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From "Marco Massenzio" <ma...@mesosphere.io>
Subject Re: Review Request 36424: Created a command executor helper method.
Date Wed, 15 Jul 2015 19:09:42 GMT


> On July 14, 2015, 12:20 a.m., Paul Brett wrote:
> > 3rdparty/libprocess/include/process/subprocess.hpp, line 307
> > <https://reviews.apache.org/r/36424/diff/1/?file=1008891#file1008891line307>
> >
> >     How about returning a tuple/struct of stdout, stderr and return code and let
the caller decide what they want?
> 
> Marco Massenzio wrote:
>     sure, that would be a possibility too, but it seems to me that the "approved way"
in Mesos is to return a `Try` for when something *may* go wrong.
>     This is consistent across the entire code base.
> 
> Artem Harutyunyan wrote:
>     Maybe I am missing something, I did a grep for `Future<Try..` and could not find
any occurence of it in the code base. Perhaps the reason is that it's customary to use Future's
`Failure()` to indicate an error (as opposed to returning a `Try`). If anything `Result` would
probably be more appropriate here than Try, but I'd like to hear what a shepherd has to say.
>     
>     The function could just return `Future<std::string>` and you could use `Failure()`
to indicate the error. In that case you'll need to change the return type of `.then` lamda
to `Future<std::string>` and also to replace a `return Error(...` on line 346 with `return
Failure(...` (which you might want to do anyway for the sake of consistency).

> I'd like to hear what a shepherd has to say.

eh eh, no kidding - while writing this code, I swear my brain melted :)

The one thing to bear in mind (and that's probably the reason this is a bit of a 'only child')
is that the 'error mode' here is different than elsewhere: if the command 'fails' the request
to run a command actually 'succeeded' - if I try to execute: 'ls -la /tmp/foo' well, the command
executes successfully, it's just that `foo` ain't there.

So, the semantics of a "Future of a Try" is that, yep, your request succeeded and, yay!, your
command succeeded too *or* dang! that failed and here's the error message.

(side note: that's also the reason why I return a 200 OK from the `/execute` endpoint, even
if the command fails - your Request, nonetheless succeeded).

But I can be convinced both ways, and return instead a `Failure(stderr)`

However, I like the tuple idea better because (a) the exit code carries information that we'd
be losing and (b) sometimes, to understand what really went wrong, one needs both stdout **and**
stderr, so I'm considering returning a `(code, stdout, stderr)` tuple (yay! C++11 FTW)


- Marco


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This is an automatically generated e-mail. To reply, visit:
https://reviews.apache.org/r/36424/#review91566
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On July 14, 2015, 4:21 a.m., Marco Massenzio wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/36424/
> -----------------------------------------------------------
> 
> (Updated July 14, 2015, 4:21 a.m.)
> 
> 
> Review request for mesos, Benjamin Hindman and Cody Maloney.
> 
> 
> Bugs: MESOS-3035
>     https://issues.apache.org/jira/browse/MESOS-3035
> 
> 
> Repository: mesos
> 
> 
> Description
> -------
> 
> Jira: MESOS-2902
> 
> While researching how to execute an arbitrary script
> to detect the Master IP address, it emerged clearly that
> a helper method to execute an arbitrary command/script on
> a node and obtain either stdout or stderr would have been
> useful and avoided a lot of code repetition.
> 
> This could not be ultimately used for the purpose at hand,
> but I believe it to be useful enough (particularly, to avoid
> people doing "coding by copy&paste" and/or waste time
> researching the same functionality).
> 
> This would also be beneficial in MESOS-2830, factoring out the remote command execution
logic.
> 
> 
> Diffs
> -----
> 
>   3rdparty/libprocess/include/process/subprocess.hpp 310cb4f8e4e2faa5545dffd196d7490c868bc5d6

>   3rdparty/libprocess/src/tests/subprocess_tests.cpp f6acb204582a9e696c3b09d4e4c543bb052e97d4

> 
> Diff: https://reviews.apache.org/r/36424/diff/
> 
> 
> Testing
> -------
> 
> make check
> 
> 
> Thanks,
> 
> Marco Massenzio
> 
>


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