[ViewVC] Diff of: cvs/Coro/README

Comparing Coro/README (file contents):
Revision 1.29 by root, Sat Feb 19 06:51:22 2011 UTC vs.
Revision 1.33 by root, Tue Mar 4 06:13:24 2014 UTC

       cede; # yield to coro
       print "3\n";
       cede; # and again
       # use locking
-      use Coro::Semaphore;
       my $lock = new Coro::Semaphore;
       my $locked;
       $lock->down;
       $locked = 1;
            } 1, 2, 3;
         This creates a new coro thread and puts it into the ready queue,
         meaning it will run as soon as the CPU is free for it.
-        "async" will return a coro object - you can store this for future
+        "async" will return a Coro object - you can store this for future
-        reference or ignore it, the thread itself will keep a reference to
+        reference or ignore it - a thread that is running, ready to run or
-        it's thread object - threads are alive on their own.
+        waiting for some event is alive on it's own.
         Another way to create a thread is to call the "new" constructor with
         a code-reference:
            new Coro sub {
         A lot can happen after the coro thread has started running. Quite
         usually, it will not run to the end in one go (because you could use
         a function instead), but it will give up the CPU regularly because
         it waits for external events.
-        As long as a coro thread runs, it's coro object is available in the
+        As long as a coro thread runs, its Coro object is available in the
         global variable $Coro::current.
         The low-level way to give up the CPU is to call the scheduler, which
         selects a new coro thread to run:
            async {
               Coro::terminate "return value 1", "return value 2";
            };
-        And yet another way is to "->cancel" the coro thread from another
+        Yet another way is to "->cancel" (or "->safe_cancel") the coro
-        thread:
+        thread from another thread:
            my $coro = async {
               exit 1;
            };
-           $coro->cancel; # an also accept values for ->join to retrieve
+           $coro->cancel; # also accepts values for ->join to retrieve
         Cancellation *can* be dangerous - it's a bit like calling "exit"
         without actually exiting, and might leave C libraries and XS modules
         in a weird state. Unlike other thread implementations, however, Coro
         is exceptionally safe with regards to cancellation, as perl will
-        always be in a consistent state.
+        always be in a consistent state, and for those cases where you want
+        to do truly marvellous things with your coro while it is being
+        cancelled - that is, make sure all cleanup code is executed from the
+        thread being cancelled - there is even a "->safe_cancel" method.
         So, cancelling a thread that runs in an XS event loop might not be
         the best idea, but any other combination that deals with perl only
         (cancelling when a thread is in a "tie" method or an "AUTOLOAD" for
         example) is safe.
+        Last not least, a coro thread object that isn't referenced is
+        "->cancel"'ed automatically - just like other objects in Perl. This
+        is not such a common case, however - a running thread is referencedy
+        by $Coro::current, a thread ready to run is referenced by the ready
+        queue, a thread waiting on a lock or semaphore is referenced by
+        being in some wait list and so on. But a thread that isn't in any of
+        those queues gets cancelled:
+           async {
+              schedule; # cede to other coros, don't go into the ready queue
+           };
+           cede;
+           # now the async above is destroyed, as it is not referenced by anything.
+        A slightly embellished example might make it clearer:
+           async {
+              my $guard = Guard::guard { print "destroyed\n" };
+              schedule while 1;
+           };
+           cede;
+        Superficially one might not expect any output - since the "async"
+        implements an endless loop, the $guard will not be cleaned up.
+        However, since the thread object returned by "async" is not stored
+        anywhere, the thread is initially referenced because it is in the
+        ready queue, when it runs it is referenced by $Coro::current, but
+        when it calls "schedule", it gets "cancel"ed causing the guard
+        object to be destroyed (see the next section), and printing it's
+        message.
+        If this seems a bit drastic, remember that this only happens when
+        nothing references the thread anymore, which means there is no way
+        to further execute it, ever. The only options at this point are
+        leaking the thread, or cleaning it up, which brings us to...
     5. Cleanup
         Threads will allocate various resources. Most but not all will be
         returned when a thread terminates, during clean-up.
