registry, or with
    the registry of an auto releaser (which will contain a subset of the
    objects in the client's object registry).
 
    The function will release all objects of a given type in the registry
    submitted.The objects must be present and in use. If the objects become
    unused, they are added to the free list in the shared cache, which is
    then rectified to enforce its capacity constraints. The objects are also
    removed from the client's object registry.
 
    @tparam      T        Dictionary object type.
    @param       registry Object registry tp release from.
 
    @return Number of objects released.
  */
 
  template <typename T>
  size_t release(Object_registry *registry);
 
  /**
    Release all objects in the submitted object registry.
 
    This function will release all objects from the client's registry, or
    from the registry of an auto releaser.
 
    @param       registry Object registry tp release from.
 
    @return Number of objects released.
  */
 
  size_t release(Object_registry *registry);
 
  /**
    Register an uncached object to be auto deleted.
 
    @tparam T       Dictionary object type.
    @param  object  Dictionary object to auto delete.
  */
 
  template <typename T>
  void auto_delete(T *object) {
#ifndef NDEBUG
    // Make sure we do not sign up a shared object for auto delete.
    Cache_element<typename T::Cache_partition> *element = nullptr;
    m_registry_committed.get(
        static_cast<const typename T::Cache_partition *>(object), &element);
    assert(element == nullptr);
 
    // Make sure we do not sign up an uncommitted object for auto delete.
    m_registry_uncommitted.get(
        static_cast<const typename T::Cache_partition *>(object), &element);
    assert(element == nullptr);
 
    // We must require a top level non-default releaser to ensure a
    // predictable life span of the objects.
    assert(m_current_releaser != &m_default_releaser);
#endif
 
    m_uncached_objects.push_back(object);
  }
 
  /**
    Remove an object from the auto delete vector.
 
    @tparam T       Dictionary object type.
    @param  object  Dictionary object to keep.
  */
 
  template <typename T>
  void no_auto_delete(T *object) {
#ifndef NDEBUG
    // Make sure the object has been registered as uncommitted.
    Cache_element<typename T::Cache_partition> *element = nullptr;
    m_registry_uncommitted.get(
        static_cast<const typename T::Cache_partition *>(object), &element);
    assert(element != nullptr);
#endif
    m_uncached_objects.erase(std::remove(m_uncached_objects.begin(),
                                         m_uncached_objects.end(), object),
                             m_uncached_objects.end());
  }
 
  /**
    Transfer object ownership from caller to Dictionary_client,
    and register the object as uncommitted.
 
    This is intended for objects created by the caller that should
    be managed by Dictionary_client. Transferring an object in this
    way will make it accessible by calling acquire().
 
    This method takes a non-const argument as it only makes
    sense to register objects not acquired from the shared cache.
 
    @tparam          T          Dictionary object type.
    @param           object     Object to transfer ownership.
  */
 
  template <typename T>
  void register_uncommitted_object(T *object);
 
  /**
    Transfer object ownership from caller to Dictionary_client,
    and register the object as dropped.
 
    This method is used internally by the Dictionary_client for
    keeping track of dropped objects. This is needed before
    transaction commit if an attempt is made to acquire the
    dropped object, to avoid consulting the shared cache, which
    might contaminate the cache due to a cache miss (handled with
    isolation level READ_COMMITTED). Instead of consulting the
    shared cache, this Dictionary_client will recognize that the
    object is dropped, and return a nullptr.
 
    This method takes a non-const argument as it only makes
    sense to register objects not acquired from the shared cache.
 
    @tparam          T          Dictionary object type.
    @param           object     Object to transfer ownership.
  */
 
  template <typename T>
  void register_dropped_object(T *object);
 
  /**
    Remove the uncommitted objects from the client and (depending
    on the parameter) put them into the shared cache,
    thereby making them visible to other clients. Should be called
    after commit to disk but before metadata locks are released.
 
    Can also be called after rollback in order to explicitly throw
    the modified objects away before making any actions to compensate
    for a partially completed statement. Note that uncommitted objects
    are automatically removed once the topmost stack-allocated auto
    releaser goes out of scope, so calling this function in case of
    abort is only needed to make acquire return the old object again
    later in the same statement.
 
