Lambdas: caveats, do's and don'ts

[Alex Rukletsov <al...@mesosphere.com>]

Folks,

We've recently seen several bugs and potential issues related to lambda
captures in our codebase. To help us avoid these problems in the future, we
would like to touch on few general guidelines now. After 1.0 release, we
will take some time and revisit our lambda guidelines and think about what
primitives we can add in order to enforce the rules.

1) Use `defer` to execute on the same context.
If your lambda captures `this` to capture the actor context, most probably
you want it to be executed in the actor’s context. you should use `defer()`
to achieve this. For an example, see [1].

2) `this` should typically point to the actor instance.
When you capture `this`, make sure it represents the actor instance onto
which you `defer()` or `dispatch()`. If you are doing otherwise, make sure
you capture the reason in a comment. For an example, see [2], the second
and the third hunks.

3) Avoid capturing by reference.
While capturing by reference is allowed, captured objects must not be
destructed before they are used in the lambda. For an example, see [3].
Except common cases, like using a library that always executes lambdas
immediately [4], you should always write a comment explaining that capture
by reference is done on purpose, e.g. [5]. IMPORTANT: capturing `this` by
value and using a member variable is effectively capture by reference!

AlexR & MPark.

[1]
https://github.com/apache/mesos/commit/97124ac72bbc42f809b800fe2da383dc43b1b34e
[2]
https://github.com/apache/mesos/commit/89d072d8567ccdcf11d1b1093350f393885fc593
[3]
https://github.com/apache/mesos/commit/54b82e5713243f44e7e2a617f53bce872381b36a
[4]
https://github.com/apache/mesos/blob/149f4c55b951e5eaea1cb5faf400089dd36c355a/src/master/http.cpp#L1251
[5]
https://github.com/apache/mesos/blob/149f4c55b951e5eaea1cb5faf400089dd36c355a/src/master/http.cpp#L1239-L1241

Re: Lambdas: caveats, do's and don'ts

[Alexander Rojas <al...@mesosphere.io>]

Now that you touch the topic, I think that we need to expand our style guide 
concerning lambdas. The rules do not cover all the cases, e.g.

```
auto lambda = [a, long, number, of, captures](a, long, number, of, parameters) -> ExplicitReturnType {
}
```

so far, the formatting has been done based on what the shepherd thinks the
formatting should be, but is not always the same which causes sometimes
calling out other just to ask what the right formatting in a given context and
with a particular shepherd should be.


> On 06 Jul 2016, at 15:35, Alex Rukletsov <al...@mesosphere.com> wrote:
> 
> Folks,
> 
> We've recently seen several bugs and potential issues related to lambda
> captures in our codebase. To help us avoid these problems in the future, we
> would like to touch on few general guidelines now. After 1.0 release, we
> will take some time and revisit our lambda guidelines and think about what
> primitives we can add in order to enforce the rules.
> 
> 1) Use `defer` to execute on the same context.
> If your lambda captures `this` to capture the actor context, most probably
> you want it to be executed in the actor’s context. you should use `defer()`
> to achieve this. For an example, see [1].
> 
> 2) `this` should typically point to the actor instance.
> When you capture `this`, make sure it represents the actor instance onto
> which you `defer()` or `dispatch()`. If you are doing otherwise, make sure
> you capture the reason in a comment. For an example, see [2], the second
> and the third hunks.
> 
> 3) Avoid capturing by reference.
> While capturing by reference is allowed, captured objects must not be
> destructed before they are used in the lambda. For an example, see [3].
> Except common cases, like using a library that always executes lambdas
> immediately [4], you should always write a comment explaining that capture
> by reference is done on purpose, e.g. [5]. IMPORTANT: capturing `this` by
> value and using a member variable is effectively capture by reference!
> 
> AlexR & MPark.
> 
> [1]
> https://github.com/apache/mesos/commit/97124ac72bbc42f809b800fe2da383dc43b1b34e
> [2]
> https://github.com/apache/mesos/commit/89d072d8567ccdcf11d1b1093350f393885fc593
> [3]
> https://github.com/apache/mesos/commit/54b82e5713243f44e7e2a617f53bce872381b36a
> [4]
> https://github.com/apache/mesos/blob/149f4c55b951e5eaea1cb5faf400089dd36c355a/src/master/http.cpp#L1251
> [5]
> https://github.com/apache/mesos/blob/149f4c55b951e5eaea1cb5faf400089dd36c355a/src/master/http.cpp#L1239-L1241