--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/CSP2/CSP2_env/env-d9b9114564458d9d-741b3de822f2aaca6c6caa4325c4afce/include/kj/mutex.h	Tue Mar 18 17:55:14 2025 -0400
@@ -0,0 +1,778 @@
+// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
+// Licensed under the MIT License:
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+#pragma once
+
+#include "debug.h"
+#include "memory.h"
+#include <inttypes.h>
+#include "time.h"
+#include "source-location.h"
+#include "one-of.h"
+
+KJ_BEGIN_HEADER
+
+#if __linux__ && !defined(KJ_USE_FUTEX)
+#define KJ_USE_FUTEX 1
+#endif
+
+#if !KJ_USE_FUTEX && !_WIN32 && !__CYGWIN__
+// We fall back to pthreads when we don't have a better platform-specific primitive. pthreads
+// mutexes are bloated, though, so we like to avoid them. Hence on Linux we use futex(), and on
+// Windows we use SRW locks and friends. On Cygwin we prefer the Win32 primitives both because they
+// are more efficient and because I ran into problems with Cygwin's implementation of RW locks
+// seeming to allow multiple threads to lock the same mutex (but I didn't investigate very
+// closely).
+//
+// TODO(someday):  Write efficient low-level locking primitives for other platforms.
+#include <pthread.h>
+#endif
+
+// There are 3 macros controlling lock tracking:
+// KJ_TRACK_LOCK_BLOCKING will set up async signal safe TLS variables that can be used to identify
+// the KJ primitive blocking the current thread.
+// KJ_SAVE_ACQUIRED_LOCK_INFO will allow introspection of a Mutex to get information about what is
+// currently holding the lock.
+// KJ_TRACK_LOCK_ACQUISITION is automatically enabled by either one of them.
+
+#if KJ_TRACK_LOCK_BLOCKING
+// Lock tracking is required to keep track of what blocked.
+#define KJ_TRACK_LOCK_ACQUISITION 1
+#endif
+
+#if KJ_SAVE_ACQUIRED_LOCK_INFO
+#define KJ_TRACK_LOCK_ACQUISITION 1
+#include <unistd.h>
+#endif
+
+namespace kj {
+#if KJ_TRACK_LOCK_ACQUISITION
+#if !KJ_USE_FUTEX
+#error Lock tracking is only currently supported for futex-based mutexes.
+#endif
+
+#if !KJ_COMPILER_SUPPORTS_SOURCE_LOCATION
+#error C++20 or newer is required (or the use of clang/gcc).
+#endif
+
+using LockSourceLocation = SourceLocation;
+using LockSourceLocationArg = const SourceLocation&;
+// On x86-64 the codegen is optimal if the argument has type const& for the location. However,
+// since this conflicts with the optimal call signature for NoopSourceLocation,
+// LockSourceLocationArg is used to conditionally select the right type without polluting the usage
+// themselves. Interestingly this makes no difference on ARM.
+// https://godbolt.org/z/q6G8ee5a3
+#else
+using LockSourceLocation = NoopSourceLocation;
+using LockSourceLocationArg = NoopSourceLocation;
+#endif
+
+
+class Exception;
+
+// =======================================================================================
+// Private details -- public interfaces follow below.
+
+namespace _ {  // private
+
+#if KJ_SAVE_ACQUIRED_LOCK_INFO
+class HoldingExclusively {
+  // The lock is being held in exclusive mode.
+public:
+  constexpr HoldingExclusively(pid_t tid, const SourceLocation& location)
+      : heldBy(tid), acquiredAt(location) {}
+
+  pid_t threadHoldingLock() const { return heldBy; }
+  const SourceLocation& lockAcquiredAt() const { return acquiredAt; }
+
+private:
+  pid_t heldBy;
+  SourceLocation acquiredAt;
+};
+
+class HoldingShared {
+  // The lock is being held in shared mode currently. Which threads are holding this lock open
+  // is unknown.
+public:
+  constexpr HoldingShared(const SourceLocation& location) : acquiredAt(location) {}
+
+  const SourceLocation& lockAcquiredAt() const { return acquiredAt; }
+
+private:
+  SourceLocation acquiredAt;
+};
+#endif
+
+class Mutex {
+  // Internal implementation details.  See `MutexGuarded<T>`.