         Cleanup is quite similar to throwing an uncaught exception: perl
            my $sem = new Coro::Semaphore;
            async {
               my $lock_guard = $sem->guard;
-              # if we reutrn, or die or get cancelled, here,
+              # if we return, or die or get cancelled, here,
               # then the semaphore will be "up"ed.
            };
         The "Guard::guard" function comes in handy for any custom cleanup
-        you might want to do:
+        you might want to do (but you cannot switch to other coroutines from
+        those code blocks):
            async {
               my $window = new Gtk2::Window "toplevel";
               # The window will not be cleaned up automatically, even when $window
               # gets freed, so use a guard to ensure it's destruction
               # if we return or die here, the description will be restored
            }
     6. Viva La Zombie Muerte
-        Even after a thread has terminated and cleaned up it's resources,
+        Even after a thread has terminated and cleaned up its resources, the
-        the coro object still is there and stores the return values of the
+        Coro object still is there and stores the return values of the
-        thread. Only in this state will the coro object be "reference
+        thread.
-        counted" in the normal perl sense: the thread code keeps a reference
-        to it when it is active, but not after it has terminated.
-        The means the coro object gets freed automatically when the thread
+        When there are no other references, it will simply be cleaned up and
-        has terminated and cleaned up and there arenot other references.
+        freed.
-        If there are, the coro object will stay around, and you can call
+        If there areany references, the Coro object will stay around, and
-        "->join" as many times as you wish to retrieve the result values:
+        you can call "->join" as many times as you wish to retrieve the
+        result values:
            async {
               print "hi\n";
               1
            };
            };
 GLOBAL VARIABLES
     $Coro::main
         This variable stores the Coro object that represents the main
-        program. While you cna "ready" it and do most other things you can
+        program. While you can "ready" it and do most other things you can
         do to coro, it is mainly useful to compare again $Coro::current, to
         see whether you are running in the main program or not.
     $Coro::current
         The Coro object representing the current coro (the last coro that
         *any* coro, regardless of priority. This is useful sometimes to
         ensure progress is made.
     terminate [arg...]
         Terminates the current coro with the given status values (see
-        cancel).
+        cancel). The values will not be copied, but referenced directly.
     Coro::on_enter BLOCK, Coro::on_leave BLOCK
         These function install enter and leave winders in the current scope.
         The enter block will be executed when on_enter is called and
         whenever the current coro is re-entered by the scheduler, while the
               Coro::on_enter {
                  # on entering the thread, we set an VTALRM handler to cede
                  $SIG{VTALRM} = sub { cede };
                  # and then start the interval timer
                  Time::HiRes::setitimer &Time::HiRes::ITIMER_VIRTUAL, 0.01, 0.01;
               };
               Coro::on_leave {
                  # on leaving the thread, we stop the interval timer again
                  Time::HiRes::setitimer &Time::HiRes::ITIMER_VIRTUAL, 0, 0;
               };
               &{+shift};
            }
            # use like this:
            timeslice {
               # The following is an endless loop that would normally
               # monopolise the process. Since it runs in a timesliced
         To avoid this, it is best to put a suspended coro into the ready
         queue unconditionally, as every synchronisation mechanism must
         protect itself against spurious wakeups, and the one in the Coro
         family certainly do that.
+    $state->is_new
+        Returns true iff this Coro object is "new", i.e. has never been run
+        yet. Those states basically consist of only the code reference to
+        call and the arguments, but consumes very little other resources.
+        New states will automatically get assigned a perl interpreter when
+        they are transfered to.
+    $state->is_zombie
+        Returns true iff the Coro object has been cancelled, i.e. it's
+        resources freed because they were "cancel"'ed, "terminate"'d,
+        "safe_cancel"'ed or simply went out of scope.
+        The name "zombie" stems from UNIX culture, where a process that has
+        exited and only stores and exit status and no other resources is
+        called a "zombie".
     $is_ready = $coro->is_ready
         Returns true iff the Coro object is in the ready queue. Unless the
         Coro object gets destroyed, it will eventually be scheduled by the
         scheduler.
     $is_suspended = $coro->is_suspended
         Returns true iff this Coro object has been suspended. Suspended
         Coros will not ever be scheduled.
     $coro->cancel (arg...)