    @param           commit_to_shared_cache
                                If true, uncommitted objects will
                                be put into the shared cache.
    @tparam          T          Dictionary object type.
  */
 
  template <typename T>
  void remove_uncommitted_objects(bool commit_to_shared_cache);
 
  template <typename T>
  using Const_ptr_vec = std::vector<const T *>;
 
  /**
    Fetch objects from DD tables that match the supplied key.
 
    @tparam Object_type Type of object to fetch.
    @param coll         Vector to fill with objects.
    @param object_key   The search key. If key is not supplied, then
                        we do full index scan.
 
    @return false       Success.
    @return true        Failure (error is reported).
  */
 
  template <typename Object_type>
  [[nodiscard]] bool fetch(Const_ptr_vec<Object_type> *coll,
                           const Object_key *object_key);
 
  /**
    Auxiliary function to retrieve an object by its object id without caching
    it.It is responsibility of the caller to delete the retrieved object.
  */
  template <typename T>
  bool acquire_uncached_impl(Object_id id, T **object);
 
  /**
    Auxiliary function to retrieve a possibly uncommitted object by its object
    id without caching it. It is responsibility of the caller to delete the
    retrieved object.
  */
  template <typename T>
  bool acquire_uncached_uncommitted_impl(Object_id id, T **object);
 
 public:
  // Initialize an instance with a default auto releaser.
  explicit Dictionary_client(THD *thd);
 
  // Make sure all objects are released.
  ~Dictionary_client();
 
  MY_COMPILER_DIAGNOSTIC_PUSH()
  MY_COMPILER_CLANG_WORKAROUND_TPARAM_DOCBUG()
  /**
    Retrieve an object by its object id.
 
    @tparam       T       Dictionary object type.
    @param        id      Object id to retrieve.
    @param [out]  object  Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from handling a cache miss).
  */
  MY_COMPILER_DIAGNOSTIC_POP()
 
  template <typename T>
  [[nodiscard]] bool acquire(Object_id id, const T **object);
 
  /**
    Retrieve an object by its object id.
 
    This function returns a cloned object that can be modified.
 
    @tparam       T       Dictionary object type.
    @param        id      Object id to retrieve.
    @param [out]  object  Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from handling a cache miss).
  */
 
  template <typename T>
  [[nodiscard]] bool acquire_for_modification(Object_id id, T **object);
 
  /**
    Retrieve an object by its object id without caching it.
 
    The object is not cached but owned by the dictionary client, who
    makes sure it is deleted. The object must not be released, and may not
    be used as a parameter to the other dictionary client methods since it is
    not known by the object registry.
 
    @tparam       T       Dictionary object type.
    @param        id      Object id to retrieve.
    @param [out]  object  Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from reading the dictionary tables).
   */
 
  template <typename T>
  [[nodiscard]] bool acquire_uncached(Object_id id, T **object);
 
  /**
    Retrieve an object by its object id without caching it.
 
    The object is not cached and owned by the caller through unique_ptr
    it provides. The object may not be used as a parameter to the other
    dictionary client methods since it is not known by the object registry.
 
    @tparam       T           Dictionary object type.
    @param        id          Object id to retrieve.
    @param [out]  object_ptr  Smart pointer with dictionary object,
                              if present or with nullptr otherwise.
 
    @retval       false   No error.
    @retval       true    Error (from reading the dictionary tables).
  */
  template <typename T>
  [[nodiscard]] bool acquire_uncached(Object_id id,
                                      std::unique_ptr<T> *object_ptr);
 
  /**
    Retrieve a possibly uncommitted object by its object id without caching it.
 
    The object is not cached but owned by the dictionary client, who
    makes sure it is deleted. The object must not be released, and may not
    be used as a parameter to the other dictionary client methods since it is
    not known by the object registry.
 
    When the object is read from the persistent tables, the transaction
    isolation level is READ UNCOMMITTED. This is necessary to be able to
    read uncommitted data from an earlier stage of the same session.
 
    @tparam       T       Dictionary object type.
    @param        id      Object id to retrieve.
    @param [out]  object  Dictionary object, if present; otherwise nullptr.
 