+
+  struct Waiter;
+
+public:
+  Mutex();
+  ~Mutex();
+  KJ_DISALLOW_COPY_AND_MOVE(Mutex);
+
+  enum Exclusivity {
+    EXCLUSIVE,
+    SHARED
+  };
+
+  bool lock(Exclusivity exclusivity, Maybe<Duration> timeout, LockSourceLocationArg location);
+  void unlock(Exclusivity exclusivity, Waiter* waiterToSkip = nullptr);
+
+  void assertLockedByCaller(Exclusivity exclusivity) const;
+  // In debug mode, assert that the mutex is locked by the calling thread, or if that is
+  // non-trivial, assert that the mutex is locked (which should be good enough to catch problems
+  // in unit tests).  In non-debug builds, do nothing.
+
+  class Predicate {
+  public:
+    virtual bool check() = 0;
+  };
+
+  void wait(Predicate& predicate, Maybe<Duration> timeout, LockSourceLocationArg location);
+  // If predicate.check() returns false, unlock the mutex until predicate.check() returns true, or
+  // when the timeout (if any) expires. The mutex is always re-locked when this returns regardless
+  // of whether the timeout expired, and including if it throws.
+  //
+  // Requires that the mutex is already exclusively locked before calling.
+
+  void induceSpuriousWakeupForTest();
+  // Utility method for mutex-test.c++ which causes a spurious thread wakeup on all threads that
+  // are waiting for a wait() condition. Assuming correct implementation, all those threads
+  // should immediately go back to sleep.
+
+#if KJ_USE_FUTEX
+  uint numReadersWaitingForTest() const;
+  // The number of reader locks that are currently blocked on this lock (must be called while
+  // holding the writer lock). This is really only a utility method for mutex-test.c++ so it can
+  // validate certain invariants.
+#endif
+
+#if KJ_SAVE_ACQUIRED_LOCK_INFO
+  using AcquiredMetadata = kj::OneOf<HoldingExclusively, HoldingShared>;
+  KJ_DISABLE_TSAN AcquiredMetadata lockedInfo() const;
+  // Returns metadata about this lock when its held. This method is async signal safe. It must also
+  // be called in a state where it's guaranteed that the lock state won't be released by another
+  // thread. In other words this has to be called from the signal handler within the thread that's
+  // holding the lock.
+#endif
+
+private:
+#if KJ_USE_FUTEX
+  uint futex;
+  // bit 31 (msb) = set if exclusive lock held
+  // bit 30 (msb) = set if threads are waiting for exclusive lock
+  // bits 0-29 = count of readers; If an exclusive lock is held, this is the count of threads
+  //   waiting for a read lock, otherwise it is the count of threads that currently hold a read
+  //   lock.
+
+#ifdef KJ_CONTENTION_WARNING_THRESHOLD
+  bool printContendedReader = false;
+#endif
+
+  static constexpr uint EXCLUSIVE_HELD = 1u << 31;
+  static constexpr uint EXCLUSIVE_REQUESTED = 1u << 30;
+  static constexpr uint SHARED_COUNT_MASK = EXCLUSIVE_REQUESTED - 1;
+
+#elif _WIN32 || __CYGWIN__
+  uintptr_t srwLock;  // Actually an SRWLOCK, but don't want to #include <windows.h> in header.
+
+#else
+  mutable pthread_rwlock_t mutex;
+#endif
+
+#if KJ_SAVE_ACQUIRED_LOCK_INFO
+  pid_t lockedExclusivelyByThread = 0;
+  SourceLocation lockAcquiredLocation;
+
+  KJ_DISABLE_TSAN void acquiredExclusive(pid_t tid, const SourceLocation& location) noexcept {
+    lockAcquiredLocation = location;
+    __atomic_store_n(&lockedExclusivelyByThread, tid, __ATOMIC_RELAXED);
+  }
+
+  KJ_DISABLE_TSAN void acquiredShared(const SourceLocation& location) noexcept {
+    lockAcquiredLocation = location;
+  }
+
+  KJ_DISABLE_TSAN SourceLocation releasingExclusive() noexcept {
+    auto tmp = lockAcquiredLocation;
+    lockAcquiredLocation = SourceLocation{};
+    lockedExclusivelyByThread = 0;
+    return tmp;
+  }
+#else
+  static constexpr void acquiredExclusive(uint, LockSourceLocationArg) {}
+  static constexpr void acquiredShared(LockSourceLocationArg) {}
+  static constexpr NoopSourceLocation releasingExclusive() { return NoopSourceLocation{}; }
+#endif
+  struct Waiter {
+    kj::Maybe<Waiter&> next;
+    kj::Maybe<Waiter&>* prev;
+    Predicate& predicate;
+    Maybe<Own<Exception>> exception;
+#if KJ_USE_FUTEX
+    uint futex;
+    bool hasTimeout;
+#elif _WIN32 || __CYGWIN__
+    uintptr_t condvar;
+    // Actually CONDITION_VARIABLE, but don't want to #include <windows.h> in header.
