lib/channel/channel_args.h

//
//
// Copyright 2015 gRPC authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
//

#ifndef GRPC_SRC_CORE_LIB_CHANNEL_CHANNEL_ARGS_H
#define GRPC_SRC_CORE_LIB_CHANNEL_CHANNEL_ARGS_H

#include <grpc/support/port_platform.h>

#include <stddef.h>
#include <stdint.h>

#include <algorithm>  // IWYU pragma: keep
#include <iosfwd>
#include <memory>
#include <string>
#include <type_traits>
#include <utility>

#include "absl/meta/type_traits.h"
#include "absl/strings/string_view.h"
#include "absl/types/optional.h"

#include <grpc/event_engine/event_engine.h>
#include <grpc/grpc.h>

#include "src/core/lib/avl/avl.h"
#include "src/core/lib/gpr/useful.h"
#include "src/core/lib/gprpp/debug_location.h"
#include "src/core/lib/gprpp/dual_ref_counted.h"
#include "src/core/lib/gprpp/ref_counted.h"
#include "src/core/lib/gprpp/ref_counted_ptr.h"
#include "src/core/lib/gprpp/ref_counted_string.h"
#include "src/core/lib/gprpp/time.h"
#include "src/core/lib/surface/channel_stack_type.h"

// TODO(hork): When we're ready to allow setting via a channel arg from the
// application, replace this with a macro in
// include/grpc/impl/codegen/grpc_types.h.
#define GRPC_INTERNAL_ARG_EVENT_ENGINE "grpc.internal.event_engine"

// Channel args are intentionally immutable, to avoid the need for locking.

namespace grpc_core {

// Define a traits object for vtable lookup - allows us to integrate with
// existing code easily (just define the trait!) and allows some magic in
// ChannelArgs to automatically derive a vtable from a T*.
// To participate as a pointer, instances should expose the function:
//   // Gets the vtable for this type
//   static const grpc_arg_pointer_vtable* VTable();
//   // Performs any mutations required for channel args to own a pointer
//   // Only needed if ChannelArgs::Set is to be called with a raw pointer.
//   static void* TakeUnownedPointer(T* p);
template <typename T, typename Ignored = void /* for SFINAE */>
struct ChannelArgTypeTraits;

namespace channel_args_detail {
inline int PointerCompare(void* a_ptr, const grpc_arg_pointer_vtable* a_vtable,
                          void* b_ptr,
                          const grpc_arg_pointer_vtable* b_vtable) {
  int c = QsortCompare(a_ptr, b_ptr);
  if (c == 0) return 0;
  c = QsortCompare(a_vtable, b_vtable);
  if (c != 0) return c;
  return a_vtable->cmp(a_ptr, b_ptr);
}

// The type returned by calling Ref() on a T - used to determine the basest-type
// before the crt refcount base class.
template <typename T>
using RefType = absl::remove_cvref_t<decltype(*std::declval<T>().Ref())>;
}  // namespace channel_args_detail

// Specialization for ref-counted pointers.
// Types should expose:
// static int ChannelArgsCompare(const T* a, const T* b);
template <typename T>
struct ChannelArgTypeTraits<
    T, absl::enable_if_t<
           std::is_base_of<RefCounted<channel_args_detail::RefType<T>>,
                           channel_args_detail::RefType<T>>::value ||
               std::is_base_of<RefCounted<channel_args_detail::RefType<T>,
                                          NonPolymorphicRefCount>,
                               channel_args_detail::RefType<T>>::value ||
               std::is_base_of<DualRefCounted<channel_args_detail::RefType<T>>,
                               channel_args_detail::RefType<T>>::value,
           void>> {
  static const grpc_arg_pointer_vtable* VTable() {
    static const grpc_arg_pointer_vtable tbl = {
        // copy
        [](void* p) -> void* {
          return p == nullptr ? nullptr
                              : static_cast<T*>(p)
                                    ->Ref(DEBUG_LOCATION, "ChannelArgs copy")
                                    .release();
        },
        // destroy
        [](void* p) {
          if (p != nullptr) {
            static_cast<T*>(p)->Unref(DEBUG_LOCATION, "ChannelArgs destroy");
          }
        },
        // compare
        [](void* p1, void* p2) {
          return T::ChannelArgsCompare(static_cast<const T*>(p1),
                                       static_cast<const T*>(p2));
        },
    };
    return &tbl;
  };
};

