DirectoryLock Class Reference

This class can be used to implement file-based directory locks. More...

#include <directorylock.h>

Collaboration diagram for DirectoryLock:

Public Types
enum class	LockStatus
	The return type of getStatus() More...
typedef std::function< bool(const FilePath &dir, LockStatus status, const QString &user)>	LockHandlerCallback
	Callback type used to determine whether a lock should be overridden or not. More...

Public Member Functions
	DirectoryLock () noexcept
	The default constructor. More...
	DirectoryLock (const DirectoryLock &other)=delete
	Copy constructor. More...
	DirectoryLock (const FilePath &dir) noexcept
	A constructor which will call setDirToLock() More...
	~DirectoryLock () noexcept
	The destructor (this may also unlock the locked file) More...
void	setDirToLock (const FilePath &dir) noexcept
	Specify the directory for which you need the lock. More...
const FilePath &	getDirToLock () const noexcept
	Get the filepath of the directory to lock (passed by setDirToLock()) More...
const FilePath &	getLockFilepath () const noexcept
	Get the filepath of the lock file (NOT the directory to lock!) More...
LockStatus	getStatus (QString *lockedByUser=nullptr) const
	Get the lock status of the specified directory. More...
void	tryLock (LockHandlerCallback lockHandler=nullptr)
	Lock the specified directory if not already locked. More...
bool	unlockIfLocked ()
	Unlock the specified directory if it was locked by this object. More...
void	lock ()
	Lock the specified directory (create/update the lock file) More...
void	unlock ()
	Unlock the specified directory (remove the lock file) More...
DirectoryLock &	operator= (const DirectoryLock &rhs)=delete

Static Private Member Functions
static QSet< FilePath > &	dirsLockedByThisAppInstance () noexcept
	Get the global set of filepaths locked by this application instance. More...

Private Attributes
FilePath	mDirToLock
	The filepath to the directory to lock (passed by setDirToLock()) More...
FilePath	mLockFilePath
	The filepath to the lock file. More...
bool	mLockedByThisObject
	This attribute defines if the lock is active by this object. More...

Detailed Description

This class can be used to implement file-based directory locks.

Many classes of this project open some directories (workspaces, projects, library elements, ...). But it's very dangerous if a directory is opened multiple times simultaneously (by the same or another instance of the application, maybe even on different computers if the directories are located on a network drive). To avoid such problems, this class provides a mechanism to create directory locks.

How such a directory lock works:

Let's say that you want to open the directory "/foo/bar/". Then a lock file with the filepath "/foo/bar/.lock" will be created. After closing the directory, the lock file will be removed. So, while the directory (e.g. a project) is open, there will be a lock file in the same directory. If the same or another instance of the application now wants to open the same directory at the same time, the lock file is detected and opening the directory will be denied.

The lock file is a simple UTF-8 encoded text file with 5 lines with following values:

1. The full name (first name + last name) of the user which holds the lock
2. The username (logon name) of the user which holds the lock
3. The hostname of the user's computer which holds the lock
4. The process id (PID) of the application instance which holds the lock
5. The process name of the application instance which holds the lock
6. The datetime when the lock file was created/updated (UTC and ISO format!)

Example:

Homer Simpson
homer
homer-workstation
1234
librepcb
2013-04-13T12:43:52Z

The lock file (and especially its content) is also used to detect application crashes. If the application crashes while a directory was locked, the lock file will still exist after the application crashed. Now, if the user tries to open the locked directory again, the content of the lock file will be parsed. If the username and the hostname in the lock file is equal to the current user which tries to get the lock, it's clear that the lock file does NOT exist because the locked directory is already open, but that the application crashed while the directory was locked. If there exists a backup of the locked directory (e.g. project auto-save), this allows to ask the user whether the backup should be restored or not.