-        Terminates the given Coro and makes it return the given arguments as
+        Terminates the given Coro thread and makes it return the given
-        status (default: the empty list). Never returns if the Coro is the
+        arguments as status (default: an empty list). Never returns if the
-        current Coro.
+        Coro is the current Coro.
+        This is a rather brutal way to free a coro, with some limitations -
+        if the thread is inside a C callback that doesn't expect to be
+        canceled, bad things can happen, or if the cancelled thread insists
+        on running complicated cleanup handlers that rely on its thread
+        context, things will not work.
+        Any cleanup code being run (e.g. from "guard" blocks) will be run
+        without a thread context, and is not allowed to switch to other
+        threads. On the plus side, "->cancel" will always clean up the
+        thread, no matter what. If your cleanup code is complex or you want
+        to avoid cancelling a C-thread that doesn't know how to clean up
+        itself, it can be better to "->throw" an exception, or use
+        "->safe_cancel".
+        The arguments to "->cancel" are not copied, but instead will be
+        referenced directly (e.g. if you pass $var and after the call change
+        that variable, then you might change the return values passed to
+        e.g. "join", so don't do that).
+        The resources of the Coro are usually freed (or destructed) before
+        this call returns, but this can be delayed for an indefinite amount
+        of time, as in some cases the manager thread has to run first to
+        actually destruct the Coro object.
+    $coro->safe_cancel ($arg...)
+        Works mostly like "->cancel", but is inherently "safer", and
+        consequently, can fail with an exception in cases the thread is not
+        in a cancellable state.
+        This method works a bit like throwing an exception that cannot be
+        caught - specifically, it will clean up the thread from within
+        itself, so all cleanup handlers (e.g. "guard" blocks) are run with
+        full thread context and can block if they wish. The downside is that
+        there is no guarantee that the thread can be cancelled when you call
+        this method, and therefore, it might fail. It is also considerably
+        slower than "cancel" or "terminate".
+        A thread is in a safe-cancellable state if it either hasn't been run
+        yet, or it has no C context attached and is inside an SLF function.
+        The latter two basically mean that the thread isn't currently inside
+        a perl callback called from some C function (usually via some XS
+        modules) and isn't currently executing inside some C function itself
+        (via Coro's XS API).
+        This call returns true when it could cancel the thread, or croaks
+        with an error otherwise (i.e. it either returns true or doesn't
+        return at all).
+        Why the weird interface? Well, there are two common models on how
+        and when to cancel things. In the first, you have the expectation
+        that your coro thread can be cancelled when you want to cancel it -
+        if the thread isn't cancellable, this would be a bug somewhere, so
+        "->safe_cancel" croaks to notify of the bug.
+        In the second model you sometimes want to ask nicely to cancel a
+        thread, but if it's not a good time, well, then don't cancel. This
+        can be done relatively easy like this:
+           if (! eval { $coro->safe_cancel }) {
+              warn "unable to cancel thread: $@";
+           }
+        However, what you never should do is first try to cancel "safely"
+        and if that fails, cancel the "hard" way with "->cancel". That makes
+        no sense: either you rely on being able to execute cleanup code in
+        your thread context, or you don't. If you do, then "->safe_cancel"
+        is the only way, and if you don't, then "->cancel" is always faster
+        and more direct.
     $coro->schedule_to
         Puts the current coro to sleep (like "Coro::schedule"), but instead
         of continuing with the next coro from the ready queue, always switch
         to the given coro object (regardless of priority etc.). The
         Otherwise clears the exception object.
         Coro will check for the exception each time a schedule-like-function
         returns, i.e. after each "schedule", "cede",
         "Coro::Semaphore->down", "Coro::Handle->readable" and so on. Most of
-        these functions detect this case and return early in case an
+        those functions (all that are part of Coro itself) detect this case
-        exception is pending.
+        and return early in case an exception is pending.
         The exception object will be thrown "as is" with the specified
         scalar in $@, i.e. if it is a string, no line number or newline will
         be appended (unlike with "die").
-        This can be used as a softer means than "cancel" to ask a coro to
+        This can be used as a softer means than either "cancel" or
-        end itself, although there is no guarantee that the exception will
+        "safe_cancel "to ask a coro to end itself, although there is no
-        lead to termination, and if the exception isn't caught it might well
+        guarantee that the exception will lead to termination, and if the
-        end the whole program.