+#else
+    pthread_cond_t condvar;
+
+    pthread_mutex_t stupidMutex;
+    // pthread condvars are only compatible with basic pthread mutexes, not rwlocks, for no
+    // particularly good reason. To work around this, we need an extra mutex per condvar.
+#endif
+  };
+
+  kj::Maybe<Waiter&> waitersHead = nullptr;
+  kj::Maybe<Waiter&>* waitersTail = &waitersHead;
+  // linked list of waiters; can only modify under lock
+
+  inline void addWaiter(Waiter& waiter);
+  inline void removeWaiter(Waiter& waiter);
+  bool checkPredicate(Waiter& waiter);
+#if _WIN32 || __CYGWIN__
+  void wakeReadyWaiter(Waiter* waiterToSkip);
+#endif
+};
+
+class Once {
+  // Internal implementation details.  See `Lazy<T>`.
+
+public:
+#if KJ_USE_FUTEX
+  inline Once(bool startInitialized = false)
+      : futex(startInitialized ? INITIALIZED : UNINITIALIZED) {}
+#else
+  Once(bool startInitialized = false);
+  ~Once();
+#endif
+  KJ_DISALLOW_COPY_AND_MOVE(Once);
+
+  class Initializer {
+  public:
+    virtual void run() = 0;
+  };
+
+  void runOnce(Initializer& init, LockSourceLocationArg location);
+
+#if _WIN32 || __CYGWIN__  // TODO(perf): Can we make this inline on win32 somehow?
+  bool isInitialized() noexcept;
+
+#else
+  inline bool isInitialized() noexcept {
+    // Fast path check to see if runOnce() would simply return immediately.
+#if KJ_USE_FUTEX
+    return __atomic_load_n(&futex, __ATOMIC_ACQUIRE) == INITIALIZED;
+#else
+    return __atomic_load_n(&state, __ATOMIC_ACQUIRE) == INITIALIZED;
+#endif
+  }
+#endif
+
+  void reset();
+  // Returns the state from initialized to uninitialized.  It is an error to call this when
+  // not already initialized, or when runOnce() or isInitialized() might be called concurrently in
+  // another thread.
+
+private:
+#if KJ_USE_FUTEX
+  uint futex;
+
+  enum State {
+    UNINITIALIZED,
+    INITIALIZING,
+    INITIALIZING_WITH_WAITERS,
+    INITIALIZED
+  };
+
+#elif _WIN32 || __CYGWIN__
+  uintptr_t initOnce;  // Actually an INIT_ONCE, but don't want to #include <windows.h> in header.
+
+#else
+  enum State {
+    UNINITIALIZED,
+    INITIALIZED
+  };
+  State state;
+  pthread_mutex_t mutex;
+#endif
+};
+
+}  // namespace _ (private)
+
+// =======================================================================================
+// Public interface
+
+template <typename T>
+class Locked {
+  // Return type for `MutexGuarded<T>::lock()`.  `Locked<T>` provides access to the bounded object
+  // and unlocks the mutex when it goes out of scope.