How to use this class:

First, you need to create an instance of this class for the directory you want to protect with a lock. There are two different constructors for this purpose. If you use the default constructor, you need to call setDirToLock() afterwards. Now you can read the lock status of the specified directory with getStatus(). With lock() you can create the lock file, and with unlock() you can remove the lock file. There are also the two convenience methods tryLock() and unlockIfLocked(), just read their documentation for more information.

Note

The destructor will automatically call unlockIfLocked(). This allows a reliable implementation of a directory lock, because you can add a DirectoryLock instance to the attributes of your class which access a directory which should be locked. This will ensure that the lock will be released when your object gets destroyed (RAII). See the code example below.

Code Example:

// a class which opens a directory and needs to lock it
class MyDirectoryOpeningClass {
public:
  MyDirectoryOpeningClass()  // constructor
    : mLock(FilePath("C:/myDirectory")) { // variant 1 to set the filepath
    mLock.setDirToLock(FilePath("C:/myDirectory"));  // variant 2
    switch (mLock.getStatus()) { // Note: this line can throw an exception!
      case DirectoryLock::LockStatus::Unlocked:
        // No lock exists --> lock the directory now
        mLock.lock();  // Note: this line can throw an exception!
        break;
      case DirectoryLock::LockStatus::LockedByThisApp:
      case DirectoryLock::LockStatus::LockedByOtherApp:
      case DirectoryLock::LockStatus::LockedByOtherUser:
      case DirectoryLock::LockStatus::LockedByUnknownApp:
        // The directory is (probably) already locked by another instance!
        throw Exception("Directory is locked!");
      case DirectoryLock::LockStatus::StaleLock:
        // The application crashed while the lock was active.
        // Ask the user whether a backup should be restored or not.
        break;
      default:
        // Should not happen...
        break;
    }
    // if you don't care about stale locks, you could just do this instead:
    // myLock.tryLock(); // Note: this line can throw an exception!
  }
 
  ~MyDirectoryOpeningClass() {
    // You do not have to (but you could) call myLock.unlockIfLocked(),
    // as it will be called automatically in the destructor of myLock.
    // try { myLock.unlockIfLocked(); } catch (...) { }
  }
 
private:
  // an instance, not only a pointer (important for RAII)!
  DirectoryLock mLock;
};

Member Typedef Documentation

LockHandlerCallback

typedef std::function<bool(const FilePath& dir, LockStatus status, const QString& user)> LockHandlerCallback

Callback type used to determine whether a lock should be overridden or not.

Parameters

dir	The directory to be locked.
status	The current status of the lock (see getStatus()).
user	Name of the user which currently holds the lock.

Return values

true	Override lock.
false	Do not override lock.

Exceptions

librepcb::UserCanceled

to abort locking the directory.

Member Enumeration Documentation

LockStatus

enum class LockStatus

strong

The return type of getStatus()

Enumerator
Unlocked	The directory is not locked (lock file does not exist).
StaleLock	The directory is locked by a crashed application instance.
LockedByThisApp	The directory is locked by this application instance.
LockedByOtherApp	The directory is locked by another application instance on this machine.
LockedByOtherUser	The directory is locked by another user or machine.
LockedByUnknownApp	The directory is locked by an unknown application (may be stale).

Constructor & Destructor Documentation

DirectoryLock() [1/3]

DirectoryLock ( )

noexcept

The default constructor.

Warning

If you use this constructor, you need to call setDirToLock() afterwards (before calling any other method of this class)!

DirectoryLock() [2/3]

DirectoryLock ( const DirectoryLock &  other )

delete

Copy constructor.

Parameters

other

The object to copy

DirectoryLock() [3/3]

DirectoryLock ( const FilePath &  dir )

explicitnoexcept

A constructor which will call setDirToLock()

Parameters

dir	See setDirToLock()

~DirectoryLock()

~DirectoryLock ( )

noexcept

The destructor (this may also unlock the locked file)

Note

The destructor will also try to unlock the directory if it was locked with this object.

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Member Function Documentation

setDirToLock()

void setDirToLock ( const FilePath &  dir )

noexcept

Specify the directory for which you need the lock.

