grpc-swift/Sources/GRPCCore/Internal/Concurrency Primities/Lock.swift

/*
 * Copyright 2023, gRPC Authors All rights reserved.
 *
 * 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.
 */
//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftNIO open source project
//
// Copyright (c) 2017-2022 Apple Inc. and the SwiftNIO project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//

#if canImport(Darwin)
import Darwin
#elseif canImport(Glibc)
import Glibc
#endif

@usableFromInline
typealias LockPrimitive = pthread_mutex_t

@usableFromInline
enum LockOperations {}

extension LockOperations {
  @inlinable
  static func create(_ mutex: UnsafeMutablePointer<LockPrimitive>) {
    mutex.assertValidAlignment()

    var attr = pthread_mutexattr_t()
    pthread_mutexattr_init(&attr)

    let err = pthread_mutex_init(mutex, &attr)
    precondition(err == 0, "\(#function) failed in pthread_mutex with error \(err)")
  }

  @inlinable
  static func destroy(_ mutex: UnsafeMutablePointer<LockPrimitive>) {
    mutex.assertValidAlignment()

    let err = pthread_mutex_destroy(mutex)
    precondition(err == 0, "\(#function) failed in pthread_mutex with error \(err)")
  }

  @inlinable
  static func lock(_ mutex: UnsafeMutablePointer<LockPrimitive>) {
    mutex.assertValidAlignment()

    let err = pthread_mutex_lock(mutex)
    precondition(err == 0, "\(#function) failed in pthread_mutex with error \(err)")
  }

  @inlinable
  static func unlock(_ mutex: UnsafeMutablePointer<LockPrimitive>) {
    mutex.assertValidAlignment()

    let err = pthread_mutex_unlock(mutex)
    precondition(err == 0, "\(#function) failed in pthread_mutex with error \(err)")
  }
}

// Tail allocate both the mutex and a generic value using ManagedBuffer.
// Both the header pointer and the elements pointer are stable for
// the class's entire lifetime.
//
// However, for safety reasons, we elect to place the lock in the "elements"
// section of the buffer instead of the head. The reasoning here is subtle,
// so buckle in.
//
// _As a practical matter_, the implementation of ManagedBuffer ensures that
// the pointer to the header is stable across the lifetime of the class, and so
// each time you call `withUnsafeMutablePointers` or `withUnsafeMutablePointerToHeader`
// the value of the header pointer will be the same. This is because ManagedBuffer uses
// `Builtin.addressOf` to load the value of the header, and that does ~magic~ to ensure
// that it does not invoke any weird Swift accessors that might copy the value.
//
// _However_, the header is also available via the `.header` field on the ManagedBuffer.
// This presents a problem! The reason there's an issue is that `Builtin.addressOf` and friends
// do not interact with Swift's exclusivity model. That is, the various `with` functions do not
// conceptually trigger a mutating access to `.header`. For elements this isn't a concern because
// there's literally no other way to perform the access, but for `.header` it's entirely possible
// to accidentally recursively read it.
//
// Our implementation is free from these issues, so we don't _really_ need to worry about it.
// However, out of an abundance of caution, we store the Value in the header, and the LockPrimitive
// in the trailing elements. We still don't use `.header`, but it's better to be safe than sorry,
// and future maintainers will be happier that we were cautious.
//
// See also: https://github.com/apple/swift/pull/40000
@usableFromInline
final class LockStorage<Value>: ManagedBuffer<Value, LockPrimitive> {

  @inlinable
  static func create(value: Value) -> Self {
    let buffer = Self.create(minimumCapacity: 1) { _ in
      return value
    }
    let storage = unsafeDowncast(buffer, to: Self.self)

    storage.withUnsafeMutablePointers { _, lockPtr in
      LockOperations.create(lockPtr)
    }

    return storage
  }

  @inlinable
  func lock() {
    self.withUnsafeMutablePointerToElements { lockPtr in
      LockOperations.lock(lockPtr)
    }
  }

  @inlinable
  func unlock() {
    self.withUnsafeMutablePointerToElements { lockPtr in
      LockOperations.unlock(lockPtr)
    }
  }

  @inlinable
  deinit {
    self.withUnsafeMutablePointerToElements { lockPtr in
      LockOperations.destroy(lockPtr)
    }
  }

  @inlinable
  func withLockPrimitive<T>(
    _ body: (UnsafeMutablePointer<LockPrimitive>) throws -> T
  ) rethrows -> T {
    try self.withUnsafeMutablePointerToElements { lockPtr in
      return try body(lockPtr)
    }
  }

  @inlinable
  func withLockedValue<T>(_ mutate: (inout Value) throws -> T) rethrows -> T {
    try self.withUnsafeMutablePointers { valuePtr, lockPtr in
      LockOperations.lock(lockPtr)
      defer { LockOperations.unlock(lockPtr) }
      return try mutate(&valuePtr.pointee)
    }
  }
}

extension LockStorage: @unchecked Sendable {}

/// A threading lock based on `libpthread` instead of `libdispatch`.
///
/// - note: ``Lock`` has reference semantics.
///
/// This object provides a lock on top of a single `pthread_mutex_t`. This kind
/// of lock is safe to use with `libpthread`-based threading models, such as the
/// one used by NIO. On Windows, the lock is based on the substantially similar
/// `SRWLOCK` type.
@usableFromInline
struct Lock {
  @usableFromInline
  internal let _storage: LockStorage<Void>

  /// Create a new lock.
  @inlinable
  init() {
    self._storage = .create(value: ())
  }

  /// Acquire the lock.
  ///
  /// Whenever possible, consider using `withLock` instead of this method and
  /// `unlock`, to simplify lock handling.
  @inlinable
  func lock() {
    self._storage.lock()
  }

  /// Release the lock.
  ///
  /// Whenever possible, consider using `withLock` instead of this method and
  /// `lock`, to simplify lock handling.
  @inlinable
  func unlock() {
    self._storage.unlock()
  }

  @inlinable
  internal func withLockPrimitive<T>(
    _ body: (UnsafeMutablePointer<LockPrimitive>) throws -> T
  ) rethrows -> T {
    return try self._storage.withLockPrimitive(body)
  }
}

extension Lock {
  /// Acquire the lock for the duration of the given block.
  ///
  /// This convenience method should be preferred to `lock` and `unlock` in
  /// most situations, as it ensures that the lock will be released regardless
  /// of how `body` exits.
  ///
  /// - Parameter body: The block to execute while holding the lock.
  /// - Returns: The value returned by the block.
  @inlinable
  func withLock<T>(_ body: () throws -> T) rethrows -> T {
    self.lock()
    defer {
      self.unlock()
    }
    return try body()
  }
}

extension Lock: Sendable {}

extension UnsafeMutablePointer {
  @inlinable
  func assertValidAlignment() {
    assert(UInt(bitPattern: self) % UInt(MemoryLayout<Pointee>.alignment) == 0)
  }
}

@usableFromInline
struct LockedValueBox<Value> {
  @usableFromInline
  let storage: LockStorage<Value>

  @inlinable
  init(_ value: Value) {
    self.storage = .create(value: value)
  }

  @inlinable
  func withLockedValue<T>(_ mutate: (inout Value) throws -> T) rethrows -> T {
    return try self.storage.withLockedValue(mutate)
  }
}

extension LockedValueBox: Sendable where Value: Sendable {}