// Define a check for shared_ptr supported types, which must extend
// enable_shared_from_this.
template <typename T>
struct SupportedSharedPtrType
    : std::integral_constant<
          bool, std::is_base_of<std::enable_shared_from_this<T>, T>::value> {};
template <>
struct SupportedSharedPtrType<grpc_event_engine::experimental::EventEngine>
    : std::true_type {};

// Specialization for shared_ptr
// Incurs an allocation because shared_ptr.release is not a thing.
template <typename T>
struct is_shared_ptr : std::false_type {};
template <typename T>
struct is_shared_ptr<std::shared_ptr<T>> : std::true_type {};
template <typename T>
struct ChannelArgTypeTraits<T,
                            absl::enable_if_t<is_shared_ptr<T>::value, void>> {
  static void* TakeUnownedPointer(T* p) { return p; }
  static const grpc_arg_pointer_vtable* VTable() {
    static const grpc_arg_pointer_vtable tbl = {
        // copy
        [](void* p) -> void* { return new T(*static_cast<T*>(p)); },
        // destroy
        [](void* p) { delete static_cast<T*>(p); },
        // compare
        [](void* p1, void* p2) {
          return QsortCompare(static_cast<const T*>(p1)->get(),
                              static_cast<const T*>(p2)->get());
        },
    };
    return &tbl;
  };
};

// If a type declares some member 'struct RawPointerChannelArgTag {}' then
// we automatically generate a vtable for it that does not do any ownership
// management and compares the type by pointer identity.
// This is intended to be relatively ugly because *most types should worry about
// ownership*.
template <typename T>
struct ChannelArgTypeTraits<T,
                            absl::void_t<typename T::RawPointerChannelArgTag>> {
  static void* TakeUnownedPointer(T* p) { return p; }
  static const grpc_arg_pointer_vtable* VTable() {
    static const grpc_arg_pointer_vtable tbl = {
        // copy
        [](void* p) -> void* { return p; },
        // destroy
        [](void*) {},
        // compare
        [](void* p1, void* p2) { return QsortCompare(p1, p2); },
    };
    return &tbl;
  };
};

// Determine if the pointer for a channel arg name should be const or not
template <typename T, typename SfinaeVoid = void>
struct ChannelArgPointerShouldBeConst {
  static constexpr bool kValue = false;
};

template <typename T>
struct ChannelArgPointerShouldBeConst<
    T, absl::void_t<decltype(T::ChannelArgUseConstPtr())>> {
  static constexpr bool kValue = T::ChannelArgUseConstPtr();
};

// GetObject support for shared_ptr and RefCountedPtr
template <typename T, typename Ignored = void /* for SFINAE */>
struct GetObjectImpl;
// std::shared_ptr implementation
template <typename T>
struct GetObjectImpl<
    T, absl::enable_if_t<!ChannelArgPointerShouldBeConst<T>::kValue &&
                             SupportedSharedPtrType<T>::value,
                         void>> {
  using Result = T*;
  using ReffedResult = std::shared_ptr<T>;
  using StoredType = std::shared_ptr<T>*;
  static Result Get(StoredType p) {
    if (p == nullptr) return nullptr;
    return p->get();
  };
  static ReffedResult GetReffed(StoredType p) {
    if (p == nullptr) return nullptr;
    return ReffedResult(*p);
  };
  static ReffedResult GetReffed(StoredType p,
                                const DebugLocation& /* location */,
                                const char* /* reason */) {
    return GetReffed(*p);
  };
};
// RefCountedPtr
template <typename T>
struct GetObjectImpl<
    T, absl::enable_if_t<!ChannelArgPointerShouldBeConst<T>::kValue &&
                             !SupportedSharedPtrType<T>::value,
                         void>> {
  using Result = T*;
  using ReffedResult = RefCountedPtr<T>;
  using StoredType = Result;
  static Result Get(StoredType p) { return p; };
  static ReffedResult GetReffed(StoredType p) {
    if (p == nullptr) return nullptr;
    return p->template RefAsSubclass<T>();
  };
  static ReffedResult GetReffed(StoredType p, const DebugLocation& location,
                                const char* reason) {
    if (p == nullptr) return nullptr;
    return p->template RefAsSubclass<T>(location, reason);
  };
};

template <typename T>
struct GetObjectImpl<
    T, absl::enable_if_t<ChannelArgPointerShouldBeConst<T>::kValue &&
                             !SupportedSharedPtrType<T>::value,
                         void>> {
  using Result = const T*;
  using ReffedResult = RefCountedPtr<const T>;
  using StoredType = Result;
  static Result Get(StoredType p) { return p; };
  static ReffedResult GetReffed(StoredType p) {
    if (p == nullptr) return nullptr;
    return p->Ref();
  };
  static ReffedResult GetReffed(StoredType p, const DebugLocation& location,
                                const char* reason) {
    if (p == nullptr) return nullptr;
    return p->Ref(location, reason);
  };
};

