// bslmt_writelockguard.h                                             -*-C++-*-
#ifndef INCLUDED_BSLMT_WRITELOCKGUARD
#define INCLUDED_BSLMT_WRITELOCKGUARD

#include <bsls_ident.h>
BSLS_IDENT("$Id: $")

//@PURPOSE: Provide generic scoped guards for write synchronization objects.
//
//@CLASSES:
//  bslmt::WriteLockGuard: automatic locking-unlocking for write access
//  bslmt::WriteLockGuardUnlock: automatic unlocking-locking for write access
//  bslmt::WriteLockGuardTryLock: automatic non-blocking locking-unlocking
//  bslmt::LockWriteGuard: DEPRECATED
//
//@SEE_ALSO: bslmt_lockguard, bslmt_readlockguard
//
//@DESCRIPTION: This component provides generic guards,
// `bslmt::WriteLockGuard`, `bslmt::WriteLockGuardUnlock`, and
// `bslmt::WriteLockGuardTryLock`, to automatically lock and unlock an external
// synchronization object for writing.  The synchronization object can be any
// type (e.g., `bslmt::ReaderWriterLock`) that provides the following methods:
// ```
// void lockWrite();
// void unlock();
// ```
// Both `bslmt::WriteLockGuard` and `bslmt::WriteLockGuardUnlock` implement the
// "construction is acquisition, destruction is release" idiom.  During
// construction, `bslmt::WriteLockGuard` automatically calls `lockWrite` on the
// user-supplied object, and `unlock` when it is destroyed (unless released).
// `bslmt::WriteLockGuardUnlock` does the opposite -- it invokes the `unlock`
// method when constructed and the `lockWrite` method when destroyed.
//
// A third type of guard, `bslmt::WriteLockGuardTryLock`, attempts to acquire a
// lock, and if acquisition succeeds, releases it upon destruction.  Since the
// acquisition is done at construction time, it is not possible to return a
// value to indicate success.  Rather, the `bslmt::WriteLockGuardTryLock`
// contains a pointer to the synchronization object if `tryLock` succeeds, and
// is null otherwise.  The synchronization object can be any type (e.g.,
// `bslmt::Mutex` or `bslmt::RecursiveMutex`) that provides the following
// methods:
// ```
// int tryLockWrite();
// void unlock();
// ```
// Note that objects of neither guard type assumes ownership of the
// synchronization object passed at construction.  Also note that objects of
// all of the guard types may be constructed with a null `lock` whereby the
// constructed guard objects guard no lock.  The destructor of each of the
// guard types has no effect if no lock is under management.
//
///Behavior of the `release` Method
///--------------------------------
// Like all BDE guard classes, each of the three `bslmt::WriteLockGuard*`
// classes provides a `release` method that terminates the guard's management
// of any lock object that the guard holds.  The `release` method has *no*
// *effect* on the state of the lock object.
//
// In particular, `bslmt::WriteLockGuard::release` does not unlock the lock
// object under management.  If a user wants to release the lock object *and*
// unlock the lock object (because the lock is no longer required before the
// guard goes out of scope), the following idiom can be used:
// ```
// // 'guard' is an existing guard of type 'bslmt::WriteLockGuard<my_RLock>',
// // created in a scope that we do not control.
//
// {
//     // ... Do work that requires the lock.
//
//     // We know that the lock is no longer needed.
//
//     my_RLock *rlock = guard.release();
//
//     // 'rlock' is no longer managed, but is *still* *locked*.
//
//     rlock->unlock();
//
//     // ... Do work that does not require the lock.
// }
// ```
//
///Usage
///-----
// This section illustrates intended use of this component.
//
///Example 1: Basic Usage
/// - - - - - - - - - - -
// Use this component to ensure that in the event of an exception or exit from
// any point in a given scope, the synchronization object will be properly
// unlocked.  The following function, `errorProneFunc`, is overly complex, not
// exception safe, and contains a bug.
// ```
// static void errorProneFunc(my_Object *obj, my_RWLock *rwlock)
// {
//     rwlock->lockWrite();
//     if (someUpgradeCondition) {
//         obj->someUpgradeMethod();
//         rwlock->unlock();
//         return;                                                   // RETURN
//     } else if (someOtherUpgradeCondition) {
//         obj->someOtherUpgradeMethod();
//         // MISTAKE! forgot to unlock rwlock
//         return;                                                   // RETURN
//     }
//     obj->defaultUpgradeMethod();
//     rwlock->unlock();
//     return;
// }
// ```
// The function can be rewritten with a cleaner and safer implementation using
// a guard object.  The `safeFunc` function is simpler than `errorProneFunc`,
// is exception safe, and avoids the multiple calls to unlock that can be a
// source of errors.
// ```
// static void safeFunc(my_Object *obj, my_RWLock *rwlock)
// {
//     bslmt::WriteLockGuard<my_RWLock> guard(rwlock);
//     if (someUpgradeCondition) {
//         obj->someUpgradeMethod();
//         return;                                                   // RETURN
//     } else if (someOtherUpgradeCondition) {
//         obj->someOtherUpgradeMethod();
//         // OK, rwlock is automatically unlocked
//         return;                                                   // RETURN
//     }
//     obj->defaultUpgradeMethod();
//     return;
// }
// ```
// When blocking while acquiring the lock is not desirable, one may instead use
// a `bslmt::WriteLockGuardTryLock` in the typical following fashion:
// ```
// /// Perform upgrade and return positive value if locking succeeds.
// /// Return 0 if locking fails.
// static int safeButNonBlockingFunc(my_Object *obj, my_RWLock *rwlock)
// {
//     const int RETRIES = 1; // use higher values for higher success rate
//     bslmt::WriteLockGuardTryLock<my_RWLock> guard(rwlock, RETRIES);
//     if (guard.ptr()) { // rwlock is locked
//         if (someUpgradeCondition) {
//             obj->someUpgradeMethod();
//             return 2;                                             // RETURN
//         } else if (someOtherUpgradeCondition) {
//             obj->someOtherUpgradeMethod();
//             return 3;                                             // RETURN
//         }
//         obj->defaultUpgradeMethod();
//         return 1;                                                 // RETURN
//     }
//     return 0;
// }
// ```
// If the underlying lock object provides an upgrade from a lock for read to a
// lock for write (as does `bslmt::ReaderWriterLock` with the
// `upgradeToWriteLock` function, for example), and the lock is already guarded
// by a `bslmt::ReadLockGuard`, then it is not necessary to transfer the guard
// to a `bslmt::WriteLockGuard`.  In fact, a combination of
// `bslmt::ReadLockGuard` and `bslmt::WriteLockGuard` guarding a common lock
// object should probably never be needed.
//
// Care must be taken so as not to interleave guard objects in such a way as to
// cause an illegal sequence of calls on a lock (two sequential lock calls or
// two sequential unlock calls on a non-recursive read/write lock).