+
+public:
+  KJ_DISALLOW_COPY(Locked);
+  inline Locked(): mutex(nullptr), ptr(nullptr) {}
+  inline Locked(Locked&& other): mutex(other.mutex), ptr(other.ptr) {
+    other.mutex = nullptr;
+    other.ptr = nullptr;
+  }
+  inline ~Locked() {
+    if (mutex != nullptr) mutex->unlock(isConst<T>() ? _::Mutex::SHARED : _::Mutex::EXCLUSIVE);
+  }
+
+  inline Locked& operator=(Locked&& other) {
+    if (mutex != nullptr) mutex->unlock(isConst<T>() ? _::Mutex::SHARED : _::Mutex::EXCLUSIVE);
+    mutex = other.mutex;
+    ptr = other.ptr;
+    other.mutex = nullptr;
+    other.ptr = nullptr;
+    return *this;
+  }
+
+  inline void release() {
+    if (mutex != nullptr) mutex->unlock(isConst<T>() ? _::Mutex::SHARED : _::Mutex::EXCLUSIVE);
+    mutex = nullptr;
+    ptr = nullptr;
+  }
+
+  inline T* operator->() { return ptr; }
+  inline const T* operator->() const { return ptr; }
+  inline T& operator*() { return *ptr; }
+  inline const T& operator*() const { return *ptr; }
+  inline T* get() { return ptr; }
+  inline const T* get() const { return ptr; }
+  inline operator T*() { return ptr; }
+  inline operator const T*() const { return ptr; }
+
+  template <typename Cond>
+  void wait(Cond&& condition, Maybe<Duration> timeout = nullptr,
+      LockSourceLocationArg location = {}) {
+    // Unlocks the lock until `condition(state)` evaluates true (where `state` is type `const T&`
+    // referencing the object protected by the lock).
+
+    // We can't wait on a shared lock because the internal bookkeeping needed for a wait requires
+    // the protection of an exclusive lock.
+    static_assert(!isConst<T>(), "cannot wait() on shared lock");
+
+    struct PredicateImpl final: public _::Mutex::Predicate {
+      bool check() override {
+        return condition(value);
+      }
+
+      Cond&& condition;
+      const T& value;
+
+      PredicateImpl(Cond&& condition, const T& value)
+          : condition(kj::fwd<Cond>(condition)), value(value) {}
+    };
+
+    PredicateImpl impl(kj::fwd<Cond>(condition), *ptr);
+    mutex->wait(impl, timeout, location);
+  }
+
+private:
+  _::Mutex* mutex;
+  T* ptr;
+
+  inline Locked(_::Mutex& mutex, T& value): mutex(&mutex), ptr(&value) {}
+
+  template <typename U>
+  friend class MutexGuarded;
+  template <typename U>
+  friend class ExternalMutexGuarded;
+
+#if KJ_MUTEX_TEST
+public:
+#endif
+  void induceSpuriousWakeupForTest() { mutex->induceSpuriousWakeupForTest(); }
+  // Utility method for mutex-test.c++ which causes a spurious thread wakeup on all threads that
+  // are waiting for a when() condition. Assuming correct implementation, all those threads should
+  // immediately go back to sleep.
+};
+
+template <typename T>
+class MutexGuarded {
+  // An object of type T, bounded by a mutex.  In order to access the object, you must lock it.
+  //
+  // Write locks are not "recursive" -- trying to lock again in a thread that already holds a lock
+  // will deadlock.  Recursive write locks are usually a sign of bad design.
+  //
+  // Unfortunately, **READ LOCKS ARE NOT RECURSIVE** either.  Common sense says they should be.
+  // But on many operating systems (BSD, OSX), recursively read-locking a pthread_rwlock is
+  // actually unsafe.  The problem is that writers are "prioritized" over readers, so a read lock
+  // request will block if any write lock requests are outstanding.  So, if thread A takes a read
+  // lock, thread B requests a write lock (and starts waiting), and then thread A tries to take
+  // another read lock recursively, the result is deadlock.
+
+public:
+  template <typename... Params>
+  explicit MutexGuarded(Params&&... params);
+  // Initialize the mutex-bounded object by passing the given parameters to its constructor.
+
+  Locked<T> lockExclusive(LockSourceLocationArg location = {}) const;
+  // Exclusively locks the object and returns it.  The returned `Locked<T>` can be passed by
+  // move, similar to `Own<T>`.
+  //
+  // This method is declared `const` in accordance with KJ style rules which say that constness
+  // should be used to indicate thread-safety.  It is safe to share a const pointer between threads,
+  // but it is not safe to share a mutable pointer.  Since the whole point of MutexGuarded is to
+  // be shared between threads, its methods should be const, even though locking it produces a
+  // non-const pointer to the contained object.
+
+  Locked<const T> lockShared(LockSourceLocationArg location = {}) const;
+  // Lock the value for shared access.  Multiple shared locks can be taken concurrently, but cannot
+  // be held at the same time as a non-shared lock.
+
+  Maybe<Locked<T>> lockExclusiveWithTimeout(Duration timeout,
+      LockSourceLocationArg location = {}) const;
+  // Attempts to exclusively lock the object. If the timeout elapses before the lock is acquired,
+  // this returns null.