// Provide the canonical name for a type's channel arg key
template <typename T>
struct ChannelArgNameTraits {
  static absl::string_view ChannelArgName() { return T::ChannelArgName(); }
};
template <typename T>
struct ChannelArgNameTraits<std::shared_ptr<T>> {
  static absl::string_view ChannelArgName() { return T::ChannelArgName(); }
};
// Specialization for the EventEngine
template <>
struct ChannelArgNameTraits<grpc_event_engine::experimental::EventEngine> {
  static absl::string_view ChannelArgName() {
    return GRPC_INTERNAL_ARG_EVENT_ENGINE;
  }
};

class ChannelArgs {
 public:
  class Pointer {
   public:
    Pointer(void* p, const grpc_arg_pointer_vtable* vtable);
    ~Pointer() { vtable_->destroy(p_); }

    Pointer(const Pointer& other);
    Pointer& operator=(Pointer other) {
      std::swap(p_, other.p_);
      std::swap(vtable_, other.vtable_);
      return *this;
    }
    Pointer(Pointer&& other) noexcept;
    Pointer& operator=(Pointer&& other) noexcept {
      std::swap(p_, other.p_);
      std::swap(vtable_, other.vtable_);
      return *this;
    }

    friend int QsortCompare(const Pointer& a, const Pointer& b) {
      return channel_args_detail::PointerCompare(a.p_, a.vtable_, b.p_,
                                                 b.vtable_);
    }

    bool operator==(const Pointer& rhs) const {
      return QsortCompare(*this, rhs) == 0;
    }
    bool operator<(const Pointer& rhs) const {
      return QsortCompare(*this, rhs) < 0;
    }
    bool operator!=(const Pointer& rhs) const {
      return QsortCompare(*this, rhs) != 0;
    }

    void* c_pointer() const { return p_; }
    const grpc_arg_pointer_vtable* c_vtable() const { return vtable_; }

   private:
    static const grpc_arg_pointer_vtable* EmptyVTable();

    void* p_;
    const grpc_arg_pointer_vtable* vtable_;
  };

  // Helper to create a `Pointer` object to an object that is not owned by the
  // `ChannelArgs` object. Useful for tests, a code smell for production code.
  template <typename T>
  static Pointer UnownedPointer(T* p) {
    static const grpc_arg_pointer_vtable vtable = {
        [](void* p) -> void* { return p; },
        [](void*) {},
        [](void* p, void* q) { return QsortCompare(p, q); },
    };
    return Pointer(p, &vtable);
  }

  class Value {
   public:
    explicit Value(int n)
        : rep_(reinterpret_cast<void*>(static_cast<intptr_t>(n)),
               &int_vtable_) {}
    explicit Value(std::string s)
        : rep_(RefCountedString::Make(s).release(), &string_vtable_) {}
    explicit Value(Pointer p) : rep_(std::move(p)) {}

    absl::optional<int> GetIfInt() const {
      if (rep_.c_vtable() != &int_vtable_) return absl::nullopt;
      return reinterpret_cast<intptr_t>(rep_.c_pointer());
    }
    RefCountedPtr<RefCountedString> GetIfString() const {
      if (rep_.c_vtable() != &string_vtable_) return nullptr;
      return static_cast<RefCountedString*>(rep_.c_pointer())->Ref();
    }
    const Pointer* GetIfPointer() const {
      if (rep_.c_vtable() == &int_vtable_) return nullptr;
      if (rep_.c_vtable() == &string_vtable_) return nullptr;
      return &rep_;
    }

    absl::string_view ToString(std::list<std::string>& backing) const;

    grpc_arg MakeCArg(const char* name) const;

    bool operator<(const Value& rhs) const { return rep_ < rhs.rep_; }
    bool operator==(const Value& rhs) const { return rep_ == rhs.rep_; }
    bool operator!=(const Value& rhs) const { return !this->operator==(rhs); }
    bool operator==(absl::string_view rhs) const {
      auto str = GetIfString();
      if (str == nullptr) return false;
      return str->as_string_view() == rhs;
    }

   private:
    static const grpc_arg_pointer_vtable int_vtable_;
    static const grpc_arg_pointer_vtable string_vtable_;

