Review Request 40559: Added a wait() function to Subprocess.

[Jie Yu]

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https://reviews.apache.org/r/40559/
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Review request for mesos, Ben Mahler and Vinod Kone.


Bugs: MESOS-3035
    https://issues.apache.org/jira/browse/MESOS-3035


Repository: mesos


Description
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Added a wait() function to Subprocess. See the example code below:

```
Try<Subprocess> s = subprocess(...);
if (s.isError()) {
  ...
}

return s.get().wait()
  .then([](const Subprocess::Result& result) {
    if (result.status.isNone()) { ... }
    if (result.status.get() != 0) { ... }
    
    handle(result.out.get());
    handle(result.err.get());
  });
```

Relevant review: https://reviews.apache.org/r/37336/


Diffs
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  3rdparty/libprocess/include/process/subprocess.hpp 
f17816e813d5efce1d3bb1ff1e1111850eeda3ba 
  3rdparty/libprocess/src/subprocess.cpp 
e51f024e89594d84f1dfb7ee6f2e1d8fb33b4a08 
  3rdparty/libprocess/src/tests/subprocess_tests.cpp 
ac600a551fb1a7782ff33cce204b7819497ef54a 

Diff: https://reviews.apache.org/r/40559/diff/


Testing
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make check


Thanks,

Jie Yu