+        exception isn't caught it might well end the whole program.
         You might also think of "throw" as being the moral equivalent of
         "kill"ing a coro with a signal (in this case, a scalar).
     $coro->join
         Wait until the coro terminates and return any values given to the
         "terminate" or "cancel" functions. "join" can be called concurrently
-        from multiple coro, and all will be resumed and given the status
+        from multiple threads, and all will be resumed and given the status
         return once the $coro terminates.
     $coro->on_destroy (\&cb)
         Registers a callback that is called when this coro thread gets
-        destroyed, but before it is joined. The callback gets passed the
+        destroyed, that is, after it's resources have been freed but before
+        it is joined. The callback gets passed the terminate/cancel
-        terminate arguments, if any, and *must not* die, under any
+        arguments, if any, and *must not* die, under any circumstances.
-        circumstances.
-        There can be any number of "on_destroy" callbacks per coro.
+        There can be any number of "on_destroy" callbacks per coro, and
+        there is currently no way to remove a callback once added.
     $oldprio = $coro->prio ($newprio)
         Sets (or gets, if the argument is missing) the priority of the coro
         thread. Higher priority coro get run before lower priority coros.
         Priorities are small signed integers (currently -4 .. +3), that you
         with a coro.
         This method simply sets the "$coro->{desc}" member to the given
         string. You can modify this member directly if you wish, and in
         fact, this is often preferred to indicate major processing states
-        that cna then be seen for example in a Coro::Debug session:
+        that can then be seen for example in a Coro::Debug session:
            sub my_long_function {
               local $Coro::current->{desc} = "now in my_long_function";
               ...
               $Coro::current->{desc} = "my_long_function: phase 1";
     But from within a coro, you often just want to write this:
        my $status = wait_for_child $pid;
     Coro offers two functions specifically designed to make this easy,
-    "Coro::rouse_cb" and "Coro::rouse_wait".
+    "rouse_cb" and "rouse_wait".
     The first function, "rouse_cb", generates and returns a callback that,
     when invoked, will save its arguments and notify the coro that created
     the callback.
     function mentioned above:
        sub wait_for_child($) {
           my ($pid) = @_;
-          my $watcher = AnyEvent->child (pid => $pid, cb => Coro::rouse_cb);
+          my $watcher = AnyEvent->child (pid => $pid, cb => rouse_cb);
-          my ($rpid, $rstatus) = Coro::rouse_wait;
+          my ($rpid, $rstatus) = rouse_wait;
           $rstatus
        }
     In the case where "rouse_cb" and "rouse_wait" are not flexible enough,
-    you can roll your own, using "schedule":
+    you can roll your own, using "schedule" and "ready":
        sub wait_for_child($) {
           my ($pid) = @_;
           # store the current coro in $current,
           my ($done, $rstatus);
           # pass a closure to ->child
           my $watcher = AnyEvent->child (pid => $pid, cb => sub {
              $rstatus = $_[1]; # remember rstatus
-             $done = 1; # mark $rstatus as valud
+             $done = 1;        # mark $rstatus as valid
+             $current->ready;  # wake up the waiting thread
           });
           # wait until the closure has been called
           schedule while !$done;
         this module from the first thread (this requirement might be removed
         in the future to allow per-thread schedulers, but Coro::State does
         not yet allow this). I recommend disabling thread support and using
         processes, as having the windows process emulation enabled under
         unix roughly halves perl performance, even when not used.
+        Attempts to use threads created in another emulated process will
+        crash ("cleanly", with a null pointer exception).
     coro switching is not signal safe
         You must not switch to another coro from within a signal handler
         (only relevant with %SIG - most event libraries provide safe
         signals), *unless* you are sure you are not interrupting a Coro

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Comparing Coro/README (file contents): Revision 1.29 by root, Sat Feb 19 06:51:22 2011 UTC vs. Revision 1.33 by root, Tue Mar 4 06:13:24 2014 UTC

Diff Legend

Comparing Coro/README (file contents):
Revision 1.29 by root, Sat Feb 19 06:51:22 2011 UTC vs.
Revision 1.33 by root, Tue Mar 4 06:13:24 2014 UTC