    Pointer rep_;
  };

  struct ChannelArgsDeleter {
    void operator()(const grpc_channel_args* p) const;
  };
  using CPtr =
      std::unique_ptr<const grpc_channel_args, ChannelArgs::ChannelArgsDeleter>;

  ChannelArgs();
  ~ChannelArgs();
  ChannelArgs(const ChannelArgs&);
  ChannelArgs& operator=(const ChannelArgs&);
  ChannelArgs(ChannelArgs&&) noexcept;
  ChannelArgs& operator=(ChannelArgs&&) noexcept;

  static ChannelArgs FromC(const grpc_channel_args* args);
  static ChannelArgs FromC(const grpc_channel_args& args) {
    return FromC(&args);
  }
  // Construct a new grpc_channel_args struct.
  CPtr ToC() const;

  // Returns the union of this channel args with other.
  // If a key is present in both, the value from this is used.
  GRPC_MUST_USE_RESULT ChannelArgs UnionWith(ChannelArgs other) const;

  // Only used in union_with_test.cc, reference version of UnionWith for
  // differential fuzzing.
  GRPC_MUST_USE_RESULT ChannelArgs
  FuzzingReferenceUnionWith(ChannelArgs other) const;

  const Value* Get(absl::string_view name) const;
  GRPC_MUST_USE_RESULT ChannelArgs Set(absl::string_view name,
                                       Pointer value) const;
  GRPC_MUST_USE_RESULT ChannelArgs Set(absl::string_view name, int value) const;
  GRPC_MUST_USE_RESULT ChannelArgs Set(absl::string_view name,
                                       absl::string_view value) const;
  GRPC_MUST_USE_RESULT ChannelArgs Set(absl::string_view name,
                                       std::string value) const;
  GRPC_MUST_USE_RESULT ChannelArgs Set(absl::string_view name,
                                       const char* value) const;
  GRPC_MUST_USE_RESULT ChannelArgs Set(grpc_arg arg) const;
  template <typename T>
  GRPC_MUST_USE_RESULT absl::enable_if_t<
      std::is_same<const grpc_arg_pointer_vtable*,
                   decltype(ChannelArgTypeTraits<T>::VTable())>::value,
      ChannelArgs>
  Set(absl::string_view name, T* value) const {
    return Set(name, Pointer(ChannelArgTypeTraits<T>::TakeUnownedPointer(value),
                             ChannelArgTypeTraits<T>::VTable()));
  }
  template <typename T>
  GRPC_MUST_USE_RESULT auto Set(absl::string_view name,
                                RefCountedPtr<T> value) const
      -> absl::enable_if_t<
          !ChannelArgPointerShouldBeConst<T>::kValue &&
              std::is_same<const grpc_arg_pointer_vtable*,
                           decltype(ChannelArgTypeTraits<
                                    absl::remove_cvref_t<T>>::VTable())>::value,
          ChannelArgs> {
    return Set(
        name, Pointer(value.release(),
                      ChannelArgTypeTraits<absl::remove_cvref_t<T>>::VTable()));
  }
  template <typename T>
  GRPC_MUST_USE_RESULT auto Set(absl::string_view name,
                                RefCountedPtr<const T> value) const
      -> absl::enable_if_t<
          ChannelArgPointerShouldBeConst<T>::kValue &&
              std::is_same<const grpc_arg_pointer_vtable*,
                           decltype(ChannelArgTypeTraits<
                                    absl::remove_cvref_t<T>>::VTable())>::value,
          ChannelArgs> {
    return Set(
        name, Pointer(const_cast<T*>(value.release()),
                      ChannelArgTypeTraits<absl::remove_cvref_t<T>>::VTable()));
  }
  template <typename T>
  GRPC_MUST_USE_RESULT absl::enable_if_t<
      std::is_same<
          const grpc_arg_pointer_vtable*,
          decltype(ChannelArgTypeTraits<std::shared_ptr<T>>::VTable())>::value,
      ChannelArgs>
  Set(absl::string_view name, std::shared_ptr<T> value) const {
    static_assert(SupportedSharedPtrType<T>::value,
                  "Type T must extend std::enable_shared_from_this to be added "
                  "into ChannelArgs as a shared_ptr<T>");
    auto* store_value = new std::shared_ptr<T>(value);
    return Set(
        name,
        Pointer(ChannelArgTypeTraits<std::shared_ptr<T>>::TakeUnownedPointer(
                    store_value),
                ChannelArgTypeTraits<std::shared_ptr<T>>::VTable()));
  }
  template <typename T>
  GRPC_MUST_USE_RESULT ChannelArgs SetIfUnset(absl::string_view name,
                                              T value) const {
    if (Contains(name)) return *this;
    return Set(name, std::move(value));
  }
  GRPC_MUST_USE_RESULT ChannelArgs Remove(absl::string_view name) const;
  bool Contains(absl::string_view name) const;