    @retval       false   No error.
    @retval       true    Error (from reading the dictionary tables).
   */
 
  template <typename T>
  [[nodiscard]] bool acquire_uncached_uncommitted(Object_id id, T **object);
 
  /**
    Retrieve a possibly uncommitted object by its object id without caching it.
 
    The object is not cached and owned by the caller through unique_ptr
    it provides. The object may not be used as a parameter to the other
    dictionary client methods since it is not known by the object registry.
 
    When the object is read from the persistent tables, the transaction
    isolation level is READ UNCOMMITTED. This is necessary to be able to
    read uncommitted data from an earlier stage of the same session.
 
    @tparam       T           Dictionary object type.
    @param        id          Object id to retrieve.
    @param [out]  object_ptr  Smart pointer with dictionary object,
                              if present or with nullptr otherwise.
 
    @retval       false   No error.
    @retval       true    Error (from reading the dictionary tables).
  */
  template <typename T>
  [[nodiscard]] bool acquire_uncached_uncommitted(
      Object_id id, std::unique_ptr<T> *object_ptr);
 
  /**
    Retrieve an object by its name.
 
    @tparam       T             Dictionary object type.
    @param        object_name   Name of the object.
    @param [out]  object        Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from handling a cache miss).
  */
 
  template <typename T>
  [[nodiscard]] bool acquire(const String_type &object_name, const T **object);
 
  /**
    Retrieve an object by its name.
 
    This function returns a cloned object that can be modified.
 
    @tparam       T             Dictionary object type.
    @param        object_name   Name of the object.
    @param [out]  object        Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from handling a cache miss).
  */
 
  template <typename T>
  [[nodiscard]] bool acquire_for_modification(const String_type &object_name,
                                              T **object);
 
  /**
    Retrieve an object by its schema- and object name.
 
    @note We will acquire an IX-lock on the schema name unless we already
          have one. This is needed for proper synchronization with schema
          DDL in cases where the table does not exist, and where the
          indirect synchronization based on table names therefore will not
          apply.
 
    @todo TODO: We should change the MDL acquisition (see above) for a more
          long term solution.
 
    @tparam       T             Dictionary object type.
    @param        schema_name   Name of the schema containing the object.
    @param        object_name   Name of the object.
    @param [out]  object        Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from handling a cache miss, or from
                                 failing to get an MDL lock).
  */
 
  template <typename T>
  [[nodiscard]] bool acquire(const String_type &schema_name,
                             const String_type &object_name, const T **object);
 
  /**
    Retrieve an object by its schema- and object name.
 
    This function returns a cloned object that can be modified.
 
    @note We will acquire an IX-lock on the schema name unless we already
          have one. This is needed for proper synchronization with schema
          DDL in cases where the table does not exist, and where the
          indirect synchronization based on table names therefore will not
          apply.
 
    @todo TODO: We should change the MDL acquisition (see above) for a more
          long term solution.
 
    @tparam       T             Dictionary object type.
    @param        schema_name   Name of the schema containing the object.
    @param        object_name   Name of the object.
    @param [out]  object        Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from handling a cache miss, or from
                                 failing to get an MDL lock).
  */
 
  template <typename T>
  [[nodiscard]] bool acquire_for_modification(const String_type &schema_name,
                                              const String_type &object_name,
                                              T **object);
 
  /**
    Retrieve an object by its schema- and object name.
 
    @note We will acquire an IX-lock on the schema name unless we already
          have one. This is needed for proper synchronization with schema
          DDL in cases where the table does not exist, and where the
          indirect synchronization based on table names therefore will not
          apply.
 
    @note This is a variant of the method above asking for an object of type
          T, and hence using T's functions for updating name keys etc.
          This function, however, returns the instance pointed to as type
          T::Cache_partition to ease handling of various subtypes
          of the same base type.
 
    @todo TODO: We should change the MDL acquisition (see above) for a more
          long term solution.
 