#include <bslscm_version.h>

namespace BloombergLP {
namespace bslmt {

                           // ====================
                           // class WriteLockGuard
                           // ====================

/// This class template implements a guard for acquisition and release of
/// write synchronization resources (i.e., writer locks).
template <class T>
class WriteLockGuard {

    // DATA
    T *d_lock_p;  // lock guarded by this object (held, not owned)

  private:
    // NOT IMPLEMENTED
    WriteLockGuard(const WriteLockGuard<T>&);
    WriteLockGuard<T>& operator=(const WriteLockGuard<T>&);

  public:
    // CREATORS

    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->lockWrite()`.  Supplying a
    /// null `lock` has no effect.  The behavior is undefined unless `lock`
    /// (if non-null) is not already locked by this thread.  Note that
    /// `lock` must remain valid throughout the lifetime of this guard, or
    /// until `release` is called.
    explicit WriteLockGuard(T *lock);

    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->lockWrite()` if the
    /// specified `alreadyLockedFlag` is `false`.  Supplying a null `lock`
    /// has no effect.  The behavior is undefined unless the state of `lock`
    /// (if non-null) is consistent with `alreadyLockedFlag`.  Note that
    /// `alreadyLockedFlag` is used to indicate whether `lock` is in an
    /// already-locked state when passed, so if `alreadyLockedFlag` is
    /// `true` the `lock` method will *not* be called on the supplied
    /// `lock`.  Also note that `lock` must remain valid throughout the
    /// lifetime of this guard, or until `release` is called.
    WriteLockGuard(T *lock, bool alreadyLockedFlag);

    /// Destroy this scoped guard and invoke the `unlock` method on the
    /// lock object under management by this guard, if any.  If no lock is
    /// currently being managed, this method has no effect.
    ~WriteLockGuard();

    // MANIPULATORS

    /// Return the address of the modifiable lock object under management by
    /// this guard, and release the lock from further management by this
    /// guard.  If no lock is currently being managed, return 0 with no
    /// other effect.  Note that this operation does *not* unlock the lock
    /// object (if any) that was under management.
    T *release();

