From type/Lock

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method lock(Lock:D:)

Acquires the lock. If it is currently not available, waits for it.

my $l = Lock.new;
$l.lock;

Since a Lock is implemented using OS-provided facilities, a thread waiting for the lock will not be scheduled until the lock is available for it. Since Lock is re-entrant, if the current thread already holds the lock, calling lock will simply bump a recursion count.

While it's easy enough to use the lock method, it's more difficult to correctly use unlock. Instead, prefer to use the protect method instead, which takes care of making sure the lock/unlock calls always both occur.

From type/IO/Handle

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method lock(IO::Handle:D:
            Bool:D :$non-blocking = False, Bool:D :$shared = False
            --> True)

Places an advisory lock on the file the filehandle if open for. If :$non-blocking is True will Fails with X::IO::Lock if lock could not be obtained, otherwise will block until the lock can be placed. If :$shared is True will place a shared (read) lock, otherwise will place an exclusive (write) lock. On success, returns True; Fails with X::IO::Lock if lock cannot be placed (e.g. when trying to place a shared lock on a filehandle opened in write mode or trying to place an exclusive lock on a filehandle opened in read mode).

You can use .lock again to replace an existing lock with another one. To remove a lock, close the filehandle or use unlock.

# One program writes, the other reads, and thanks to locks either 
# will wait for the other to finish before proceeding to read/write 
 
# Writer 
given "foo".IO.open(:w) {
    .lock;
    .spurt: "I ♥ Raku!";
    .close; # closing the handle unlocks it; we could also use `unlock` for that 
}
 
# Reader 
given "foo".IO.open {
    .lock: :shared;
    .slurp.say; # OUTPUT: «I ♥ Raku!␤» 
    .close;
}

From type/IO/CatHandle

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method lock(IO::CatHandle:D: Bool:D :$non-blocking = False, Bool:D :$shared = False --> True)

Same as IO::Handle.lock. Returns Nil if the source handle queue may get exhausted.

Locks only the currently active source handle. The .on-switch method Callable can be used to conveniently lock/unlock the handles as they're being processed by the CatHandle.

From type/Lock/Async

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method lock(Lock::Async:D: --> Promise:D)

Returns a Promise that will be kept when the lock is available. In the case that the lock is already available, an already kept Promise will be returned. Use await to wait for the lock to be available in a non-blocking manner.

my $l = Lock::Async.new;
await $l.lock;

Prefer to use protect instead of explicit calls to lock and unlock.