    @tparam       T             Dictionary object type.
    @param        schema_name   Name of the schema containing the object.
    @param        object_name   Name of the object.
    @param [out]  object        Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from handling a cache miss, or from
                                 failing to get an MDL lock).
  */
 
  template <typename T>
  [[nodiscard]] bool acquire(const String_type &schema_name,
                             const String_type &object_name,
                             const typename T::Cache_partition **object);
 
  /**
    Retrieve an object by its schema- and object name.
 
    This function returns a cloned object that can be modified.
 
    @note We will acquire an IX-lock on the schema name unless we already
          have one. This is needed for proper synchronization with schema
          DDL in cases where the table does not exist, and where the
          indirect synchronization based on table names therefore will not
          apply.
 
    @note This is a variant of the method above asking for an object of type
          T, and hence using T's functions for updating name keys etc.
          This function, however, returns the instance pointed to as type
          T::Cache_partition to ease handling of various subtypes
          of the same base type.
 
    @todo TODO: We should change the MDL acquisition (see above) for a more
          long term solution.
 
    @tparam       T             Dictionary object type.
    @param        schema_name   Name of the schema containing the object.
    @param        object_name   Name of the object.
    @param [out]  object        Dictionary object, if present; otherwise NULL.
 
    @retval       false   No error.
    @retval       true    Error (from handling a cache miss, or from
                                 failing to get an MDL lock).
  */
 
  template <typename T>
  [[nodiscard]] bool acquire_for_modification(
      const String_type &schema_name, const String_type &object_name,
      typename T::Cache_partition **object);
 
  /**
    Retrieve a table object by its se private id.
 
    @param       engine          Name of the engine storing the table.
    @param       se_private_id   SE private id of the table.
    @param [out] table           Table object, if present; otherwise NULL.
    @param       skip_spi_cache  Optionally skip looking into the missing
                                 SE private ID cache. Defaults to false.
 
    @note The object must be acquired uncached since we cannot acquire a
          metadata lock in advance since we do not know the table name.
          Thus, the returned table object is owned by the caller, who must
          make sure it is deleted.
 
    @retval      false    No error or if object was not found.
    @retval      true     Error (e.g. from reading DD tables, or if an
                                 object of a wrong type was found).
  */
 
  [[nodiscard]] bool acquire_uncached_table_by_se_private_id(
      const String_type &engine, Object_id se_private_id, Table **table,
      bool skip_spi_cache = false);
 
  /**
    Retrieve a table object by its partition se private id.
 
    @param       engine           Name of the engine storing the table.
    @param       se_partition_id  SE private id of the partition.
    @param [out] table            Table object, if present; otherwise NULL.
 
    @retval      false    No error or if object was not found.
    @retval      true     Error (from handling a cache miss).
  */
 
  [[nodiscard]] bool acquire_uncached_table_by_partition_se_private_id(
      const String_type &engine, Object_id se_partition_id, Table **table);
 
  /**
    Retrieve schema and table name by the se private id of the table.
 
    @param        engine          Name of the engine storing the table.
    @param        se_private_id   SE private id of the table.
    @param  [out] schema_name     Name of the schema containing the table.
    @param  [out] table_name      Name of the table.
 
    @retval      false    No error OR if object was not found.
                          The OUT params will be set to empty
                          string when object is not found.
    @retval      true     Error.
  */
 
  [[nodiscard]] bool get_table_name_by_se_private_id(const String_type &engine,
                                                     Object_id se_private_id,
                                                     String_type *schema_name,
                                                     String_type *table_name);
 
  /**
    Retrieve schema and table name by the se private id of the partition.
 
    @param        engine           Name of the engine storing the table.
    @param        se_partition_id  SE private id of the table partition.
    @param  [out] schema_name      Name of the schema containing the table.
    @param  [out] table_name       Name of the table.
 
    @retval      false    No error or if object was not found.
                          The OUT params will be set to empty
                          string when object is not found.
    @retval      true     Error.
  */
 
  [[nodiscard]] bool get_table_name_by_partition_se_private_id(
      const String_type &engine, Object_id se_partition_id,
      String_type *schema_name, String_type *table_name);
 
  /**
    Retrieve a table name of a given trigger name and schema.
 