    // ACCESSORS

    /// Return the address of the modifiable lock object under management by
    /// this guard, or 0 if no lock is currently being managed.
    T *ptr() const;
};

                           // ====================
                           // class LockWriteGuard
                           // ====================

/// @DEPRECATED: Use `WriteLockGuard` instead.
template <class T>
class LockWriteGuard : public WriteLockGuard<T> {

  private:
    // NOT IMPLEMENTED
    LockWriteGuard(const LockWriteGuard<T>&);
    LockWriteGuard<T>& operator=(const LockWriteGuard<T>&);

  public:
    // CREATORS

    /// @DEPRECATED: Use `WriteLockGuard` instead.
    explicit LockWriteGuard(T *lock);

    /// @DEPRECATED: Use `WriteLockGuard` instead.
    LockWriteGuard(T *lock, bool alreadyLockedFlag);

};

                        // ==========================
                        // class WriteLockGuardUnlock
                        // ==========================

/// This class template implements a guard for release and reacquisition
/// of write synchronization resources (i.e., writer locks).
template <class T>
class WriteLockGuardUnlock {

    // DATA
    T *d_lock_p;  // lock guarded by this object (held, not owned)

  private:
    // NOT IMPLEMENTED
    WriteLockGuardUnlock(const WriteLockGuardUnlock<T>&);
    WriteLockGuardUnlock<T>& operator=(const WriteLockGuardUnlock<T>&);

  public:
    // CREATORS

    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->unlock()`.  Supplying a null
    /// `lock` has no effect.  The behavior is undefined unless `lock` (if
    /// non-null) is locked by this thread.  Note that `lock` must remain
    /// valid throughout the lifetime of this guard, or until `release` is
    /// called.
    explicit WriteLockGuardUnlock(T *lock);

    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->unlock()` if the specified
    /// `alreadyUnlockedFlag` is `false`.  Supplying a null `lock` has no
    /// effect.  The behavior is undefined unless the state of `lock` (if
    /// non-null) is consistent with `alreadyUnlockedFlag`.  Note that
    /// `alreadyUnlockedFlag` is used to indicate whether `lock` is in an
    /// already-unlocked state when passed, so if `alreadyUnlockedFlag` is
    /// `true` the `unlock` method will *not* be called on the supplied
    /// `lock`.  Also note that `lock` must remain valid throughout the
    /// lifetime of this guard, or until `release` is called.
    WriteLockGuardUnlock(T *lock, bool alreadyUnlockedFlag);

    /// Destroy this scoped guard and invoke the `lockWrite` method on the
    /// lock object under management by this guard, if any.  If no lock is
    /// currently being managed, this method has no effect.
    ~WriteLockGuardUnlock();

    // MANIPULATORS

    /// Return the address of the modifiable lock object under management by
    /// this guard, and release the lock from further management by this
    /// guard.  If no lock is currently being managed, return 0 with no
    /// other effect.  Note that this operation does *not* lock the lock
    /// object (if any) that was under management.
    T *release();

    // ACCESSORS

    /// Return the address of the modifiable lock object under management by
    /// this guard, or 0 if no lock is currently being managed.
    T *ptr() const;
};

                       // ===========================
                       // class WriteLockGuardTryLock
                       // ===========================

/// This class template implements a guard for tentative acquisition and
/// release of write synchronization resources (i.e., writer locks).
template <class T>
class WriteLockGuardTryLock {

    // DATA
    T *d_lock_p;  // lock guarded by this object (held, not owned)

  private:
    // NOT IMPLEMENTED
    WriteLockGuardTryLock(const WriteLockGuardTryLock<T>&);
    WriteLockGuardTryLock<T>& operator=(const WriteLockGuardTryLock<T>&);

  public:
    // CREATORS

    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->tryLockWrite()` until the
    /// lock is acquired for writing, or until the optionally specified
    /// `attempts` have been made to acquire the lock.  If `attempts` is not
    /// specified only one attempt is made to acquire the lock.  Supplying a
    /// null `lock` has no effect.  The behavior is undefined unless `lock`
    /// (if non-null) is not already locked by this thread and
    /// `0 < attempts`.  Note that `lock` must remain valid throughout the
    /// lifetime of this guard, or until `release` is called.
    explicit WriteLockGuardTryLock(T *lock, int attempts = 1);