+
+  Maybe<Locked<const T>> lockSharedWithTimeout(Duration timeout,
+      LockSourceLocationArg location = {}) const;
+  // Attempts to lock the value for shared access. If the timeout elapses before the lock is acquired,
+  // this returns null.
+
+  inline const T& getWithoutLock() const { return value; }
+  inline T& getWithoutLock() { return value; }
+  // Escape hatch for cases where some external factor guarantees that it's safe to get the
+  // value.  You should treat these like const_cast -- be highly suspicious of any use.
+
+  inline const T& getAlreadyLockedShared() const;
+  inline T& getAlreadyLockedShared();
+  inline T& getAlreadyLockedExclusive() const;
+  // Like `getWithoutLock()`, but asserts that the lock is already held by the calling thread.
+
+  template <typename Cond, typename Func>
+  auto when(Cond&& condition, Func&& callback, Maybe<Duration> timeout = nullptr,
+      LockSourceLocationArg location = {}) const
+      -> decltype(callback(instance<T&>())) {
+    // Waits until condition(state) returns true, then calls callback(state) under lock.
+    //
+    // `condition`, when called, receives as its parameter a const reference to the state, which is
+    // locked (either shared or exclusive). `callback` receives a mutable reference, which is
+    // exclusively locked.
+    //
+    // `condition()` may be called multiple times, from multiple threads, while waiting for the
+    // condition to become true. It may even return true once, but then be called more times.
+    // It is guaranteed, though, that at the time `callback()` is finally called, `condition()`
+    // would currently return true (assuming it is a pure function of the guarded data).
+    //
+    // If `timeout` is specified, then after the given amount of time, the callback will be called
+    // regardless of whether the condition is true. In this case, when `callback()` is called,
+    // `condition()` may in fact evaluate false, but *only* if the timeout was reached.
+    //
+    // TODO(cleanup): lock->wait() is a better interface. Can we deprecate this one?
+
+    auto lock = lockExclusive();
+    lock.wait(kj::fwd<Cond>(condition), timeout, location);
+    return callback(value);
+  }
+
+private:
+  mutable _::Mutex mutex;
+  mutable T value;
+};
+
+template <typename T>
+class MutexGuarded<const T> {
+  // MutexGuarded cannot guard a const type.  This would be pointless anyway, and would complicate
+  // the implementation of Locked<T>, which uses constness to decide what kind of lock it holds.
+  static_assert(sizeof(T) < 0, "MutexGuarded's type cannot be const.");
+};
+
+template <typename T>
+class ExternalMutexGuarded {
+  // Holds a value that can only be manipulated while some other mutex is locked.
+  //
+  // The ExternalMutexGuarded<T> lives *outside* the scope of any lock on the mutex, but ensures
+  // that the value it holds can only be accessed under lock by forcing the caller to present a
+  // lock before accessing the value.
+  //
+  // Additionally, ExternalMutexGuarded<T>'s destructor will take an exclusive lock on the mutex
+  // while destroying the held value, unless the value has been release()ed before hand.
+  //
+  // The type T must have the following properties (which probably all movable types satisfy):
+  // - T is movable.
+  // - Immediately after any of the following has happened, T's destructor is effectively a no-op
+  //   (hence certainly not requiring locks):
+  //   - The value has been default-constructed.
+  //   - The value has been initialized by-move from a default-constructed T.
+  //   - The value has been moved away.
+  // - If ExternalMutexGuarded<T> is ever moved, then T must have a move constructor and move
+  //   assignment operator that do not follow any pointers, therefore do not need to take a lock.
+  //
+  // Inherits from LockSourceLocation to perform an empty base class optimization when lock tracking
+  // is compiled out. Once the minimum C++ standard for the KJ library is C++20, this optimization
+  // could be replaced by a member variable with a [[no_unique_address]] annotation.
+public:
+  ExternalMutexGuarded(LockSourceLocationArg location = {})
+      : location(location) {}
+
+  template <typename U, typename... Params>
+  ExternalMutexGuarded(Locked<U> lock, Params&&... params, LockSourceLocationArg location = {})
+      : mutex(lock.mutex),
+        value(kj::fwd<Params>(params)...),
+        location(location) {}
+  // Construct the value in-place. This constructor requires passing ownership of the lock into
+  // the constructor. Normally this should be a lock that you take on the line calling the
+  // constructor, like:
+  //
+  //     ExternalMutexGuarded<T> foo(someMutexGuarded.lockExclusive());
+  //
+  // The reason this constructor does not accept an lvalue reference to an existing lock is because
+  // this would be deadlock-prone: If an exception were thrown immediately after the constructor
+  // completed, then the destructor would deadlock, because the lock would still be held. An
+  // ExternalMutexGuarded must live outside the scope of any locks to avoid such a deadlock.