    @param        schema           Schema containing the trigger.
    @param        trigger_name     Name of the trigger.
    @param  [out] table_name       Name of the table for which
                                   trigger belongs to. Empty string if
                                   there is no such trigger.
 
    @retval      false    No error.
    @retval      true     Error.
  */
 
  [[nodiscard]] bool get_table_name_by_trigger_name(
      const Schema &schema, const String_type &trigger_name,
      String_type *table_name);
 
  /**
    Check if schema contains foreign key with specified name.
 
    @param        schema            Schema containing the foreign key.
    @param        foreign_key_name  Name of the foreign key.
    @param  [out] exists            Set to true if foreign key with
                                    the name provided exists in the
                                    schema, false otherwise.
 
    @retval      false    No error.
    @retval      true     Error.
  */
 
  [[nodiscard]] bool check_foreign_key_exists(
      const Schema &schema, const String_type &foreign_key_name, bool *exists);
 
  /**
    Check if schema contains check constraint with specified name.
 
    @param        schema            Schema containing the check constraint.
    @param        check_cons_name   Name of the check constraint.
    @param  [out] exists            Set to true if check constraint with
                                    the name provided exists in the
                                    schema, false otherwise.
 
    @retval      false    No error.
    @retval      true     Error.
  */
 
  bool check_constraint_exists(const Schema &schema,
                               const String_type &check_cons_name,
                               bool *exists);
 
  /**
    Fetch the names of the components in the schema. Hidden components are
    ignored. E.g., Object with dd::Table::hidden() == true will be ignored.
 
    @tparam        T              Type of object to retrieve names for.
    @param         schema         Schema for which to get component names.
    @param   [out] names          An std::vector containing all object names.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
  */
 
  template <typename T>
  [[nodiscard]] bool fetch_schema_component_names(
      const Schema *schema, std::vector<String_type> *names) const;
 
  /**
    Fetch the names of the tables in the schema belonging to specific
    storage engine. E.g., Object with dd::Table::hidden() == true will be
    ignored.
 
    @param         schema         Schema for which to get component names.
    @param         engine         Engine name of tables to match.
    @param   [out] names          An std::vector containing all object names.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
  */
 
  [[nodiscard]] bool fetch_schema_table_names_by_engine(
      const Schema *schema, const String_type &engine,
      std::vector<String_type> *names) const;
 
  /**
    Fetch the names of the server tables in the schema.  Ignore tables
    hidden by SE.
 
    @param         schema         Schema for which to get component names.
    @param   [out] names          An std::vector containing all object names.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
  */
 
  [[nodiscard]] bool fetch_schema_table_names_not_hidden_by_se(
      const Schema *schema, std::vector<String_type> *names) const;
 
  /** TODO.dt */
  [[nodiscard]] bool fetch_schema_base_table_names_not_hidden_by_se(
      const Schema *schema, std::vector<String_type> *names) const;
 
  /**
    Fetch all global component ids of the given type.
 
    @tparam        T              Type of components to get.
    @param   [out] ids            An std::vector containing all component ids.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
  */
 
  template <typename T>
  [[nodiscard]] bool fetch_global_component_ids(
      std::vector<Object_id> *ids) const;
 
  /**
    Fetch all global component names of the given type.
 
    @tparam        T              Type of components to get.
    @param   [out] names          An std::vector containing all component names.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
  */
 
  template <typename T>
  [[nodiscard]] bool fetch_global_component_names(
      std::vector<String_type> *names) const;
 
  /**
    Execute the submitted lambda function for each entity of the given type
    selected by the submitted key. If the lambda returns true, iteration
    stops and the function returns.
 
    @tparam Object_type Entity type to examine.
    @param  object_key  Key to use for selecting entities.
    @param  processor   Lambda to execute for each entity.
    @return      true   Failure (error is reported unless the lambda
                        returned true).
    @return      false  Success.
  */
 
  template <typename Object_type>
  [[nodiscard]] bool foreach (
      const Object_key *object_key,
      std::function<bool(std::unique_ptr<Object_type> &)> const &processor)
      const;
 
  /**
    Fetch all components in the schema.
 