    /// Destroy this scoped guard and invoke the `unlock` method on the
    /// lock object under management by this guard, if any.  If no lock is
    /// currently being managed, this method has no effect.
    ~WriteLockGuardTryLock();

    // MANIPULATORS

    /// Return the address of the modifiable lock object under management by
    /// this guard, and release the lock from further management by this
    /// guard.  If no lock is currently being managed, return 0 with no
    /// other effect.  Note that this operation does *not* unlock the lock
    /// object (if any) that was under management.
    T *release();

    // ACCESSORS

    /// Return the address of the modifiable lock object under management by
    /// this guard, or 0 if no lock is currently being managed.
    T *ptr() const;
};

// ============================================================================
//                             INLINE DEFINITIONS
// ============================================================================

                           // --------------------
                           // class WriteLockGuard
                           // --------------------

// CREATORS
template <class T>
inline
WriteLockGuard<T>::WriteLockGuard(T *lock)
: d_lock_p(lock)
{
    if (d_lock_p) {
        d_lock_p->lockWrite();
    }
}

template <class T>
inline
WriteLockGuard<T>::WriteLockGuard(T *lock, bool alreadyLockedFlag)
: d_lock_p(lock)
{
    if (d_lock_p && !alreadyLockedFlag) {
        d_lock_p->lockWrite();
    }
}

template <class T>
inline
WriteLockGuard<T>::~WriteLockGuard()
{
    if (d_lock_p) {
        d_lock_p->unlock();
    }
}

// MANIPULATORS

template <class T>
inline
T *WriteLockGuard<T>::release()
{
    T *lock  = d_lock_p;
    d_lock_p = 0;
    return lock;
}

// ACCESSORS
template <class T>
inline
T *WriteLockGuard<T>::ptr() const
{
    return d_lock_p;
}

                        // --------------------------
                        // class WriteLockGuardUnlock
                        // --------------------------

// CREATORS
template <class T>
inline
WriteLockGuardUnlock<T>::WriteLockGuardUnlock(T *lock)
: d_lock_p(lock)
{
    if (d_lock_p) {
        d_lock_p->unlock();
    }
}

template <class T>
inline
WriteLockGuardUnlock<T>::WriteLockGuardUnlock(T    *lock,
                                              bool  alreadyUnlockedFlag)
: d_lock_p(lock)
{
    if (d_lock_p && !alreadyUnlockedFlag) {
        d_lock_p->unlock();
    }
}

template <class T>
inline
WriteLockGuardUnlock<T>::~WriteLockGuardUnlock()
{
    if (d_lock_p) {
        d_lock_p->lockWrite();
    }
}

// MANIPULATORS
template <class T>
inline
T *WriteLockGuardUnlock<T>::release()
{
    T *lock  = d_lock_p;
    d_lock_p = 0;
    return lock;
}

// ACCESSORS
template <class T>
inline
T *WriteLockGuardUnlock<T>::ptr() const
{
    return d_lock_p;
}

                       // ---------------------------
                       // class WriteLockGuardTryLock
                       // ---------------------------

// CREATORS
template <class T>
WriteLockGuardTryLock<T>::WriteLockGuardTryLock(T *lock, int attempts)
: d_lock_p(0)
{
    if (lock) {
        while (attempts--) {
            if (!lock->tryLockWrite()) {
                d_lock_p = lock;
                break;
            }
        }
    }
}

template <class T>
inline
WriteLockGuardTryLock<T>::~WriteLockGuardTryLock()
{
    if (d_lock_p) {
        d_lock_p->unlock();
    }
}

// MANIPULATORS
template <class T>
inline
T *WriteLockGuardTryLock<T>::release()
{
    T *lock  = d_lock_p;
    d_lock_p = 0;
    return lock;
}

// ACCESSORS
template <class T>
inline
T *WriteLockGuardTryLock<T>::ptr() const
{
    return d_lock_p;
}

                           // --------------------
                           // class LockWriteGuard
                           // --------------------

// CREATORS
template <class T>
inline
LockWriteGuard<T>::LockWriteGuard(T *lock)
: WriteLockGuard<T>(lock)
{
}

template <class T>
inline
LockWriteGuard<T>::LockWriteGuard(T *lock, bool alreadyLockedFlag)
: WriteLockGuard<T>(lock, alreadyLockedFlag)
{
}

}  // close package namespace
}  // close enterprise namespace

#endif

// ----------------------------------------------------------------------------
// Copyright 2015 Bloomberg Finance L.P.
//
// 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.
// ----------------------------- END-OF-FILE ----------------------------------