+
+  ~ExternalMutexGuarded() noexcept(false) {
+    if (mutex != nullptr) {
+      mutex->lock(_::Mutex::EXCLUSIVE, nullptr, location);
+      KJ_DEFER(mutex->unlock(_::Mutex::EXCLUSIVE));
+      value = T();
+    }
+  }
+
+  ExternalMutexGuarded(ExternalMutexGuarded&& other)
+      : mutex(other.mutex), value(kj::mv(other.value)), location(other.location) {
+    other.mutex = nullptr;
+  }
+  ExternalMutexGuarded& operator=(ExternalMutexGuarded&& other) {
+    mutex = other.mutex;
+    value = kj::mv(other.value);
+    location = other.location;
+    other.mutex = nullptr;
+    return *this;
+  }
+
+  template <typename U>
+  void set(Locked<U>& lock, T&& newValue) {
+    KJ_IREQUIRE(mutex == nullptr);
+    mutex = lock.mutex;
+    value = kj::mv(newValue);
+  }
+
+  template <typename U>
+  T& get(Locked<U>& lock) {
+    KJ_IREQUIRE(lock.mutex == mutex);
+    return value;
+  }
+
+  template <typename U>
+  const T& get(Locked<const U>& lock) const {
+    KJ_IREQUIRE(lock.mutex == mutex);
+    return value;
+  }
+
+  template <typename U>
+  T release(Locked<U>& lock) {
+    // Release (move away) the value. This allows the destructor to skip locking the mutex.
+    KJ_IREQUIRE(lock.mutex == mutex);
+    T result = kj::mv(value);
+    mutex = nullptr;
+    return result;
+  }
+
+private:
+  _::Mutex* mutex = nullptr;
+  T value;
+  KJ_NO_UNIQUE_ADDRESS LockSourceLocation location;
+  // When built against C++20 (or clang >= 9.0), the overhead of this is elided. Otherwise this
+  // struct will be 1 byte larger than it would otherwise be.
+};
+
+template <typename T>
+class Lazy {
+  // A lazily-initialized value.
+
+public:
+  template <typename Func>
+  T& get(Func&& init, LockSourceLocationArg location = {});
+  template <typename Func>
+  const T& get(Func&& init, LockSourceLocationArg location = {}) const;
+  // The first thread to call get() will invoke the given init function to construct the value.
+  // Other threads will block until construction completes, then return the same value.
+  //
+  // `init` is a functor(typically a lambda) which takes `SpaceFor<T>&` as its parameter and returns
+  // `Own<T>`.  If `init` throws an exception, the exception is propagated out of that thread's
+  // call to `get()`, and subsequent calls behave as if `get()` hadn't been called at all yet --
+  // in other words, subsequent calls retry initialization until it succeeds.