    @tparam        T              Type of components to get.
    @param         schema         Schema for which to get components.
    @param   [out] coll           An std::vector containing all components.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
  */
 
  template <typename T>
  [[nodiscard]] bool fetch_schema_components(const Schema *schema,
                                             Const_ptr_vec<T> *coll);
 
  /**
    Fetch all global components of the given type.
 
    @tparam        T              Type of components to get.
    @param   [out] coll           An std::vector containing all components.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
  */
 
  template <typename T>
  [[nodiscard]] bool fetch_global_components(Const_ptr_vec<T> *coll);
 
  /**
     Check if a user is referenced as definer by some object of the given type.
 
     @tparam        T              Type of dictionary objects to check.
     @param         user           User name, including @ and host.
     @param   [out] is_definer     True if the user is referenced as definer
                                   by some object.
 
     @return      true   Failure (error is reported, is_definer is undefined).
     @return      false  Success.
   */
  template <typename T>
  [[nodiscard]] bool is_user_definer(const LEX_USER &user,
                                     bool *is_definer) const;
 
  /**
    Fetch Object ids of all the views referencing base table/ view/ stored
    function name specified in "schema"."name". The views are retrieved
    using READ_UNCOMMITTED reads as the views could be changed by the same
    statement (e.g. multi-table/-view RENAME TABLE).
 
    @tparam       T               Type of the object (View_table/View_routine)
                                  to retrieve view names for.
    @param        schema          Schema name.
    @param        tbl_or_sf_name  Base table/ View/ Stored function name.
    @param[out]   view_ids        Vector to store Object ids of all the views
                                  referencing schema.name.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
  */
  template <typename T>
  [[nodiscard]] bool fetch_referencing_views_object_id(
      const char *schema, const char *tbl_or_sf_name,
      std::vector<Object_id> *view_ids) const;
 
  /**
    Fetch the names of tables (children) which have foreign keys
    defined to the given table (parent).
 
    @param        parent_schema    Schema name of parent table.
    @param        parent_name      Table name of parent table.
    @param        parent_engine    Storage engine of parent table.
    @param        uncommitted      Use READ_UNCOMMITTED isolation.
    @param[out]   children_schemas Schema names of child tables.
    @param[out]   children_names   Table names of child tables.
 
    @return      true   Failure (error is reported).
    @return      false  Success.
 
    @note Child tables are identified by matching pairs of names.
 
    @note This is a temporary workaround until WL#6049. This function will
          *not* take any locks protecting against DDL changes. So the returned
          names could become invalid at any time - e.g. due to DROP DATABASE,
          DROP TABLE or DROP FOREIGN KEY.
  */
 
  [[nodiscard]] bool fetch_fk_children_uncached(
      const String_type &parent_schema, const String_type &parent_name,
      const String_type &parent_engine, bool uncommitted,
      std::vector<String_type> *children_schemas,
      std::vector<String_type> *children_names);
 
  /**
     Invalidate a cache entry.
 
     This function will acquire a table object based on the schema qualified
     table name, and call 'invalidate(table_object)'.
 
     @note This function only applies to tables yet.
 
     @param        schema_name   Name of the schema containing the table.
     @param        table_name    Name of the table.
 
     @retval       false   No error.
     @retval       true    Error (from handling a cache miss, or from
                                  failing to get an MDL lock).
   */
 
  [[nodiscard]] bool invalidate(const String_type &schema_name,
                                const String_type &table_name);
 
  /**
    Invalidate a cache entry.
 
    This function will remove and delete an object from the shared cache,
    based on the id of the object. If the object id is present in the local
    object registry and the auto releaser, it will be removed from there as
    well.
 
    @note There is no particular consideration of already dropped or modified
          objects in this method.
 
    @tparam T       Dictionary object type.
    @param  object  Object to be invalidated.
  */
 
  template <typename T>
  void invalidate(const T *object);
 
  /**
    Remove and delete an object from the cache and the dd tables.
 
    This function will remove the object from the local registry as well as
    the shared cache. This means that all keys associated with the object will
    be removed from the maps, and the cache element wrapper will be deleted.
    Afterwards, the object pointed to will also be deleted, and finally, the
    corresponding entry in the appropriate dd table is deleted. The object may
    not be accessed after calling this function.
 
