POSIX_MQ#notify block execution on message received

This is implementation uses both a short-lived POSIX thread and
a pre-spawned Ruby Thread in a manner that works properly under
both Ruby 1.8 (green threads) and 1.9 (where Ruby Threads are
POSIX threads).

The short-lived POSIX thread will write a single "\0" byte to
a pipe the Ruby Thread waits on.  This operation is atomic
on all platforms.  Once the Ruby Thread is woken up from the
pipe, it will execute th block given to it.

This dual-thread implementation is inspired by the way glibc
implements mq_notify(3) + SIGEV_THREAD under Linux where the
kernel itself cannot directly spawn POSIX threads.

1 files changed, 79 insertions, 1 deletions

diff --git a/ext/posix_mq/posix_mq.c b/ext/posix_mq/posix_mq.c
index 883aa45..70d9125 100644
--- a/ext/posix_mq/posix_mq.c
+++ b/ext/posix_mq/posix_mq.c
@@ -5,6 +5,9 @@
 #ifdef HAVE_SIGNAL_H
 #  include <signal.h>
 #endif
+#ifdef HAVE_PTHREAD_H
+#  include <pthread.h>
+#endif
 #include <ruby.h>
 
 #include <time.h>
@@ -34,13 +37,14 @@ struct posix_mq {
         mqd_t des;
         long msgsize;
         VALUE name;
+        VALUE thread;
 #ifdef MQD_TO_FD
         VALUE io;
 #endif
 };
 
 static VALUE cPOSIX_MQ, cAttr;
-static ID id_new;
+static ID id_new, id_kill, id_fileno;
 static ID sym_r, sym_w, sym_rw;
 static const mqd_t MQD_INVALID = (mqd_t)-1;
 
@@ -198,6 +202,7 @@ static void mark(void *ptr)
         struct posix_mq *mq = ptr;
 
         rb_gc_mark(mq->name);
+        rb_gc_mark(mq->thread);
         MQ_IO_MARK(mq);
 }
 
@@ -225,6 +230,7 @@ static VALUE alloc(VALUE klass)
         mq->des = MQD_INVALID;
         mq->msgsize = -1;
         mq->name = Qnil;
+        mq->thread = Qnil;
         MQ_IO_SET(mq, Qnil);
 
         return rv;
@@ -683,6 +689,53 @@ static int lookup_sig(VALUE sig)
         return NUM2INT(sig);
 }
 
+/* we spawn a thread just to write ONE byte into an fd (usually a pipe) */
+static void thread_notify_fd(union sigval sv)
+{
+        int fd = sv.sival_int;
+
+        while ((write(fd, "", 1) < 0) && (errno == EINTR || errno == EAGAIN));
+}
+
+static void setup_notify_io(struct sigevent *not, VALUE io)
+{
+        VALUE fileno = rb_funcall(io, id_fileno, 0, 0);
+        int fd = NUM2INT(fileno);
+        int flags;
+        pthread_attr_t attr;
+        int e;
+
+        /*
+         * fd going to be written to inside a native thread,
+         * make it blocking for simplicity
+         */
+        flags = fcntl(fd, F_GETFL);
+        if (flags < 0) {
+                rb_sys_fail("fcntl F_GETFL");
+        } else if (flags & O_NONBLOCK) {
+                flags = fcntl(fd, F_SETFL, flags & ~O_NONBLOCK);
+                if (flags < 0)
+                        rb_sys_fail("fcntl F_SETFL");
+        }
+
+        if ((e = pthread_attr_init(&attr)))
+                goto err;
+        if ((e = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED)))
+                goto err;
+#ifdef PTHREAD_STACK_MIN
+        (void)pthread_attr_setstacksize(&attr, PTHREAD_STACK_MIN);
+#else
+#  warning PTHREAD_STACK_MIN not available,
+#endif
+        not->sigev_notify = SIGEV_THREAD;
+        not->sigev_notify_function = thread_notify_fd;
+        not->sigev_notify_attributes = &attr;
+        not->sigev_value.sival_int = fd;
+        return;
+err:
+        rb_raise(rb_eRuntimeError, "pthread failure: %s\n", strerror(e));
+}
+
 /*
  * call-seq:
  *        mq.notify = signal        => signal
@@ -693,10 +746,16 @@ static int lookup_sig(VALUE sig)
  * request to allow other processes to register a request.
  * If +signal+ is +false+, it will register a no-op notification request
  * which will prevent other processes from registering a notification.
+ * If +signal+ is an +IO+ object, it will spawn a thread upon the
+ * arrival of the next message and write one "\\0" byte to the file
+ * descriptor belonging to that IO object.
  * Only one process may have a notification request for a queue
  * at a time, Errno::EBUSY will be raised if there is already
  * a notification request registration for the queue.
  *
+ * Notifications are only fired once and processes must reregister
+ * for subsequent notifications.
+ *
  * For readers of the mq_notify(3) manpage, passing +false+
  * is equivalent to SIGEV_NONE, and passing +nil+ is equivalent
  * of passing a NULL notification pointer to mq_notify(3).
@@ -708,6 +767,10 @@ static VALUE setnotify(VALUE self, VALUE arg)
         struct sigevent * notification = &not;
         VALUE rv = arg;
 
+        if (!NIL_P(mq->thread)) {
+                rb_funcall(mq->thread, id_kill, 0, 0);
+                mq->thread = Qnil;
+        }
         not.sigev_notify = SIGEV_SIGNAL;
 
         switch (TYPE(arg)) {
@@ -725,6 +788,9 @@ static VALUE setnotify(VALUE self, VALUE arg)
                 not.sigev_signo = lookup_sig(arg);
                 rv = INT2NUM(not.sigev_signo);
                 break;
+        case T_FILE:
+                setup_notify_io(&not, arg);
+                break;
         default:
                 /* maybe support Proc+thread via sigev_notify_function.. */
                 rb_raise(rb_eArgError, "must be a signal or nil");
@@ -784,6 +850,15 @@ static VALUE setnonblock(VALUE self, VALUE nb)
         return nb;
 }
 
+/* :nodoc: */
+static VALUE setnotifythread(VALUE self, VALUE thread)
+{
+        struct posix_mq *mq = get(self, 1);
+
+        mq->thread = thread;
+        return thread;
+}
+
 void Init_posix_mq_ext(void)
 {
         cPOSIX_MQ = rb_define_class("POSIX_MQ", rb_cObject);
@@ -822,12 +897,15 @@ void Init_posix_mq_ext(void)
         rb_define_method(cPOSIX_MQ, "name", name, 0);
         rb_define_method(cPOSIX_MQ, "notify=", setnotify, 1);
         rb_define_method(cPOSIX_MQ, "nonblock=", setnonblock, 1);
+        rb_define_method(cPOSIX_MQ, "notify_thread=", setnotifythread, 1);
         rb_define_method(cPOSIX_MQ, "nonblock?", getnonblock, 0);
 #ifdef MQD_TO_FD
         rb_define_method(cPOSIX_MQ, "to_io", to_io, 0);
 #endif
 
         id_new = rb_intern("new");
+        id_kill = rb_intern("kill");
+        id_fileno = rb_intern("fileno");
         sym_r = ID2SYM(rb_intern("r"));
         sym_w = ID2SYM(rb_intern("w"));
         sym_rw = ID2SYM(rb_intern("rw"));