+
+private:
+  mutable _::Once once;
+  mutable SpaceFor<T> space;
+  mutable Own<T> value;
+
+  template <typename Func>
+  class InitImpl;
+};
+
+// =======================================================================================
+// Inline implementation details
+
+template <typename T>
+template <typename... Params>
+inline MutexGuarded<T>::MutexGuarded(Params&&... params)
+    : value(kj::fwd<Params>(params)...) {}
+
+template <typename T>
+inline Locked<T> MutexGuarded<T>::lockExclusive(LockSourceLocationArg location)
+    const {
+  mutex.lock(_::Mutex::EXCLUSIVE, nullptr, location);
+  return Locked<T>(mutex, value);
+}
+
+template <typename T>
+inline Locked<const T> MutexGuarded<T>::lockShared(LockSourceLocationArg location) const {
+  mutex.lock(_::Mutex::SHARED, nullptr, location);
+  return Locked<const T>(mutex, value);
+}
+
+template <typename T>
+inline Maybe<Locked<T>> MutexGuarded<T>::lockExclusiveWithTimeout(Duration timeout,
+    LockSourceLocationArg location) const {
+  if (mutex.lock(_::Mutex::EXCLUSIVE, timeout, location)) {
+    return Locked<T>(mutex, value);
+  } else {
+    return nullptr;
+  }
+}
+
+template <typename T>
+inline Maybe<Locked<const T>> MutexGuarded<T>::lockSharedWithTimeout(Duration timeout,
+    LockSourceLocationArg location) const {
+  if (mutex.lock(_::Mutex::SHARED, timeout, location)) {
+    return Locked<const T>(mutex, value);
+  } else {
+    return nullptr;
+  }
+}
+
+template <typename T>
+inline const T& MutexGuarded<T>::getAlreadyLockedShared() const {
+#ifdef KJ_DEBUG
+  mutex.assertLockedByCaller(_::Mutex::SHARED);
+#endif
+  return value;
+}
+template <typename T>
+inline T& MutexGuarded<T>::getAlreadyLockedShared() {
+#ifdef KJ_DEBUG
+  mutex.assertLockedByCaller(_::Mutex::SHARED);
+#endif
+  return value;
+}
+template <typename T>
+inline T& MutexGuarded<T>::getAlreadyLockedExclusive() const {
+#ifdef KJ_DEBUG
+  mutex.assertLockedByCaller(_::Mutex::EXCLUSIVE);
+#endif
+  return const_cast<T&>(value);
+}
+
+template <typename T>
+template <typename Func>
+class Lazy<T>::InitImpl: public _::Once::Initializer {
+public:
+  inline InitImpl(const Lazy<T>& lazy, Func&& func): lazy(lazy), func(kj::fwd<Func>(func)) {}
+
+  void run() override {
+    lazy.value = func(lazy.space);
+  }
+
+private:
+  const Lazy<T>& lazy;
+  Func func;
+};
+
+template <typename T>
+template <typename Func>
+inline T& Lazy<T>::get(Func&& init, LockSourceLocationArg location) {
+  if (!once.isInitialized()) {
+    InitImpl<Func> initImpl(*this, kj::fwd<Func>(init));
+    once.runOnce(initImpl, location);
+  }
+  return *value;
+}
+
+template <typename T>
+template <typename Func>
+inline const T& Lazy<T>::get(Func&& init, LockSourceLocationArg location) const {
+  if (!once.isInitialized()) {
+    InitImpl<Func> initImpl(*this, kj::fwd<Func>(init));
+    once.runOnce(initImpl, location);
+  }
+  return *value;
+}
+
+#if KJ_TRACK_LOCK_BLOCKING
+struct BlockedOnMutexAcquisition {
+  const _::Mutex& mutex;
+  // The mutex we are blocked on.
+
+  const SourceLocation& origin;
+  // Where did the blocking operation originate from.
+};
+
+struct BlockedOnCondVarWait {
+  const _::Mutex& mutex;
+  // The mutex the condition variable is using (may or may not be locked).
+
+  const void* waiter;
+  // Pointer to the waiter that's being waited on.
+
+  const SourceLocation& origin;
+  // Where did the blocking operation originate from.
+};
+
+struct BlockedOnOnceInit {
+  const _::Once& once;
+
+  const SourceLocation& origin;
+  // Where did the blocking operation originate from.
+};
+
+using BlockedOnReason = OneOf<BlockedOnMutexAcquisition, BlockedOnCondVarWait, BlockedOnOnceInit>;
+
+Maybe<const BlockedOnReason&> blockedReason() noexcept;
+// Returns the information about the reason the current thread is blocked synchronously on KJ
+// lock primitives. Returns nullptr if the current thread is not currently blocked on such
+// primitives. This is intended to be called from a signal handler to check whether the current
+// thread is blocked. Outside of a signal handler there is little value to this function. In those
+// cases by definition the thread is not blocked. This includes the callable used as part of a
+// condition variable since that happens after the lock is acquired & the current thread is no
+// longer blocked). The utility could be made useful for non-signal handler use-cases by being able
+// to fetch the pointer to the TLS variable directly (i.e. const BlockedOnReason&*). However, there
+// would have to be additional changes/complexity to try make that work since you'd need
+// synchronization to ensure that the memory you'd try to reference is still valid. The likely
+// solution would be to make these mutually exclusive options where you can use either the fast
+// async-safe option, or a mutex-guarded TLS variable you can get a reference to that isn't
+// async-safe. That being said, maybe someone can come up with a way to make something that works
+// in both use-cases which would of course be more preferable.
+#endif
+
+
+}  // namespace kj
+
+KJ_END_HEADER