  GRPC_MUST_USE_RESULT ChannelArgs
  RemoveAllKeysWithPrefix(absl::string_view prefix) const;

  template <typename T>
  bool ContainsObject() const {
    return Get(ChannelArgNameTraits<T>::ChannelArgName()) != nullptr;
  }

  absl::optional<int> GetInt(absl::string_view name) const;
  absl::optional<absl::string_view> GetString(absl::string_view name) const;
  absl::optional<std::string> GetOwnedString(absl::string_view name) const;
  // WARNING: this is broken if `name` represents something that was stored as a
  // RefCounted<const T> - we will discard the const-ness.
  void* GetVoidPointer(absl::string_view name) const;
  template <typename T>
  typename GetObjectImpl<T>::StoredType GetPointer(
      absl::string_view name) const {
    return static_cast<typename GetObjectImpl<T>::StoredType>(
        GetVoidPointer(name));
  }
  absl::optional<Duration> GetDurationFromIntMillis(
      absl::string_view name) const;
  absl::optional<bool> GetBool(absl::string_view name) const;

  // Object based get/set.
  // Deal with the common case that we set a pointer to an object under
  // the same name in every usage.
  // Expects ChannelArgTypeTraits to exist for T, and T to expose:
  //   static string_view ChannelArgName();
  template <typename T>
  GRPC_MUST_USE_RESULT ChannelArgs SetObject(T* p) const {
    return Set(T::ChannelArgName(), p);
  }
  template <typename T>
  GRPC_MUST_USE_RESULT ChannelArgs SetObject(RefCountedPtr<T> p) const {
    return Set(T::ChannelArgName(), std::move(p));
  }
  template <typename T>
  GRPC_MUST_USE_RESULT ChannelArgs SetObject(std::shared_ptr<T> p) const {
    return Set(ChannelArgNameTraits<T>::ChannelArgName(), std::move(p));
  }
  template <typename T>
  typename GetObjectImpl<T>::Result GetObject() const {
    return GetObjectImpl<T>::Get(
        GetPointer<T>(ChannelArgNameTraits<T>::ChannelArgName()));
  }
  template <typename T>
  typename GetObjectImpl<T>::ReffedResult GetObjectRef() const {
    return GetObjectImpl<T>::GetReffed(
        GetPointer<T>(ChannelArgNameTraits<T>::ChannelArgName()));
  }
  template <typename T>
  typename GetObjectImpl<T>::ReffedResult GetObjectRef(
      const DebugLocation& location, const char* reason) const {
    return GetObjectImpl<T>::GetReffed(
        GetPointer<T>(ChannelArgNameTraits<T>::ChannelArgName()), location,
        reason);
  }

  bool operator!=(const ChannelArgs& other) const;
  bool operator<(const ChannelArgs& other) const;
  bool operator==(const ChannelArgs& other) const;

  // Helpers for commonly accessed things

  bool WantMinimalStack() const;
  std::string ToString() const;

 private:
  explicit ChannelArgs(AVL<RefCountedStringValue, Value> args);

  GRPC_MUST_USE_RESULT ChannelArgs Set(absl::string_view name,
                                       Value value) const;

  AVL<RefCountedStringValue, Value> args_;
};

std::ostream& operator<<(std::ostream& out, const ChannelArgs& args);

}  // namespace grpc_core

/// Copy the arguments in \a src into a new instance
grpc_channel_args* grpc_channel_args_copy(const grpc_channel_args* src);

/// Copy the arguments in \a src into a new instance, stably sorting keys
grpc_channel_args* grpc_channel_args_normalize(const grpc_channel_args* src);

/// Copy the arguments in \a src and append \a to_add. If \a to_add is NULL, it
/// is equivalent to calling \a grpc_channel_args_copy.
grpc_channel_args* grpc_channel_args_copy_and_add(const grpc_channel_args* src,
                                                  const grpc_arg* to_add,
                                                  size_t num_to_add);

