RFR(s) #2: 6344935: (spec) clarify specifications for Object.wait overloads

[Martin Buchholz]

My attempt to write a good wait loop led me to want only to use a
nanosecond wait method.  And in fact we have existing method
Condition.awaitNanos for inspiration.  For a moment I considered adding
Object#waitNanos but then I came to my senses.  But we could add a static
method Objects#waitNanos and that could be the recommended method to call
inside all wait loops?  Maybe it's time to replace JVM_MonitorWait (that
takes millis) with a version that takes nanos?  Object.c has not been
touched since 2003!

On Sun, Aug 20, 2017 at 8:05 PM, Martin Buchholz <martinrb at google.com>
wrote:

>
>
> On Sat, Aug 19, 2017 at 1:53 PM, Martin Buchholz <martinrb at google.com>
> wrote:
>
>> Now I see that the code snippet in TimeUnit.timedWait is also in need of fixing. Hmmmm ....
>>
>>  public synchronized Object poll(long timeout, TimeUnit unit)
>>      throws InterruptedException {
>>    while (empty) {
>>      unit.timedWait(this, timeout);
>>      ...
>>    }
>>  }
>>
>> It's surprisingly hard to write a high quality wait loop, and we've been
> doing our users a disservice by punting on providing good models in the
> javadoc.  You probably want to work entirely in nanoseconds, to avoid
> rounding errors, and because you'll end up calling nanoTime anyways.  You
> don't want to call nanoTime unless you are sure to wait.  OTOH you want to
> throw NPE on null TimeUnit even if there's no need to wait.  You must
> beware of overflow when timeout is Long.MIN_VALUE.
>
> Below is my attempt to fix the sample loop in timedWait.  Calling
> Object.wait directly is obviously even harder.  How about just not
> providing a sample loop that calls Object.wait directly (effectively,
> gently deprecate timed Object wait in favor of the loop below)  (I'm also
> pretending that the platform has better than millisecond granularity).
>
>  public E poll(long timeout, TimeUnit unit)
>      throws InterruptedException {
>    long nanosTimeout = unit.toNanos(timeout);
>    synchronized (lock) {
>      for (long deadline = 0L; isEmpty(); ) {
>        long remaining = (deadline == 0L) ? nanosTimeout
>          : deadline - System.nanoTime();
>        if (remaining <= 0)
>          return null;
>        if (deadline == 0L) { // first wait
>          deadline = System.nanoTime() + nanosTimeout;
>          if (deadline == 0L) // unlikely corner case
>            deadline = 1L;
>        }
>        NANOSECONDS.timedWait(lock, remaining);
>      }
>      return dequeueElement();
>    }
>  }
>
>