    @sa invalidate()
 
    @note The object parameter is const since the contents of the object
          is not really changed, the object is just deleted. The method
          makes sure there is an exclusive meta data lock on the object
          name.
 
    @note The argument to this function may come from acquire(), and may
          be an instance that is present in the uncommitted registry,
          or in the committed registry. These use cases are handled by
          the implementation of the function. The ownership of the
          'object' is not changed, instead, a clone is created and
          added to the dropped registry.
 
    @tparam T       Dictionary object type.
    @param  object  Object to be dropped.
 
    @retval false   The operation was successful.
    @retval true    There was an error.
  */
 
  template <typename T>
  [[nodiscard]] bool drop(const T *object);
 
  /**
    Store a new dictionary object.
 
    This function will write the object to the dd tables. The object is
    added neither to the dictionary client's object registry nor the shared
    cache.
 
    @note A precondition is that the object has not been acquired from the
          shared cache. For storing an object which is already in the cache,
          please use update().
 
    @note After calling store(), the submitted dictionary object can not be
          used for further calls to store(). It might be used as an argument
          to update(), but this is not recommended since calling update()
          will imply transferring object ownership to the dictionary client.
          Instead, please call 'acquire_for_modification()' to get a new
          object instance to use for modification and further updates.
 
    @tparam T       Dictionary object type.
    @param  object  Object to be stored.
 
    @retval false   The operation was successful.
    @retval true    There was an error.
  */
 
  template <typename T>
  [[nodiscard]] bool store(T *object);
 
  /**
    Update a persisted dictionary object, but keep the shared cache unchanged.
 
    This function will store a dictionary object to the DD tables after
    verifying that an object with the same id already exists. The old object,
    which may be present in the shared dictionary cache, is not modified. To
    make the changes visible in the shared cache, please call
    remove_uncommitted_objects().
 
    @note A precondition is that the object has been acquired from the
          shared cache indirectly by acquire_for_modification(). For storing
          an object which is not already in the cache, please use store().
 
    @note The new_object pointer submitted to this function, is owned by the
          auto delete vector. When registering the new object as an uncommitted
          object, the object must be removed from the auto delete vector.
 
    @tparam          T          Dictionary object type.
    @param           new_object New object, not present in the cache, to be
                                stored persistently.
 
    @retval          false      The operation was successful.
    @retval          true       There was an error.
  */
 
  template <typename T>
  [[nodiscard]] bool update(T *new_object);
 
  /**
    Remove the uncommitted objects from the client.
 
    Can also be used to explicitly throw the modified objects
    away before making any actions to compensate
    for a partially completed statement. Note that uncommitted objects
    are automatically removed once the topmost stack-allocated auto
    releaser goes out of scope, so calling this function in case of
    abort is only needed to make acquire return the old object again
    later in the same statement.
  */
 
  void rollback_modified_objects();
 
  /**
    Remove the uncommitted objects from the client and put them into
    the shared cache, thereby making them visible to other clients.
    Should be called after commit to disk but before metadata locks
    are released.
  */
 
  void commit_modified_objects();
 
  /**
    Remove table statistics entries from mysql.table_stats and
    mysql.index_stats.
 
    @param schema_name  Schema name of the table
    @param table_name   Table name of which stats should be cleaned.
 
    @return true  - on failure
    @return false - on success
  */
  [[nodiscard]] bool remove_table_dynamic_statistics(
      const String_type &schema_name, const String_type &table_name);
 
  /**
    Debug dump of a partition of the client and its registry to stderr.
 
    @tparam T       Dictionary object type.
  */
  template <typename T>
  void dump() const;
};
 
}  // namespace cache
}  // namespace dd
 
// Functions declarations exported only to facilitate unit testing.
namespace dd_cache_unittest {
void insert(dd::cache::SPI_lru_cache_owner_ptr &c, dd::Object_id id);
bool is_cached(const dd::cache::SPI_lru_cache_owner_ptr &c, dd::Object_id id);
}  // namespace dd_cache_unittest
 
#endif  // DD_CACHE__DICTIONARY_CLIENT_INCLUDED