/// Copies the arguments in \a src except for those whose keys are in
/// \a to_remove.
grpc_channel_args* grpc_channel_args_copy_and_remove(
    const grpc_channel_args* src, const char** to_remove, size_t num_to_remove);

/// Copies the arguments from \a src except for those whose keys are in
/// \a to_remove and appends the arguments in \a to_add.
grpc_channel_args* grpc_channel_args_copy_and_add_and_remove(
    const grpc_channel_args* src, const char** to_remove, size_t num_to_remove,
    const grpc_arg* to_add, size_t num_to_add);

/// Perform the union of \a a and \a b, prioritizing \a a entries
grpc_channel_args* grpc_channel_args_union(const grpc_channel_args* a,
                                           const grpc_channel_args* b);

/// Destroy arguments created by \a grpc_channel_args_copy
void grpc_channel_args_destroy(grpc_channel_args* a);
inline void grpc_channel_args_destroy(const grpc_channel_args* a) {
  grpc_channel_args_destroy(const_cast<grpc_channel_args*>(a));
}

int grpc_channel_args_compare(const grpc_channel_args* a,
                              const grpc_channel_args* b);

/// Returns the value of argument \a name from \a args, or NULL if not found.
const grpc_arg* grpc_channel_args_find(const grpc_channel_args* args,
                                       const char* name);

bool grpc_channel_args_want_minimal_stack(const grpc_channel_args* args);

typedef struct grpc_integer_options {
  int default_value;  // Return this if value is outside of expected bounds.
  int min_value;
  int max_value;
} grpc_integer_options;

/// Returns the value of \a arg, subject to the constraints in \a options.
int grpc_channel_arg_get_integer(const grpc_arg* arg,
                                 const grpc_integer_options options);
/// Similar to the above, but needs to find the arg from \a args by the name
/// first.
int grpc_channel_args_find_integer(const grpc_channel_args* args,
                                   const char* name,
                                   const grpc_integer_options options);

/// Returns the value of \a arg if \a arg is of type GRPC_ARG_STRING.
/// Otherwise, emits a warning log, and returns nullptr.
/// If arg is nullptr, returns nullptr, and does not emit a warning.
char* grpc_channel_arg_get_string(const grpc_arg* arg);
/// Similar to the above, but needs to find the arg from \a args by the name
/// first.
char* grpc_channel_args_find_string(const grpc_channel_args* args,
                                    const char* name);
/// If \a arg is of type GRPC_ARG_INTEGER, returns true if it's non-zero.
/// Returns \a default_value if \a arg is of other types.
bool grpc_channel_arg_get_bool(const grpc_arg* arg, bool default_value);
/// Similar to the above, but needs to find the arg from \a args by the name
/// first.
bool grpc_channel_args_find_bool(const grpc_channel_args* args,
                                 const char* name, bool default_value);

template <typename T>
T* grpc_channel_args_find_pointer(const grpc_channel_args* args,
                                  const char* name) {
  const grpc_arg* arg = grpc_channel_args_find(args, name);
  if (arg == nullptr || arg->type != GRPC_ARG_POINTER) return nullptr;
  return static_cast<T*>(arg->value.pointer.p);
}

// Helpers for creating channel args.
grpc_arg grpc_channel_arg_string_create(char* name, char* value);
grpc_arg grpc_channel_arg_integer_create(char* name, int value);
grpc_arg grpc_channel_arg_pointer_create(char* name, void* value,
                                         const grpc_arg_pointer_vtable* vtable);

// Returns a string representing channel args in human-readable form.
std::string grpc_channel_args_string(const grpc_channel_args* args);

namespace grpc_core {
// Ensure no duplicate channel args (with some backwards compatibility hacks).
// Eliminate any grpc.internal.* args.
// Return a C++ object.
ChannelArgs ChannelArgsBuiltinPrecondition(const grpc_channel_args* src);
}  // namespace grpc_core

// Takes ownership of the old_args
typedef grpc_core::ChannelArgs (
    *grpc_channel_args_client_channel_creation_mutator)(
    const char* target, const grpc_core::ChannelArgs& old_args,
    grpc_channel_stack_type type);

// Should be called only once globaly before grpc is init'ed.
void grpc_channel_args_set_client_channel_creation_mutator(
    grpc_channel_args_client_channel_creation_mutator cb);
// This will be called at the creation of each channel.
grpc_channel_args_client_channel_creation_mutator
grpc_channel_args_get_client_channel_creation_mutator();

#endif  // GRPC_SRC_CORE_LIB_CHANNEL_CHANNEL_ARGS_H