RFR 8220684 : Process.waitFor(long, TimeUnit) can return false for a process that exited within the timeout

[Ivan Gerasimov]

Hi David!


On 3/14/19 5:28 PM, David Holmes wrote:
> Hi Ivan,
>
> On 15/03/2019 5:49 am, Ivan Gerasimov wrote:
>> Hello!
>>
>> The default implementation of Process.waitFor(long, TimeUnit) does 
>> not check if the process has exited after the last portion of the 
>> timeout has expired.
>
> Please clarify. There is always a race between detecting a timeout and 
> the process actually terminating. If the process actually terminates 
> while you're deciding to report a timeout that seems just  an 
> acceptable possibility. No matter what you do the process could 
> terminate just after you decided to report the timeout.
>
>
Current code for waitFor(...) is

  212         do {
  213             try {
  214                 exitValue();
  215                 return true;
  216             } catch(IllegalThreadStateException ex) {
  217                 if (rem > 0)
  218                     Thread.sleep(
  219 Math.min(TimeUnit.NANOSECONDS.toMillis(rem) + 1, 100));
  220             }
  221             rem = unit.toNanos(timeout) - (System.nanoTime() - 
startTime);
  222         } while (rem > 0);
  223         return false;

So, if the loop has just processed the last 100 ms portion of the 
timeout, it ends right away, without checking if the process has exited 
during this period.

Not a big deal of course, but it is more accurate to check if the 
process has exited at the *end* of the specified timeout, and not 100 
milliseconds before the end.

With kind regards,
Ivan

>
> David
> -----
>
>> JDK has two implementations of Process (for Unix and Windows) and 
>> they both override waitFor(), so it's not an issue for them.
>>
>> Still, it is better to provide a more accurate default implementation.
>>
>> I'm not quite certain the regression test needs to be included in the 
>> fix.  The test does demonstrate the issue with the unfixed JDK and 
>> passed Okay on all tested platforms in Mach5. Yet, I suspect the test 
>> can still show false negative results, as there are no guaranties 
>> that even such simple application as `true` will finish in 100 ms.
>> I can tag the test as @ignored with a comment, or simply remove it 
>> from the fix.
>>
>> BUGURL: https://bugs.openjdk.java.net/browse/JDK-8220684
>> WEBREV: http://cr.openjdk.java.net/~igerasim/8220684/00/webrev/
>>
>> Thanks in advance for reviewing!
>>
>

-- 
With kind regards,
Ivan Gerasimov