Parameters

dir	The filepath to the directory to lock

Warning

This method must not be called when this object already holds a lock!

getDirToLock()

const FilePath & getDirToLock ( ) const

inlinenoexcept

Get the filepath of the directory to lock (passed by setDirToLock())

Returns

The filepath to the directory to lock (invalid if no filepath was set)

getLockFilepath()

const FilePath & getLockFilepath ( ) const

inlinenoexcept

Get the filepath of the lock file (NOT the directory to lock!)

Returns

The filepath to the lock file (invalid if no valid filepath was set)

getStatus()

DirectoryLock::LockStatus getStatus

(

QString *

lockedByUser = nullptr

)

const

Get the lock status of the specified directory.

Parameters

lockedByUser

If not nullptr and the directory is locked, the username of the current lock is written into this string.

Returns

The current lock status (see LockStatus)

Exceptions

Exception

on error (e.g. invalid filepath, no access rights, ...)

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tryLock()

void tryLock

(

LockHandlerCallback

lockHandler = nullptr

)

Lock the specified directory if not already locked.

This is a save method to get a lock without the need for first reading the lock status with getStatus(). Depending on the lock status, this method does following:

• Unlocked: Set "wasStale = false" and get the lock (calling lock())
• StaleLock: Set "wasStale = true" and get the lock (calling lock())
• Locked: Throw exception (something like "Directory already locked")

Parameters

lockHandler

If supplied and the directory is already locked, this callback gets called to determine whether the lock should be overridden or not. If not supplied and the directory is locked, an exception will be thrown.

Exceptions

Exception

on error (e.g. already locked, no access rights, ...)

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unlockIfLocked()

bool unlockIfLocked

(

)

Unlock the specified directory if it was locked by this object.

If the specified directory is locked by this object, this method calls unlock(). Otherwise this method does nothing.

Returns

True if the lock has been released by this object, false otherwise.

Exceptions

Exception

on error (e.g. invalid filepath, no access rights, ...)

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lock()

void lock

(

)

Lock the specified directory (create/update the lock file)

Warning

This method will always overwrite an already existing lock file, even if it was created by another application instance! So: Always check the lock status first with getStatus(), or use tryLock() instead!

Exceptions

Exception

on error (e.g. invalid filepath, no access rights, ...)

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unlock()

void unlock

(

)

Unlock the specified directory (remove the lock file)

Warning

This method will always remove an existing lock file, even if it was created by another application instance! So: Always check the lock status first with getStatus(), or use unlockIfLocked() instead!

Exceptions

Exception

on error (e.g. invalid filepath, no access rights, ...)

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operator=()

DirectoryLock & operator= ( const DirectoryLock &  rhs )

delete

dirsLockedByThisAppInstance()

QSet< FilePath > & dirsLockedByThisAppInstance ( )

staticprivatenoexcept

Get the global set of filepaths locked by this application instance.

Returns

Set of directory filepaths currently locked by this instance.

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Member Data Documentation

mDirToLock

FilePath mDirToLock

private

The filepath to the directory to lock (passed by setDirToLock())

mLockFilePath

FilePath mLockFilePath

private

The filepath to the lock file.

Example: If the filepath "/foo/bar" was passed to setDirToLock(), this attribute will have the value "/foo/bar/.lock".

mLockedByThisObject

bool mLockedByThisObject

private

This attribute defines if the lock is active by this object.

If lock() was called successfully, mLockedByThisObject is set to true. If unlock() was called successfully, mLockedByThisObject is set to false.

In other words: This attribute is true while this object has the ownership over the lock file (between calling lock() and unlock()).

The only goal of this attribute is to decide whether the destructor should remove the lock or not. If the destructor is called while this attribute is true, the destructor will call unlock() to remove the file lock.

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The documentation for this class was generated from the following files:

• /__w/1/s/libs/librepcb/core/fileio/directorylock.h
• /__w/1/s/libs/librepcb/core/fileio/directorylock.cpp