NAME

       "Future::Mutex" - mutual exclusion lock around code that returns Futures

SYNOPSIS

          use Future::Mutex;

          my $mutex = Future::Mutex->new;

          sub do_atomically
          {
             return $mutex->enter( sub {
                ...
                return $f;
             });
          }

DESCRIPTION

       Most Future-using code expects to run with some level of concurrency, using future
       instances to represent still-pending operations that will complete at some later time.
       There are occasions however, when this concurrency needs to be restricted - some
       operations that, once started, must not be interrupted until they are complete. Subsequent
       requests to perform the same operation while one is still outstanding must therefore be
       queued to wait until the first is finished. These situations call for a mutual-exclusion
       lock, or "mutex".

       A "Future::Mutex" instance provides one basic operation, which will execute a given block
       of code which returns a future, and itself returns a future to represent that. The mutex
       can be in one of two states; either unlocked or locked. While it is unlocked, requests to
       execute code are handled immediately. Once a block of code is invoked, the mutex is now
       considered to be locked, causing any subsequent requests to invoke code to be queued
       behind the first one, until it completes. Once the initial code indicates completion (by
       its returned future providing a result or failing), the next queued code is invoked.

       An instance may also be a counting mutex if initialised with a count greater than one. In
       this case, it can keep multiple blocks outstanding up to that limit, with subsequent
       requests queued as before. This allows it to act as a concurrency-bounding limit around
       some operation that can run concurrently, but an application wishes to apply overall
       limits to stop it growing too much, such as communications with external services or
       executing other programs.

CONSTRUCTOR

   new
          $mutex = Future::Mutex->new( count => $n )

       Returns a new "Future::Mutex" instance. It is initially unlocked.

       Takes the following named arguments:

       count => INT
               Optional number to limit outstanding concurrency. Will default to 1 if not
               supplied.

METHODS

   enter
          $f = $mutex->enter( \&code )

       Returns a new "Future" that represents the eventual result of calling the code. If the
       mutex is currently unlocked, the code will be invoked immediately. If it is currently
       locked, the code will be queued waiting for the next time it becomes unlocked.

       The code is invoked with no arguments, and is expected to return a "Future".  The eventual
       result of that future determines the result of the future that "enter" returned.

   available
          $avail = $mutex->available

       Returns true if the mutex is currently unlocked, or false if it is locked.

AUTHOR

       Paul Evans <leonerd@leonerd.org.uk>