[ruby-core:107407] Re: [Ruby master Feature#16663] Add block or filtered forms of Kernel#caller to allow early bail-out

[Gregory Cohen <gregorycohen2@gmail.com>]

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On Mon, Jan 31, 2022 at 8:39 PM jeremyevans0 (Jeremy Evans) <
noreply@ruby-lang.org> wrote:

> Issue #16663 has been updated by jeremyevans0 (Jeremy Evans).
>
>
> headius (Charles Nutter) wrote in #note-25:
> > Is there a way to prohibit the `to_enum` form? I don't think it will be
> useful to support since it may vary greatly across implementations
> (depending on what the stack looks like and what gets included).
>
> I don't know of a way to do this.  I don't think the method being called
> can tell whether it is being run via an enumerator or not.  In CRuby, you
> could probably override `to_enum`, check if the method being called is a C
> method using the same C function, and fail in that case, but that's trivial
> to work around be rebinding the Kernel `to_enum` method.
>
> > It does raise a question for me, though... is your patch eagerly
> capturing the stack? I have been trying to figure out how the stack trace
> is identical in both the block form and the to_enum form and it seems like
> the trace would have to have been captured at the same level, rather than
> at the point the enum starts to run.
>
> It isn't identical.  From the tests:
>
> ```ruby
>     cllr = caller_locations(1, 2); ary =
> Thread.to_enum(:each_caller_location).to_a[2..3]
>     assert_equal(cllr.map(&:to_s), ary.map(&:to_s))
> ```
>
> The `[2..3]` shows there are extra entries in the `to_enum` case.
>
> ----------------------------------------
> Feature #16663: Add block or filtered forms of Kernel#caller to allow
> early bail-out
> https://bugs.ruby-lang.org/issues/16663#change-96305
>
> * Author: headius (Charles Nutter)
> * Status: Open
> * Priority: Normal
> ----------------------------------------
> There are many libraries that use `caller` or `caller_locations` to gather
> stack information for logging or instrumentation. These methods generate an
> array of informational stack frames based on the current call stack.
>
> Both methods accept parameters for `level` (skip some number of Ruby
> frames) and `length` (only return this many frames). However many use cases
> are unable to provide one or both of these.
>
> Instrumentation uses, for example, may need to skip an unknown number of
> frames at the top of the trace, such as to dig out of rspec plumbing or
> active_record internals and report the first line of user code. In such
> cases, the typical pattern is to simply request *all* frames and then
> filter out the one that is desired.
>
> This leads to a great deal of wasted work gathering those frames and
> constructing objects to carry them to the user. On optimizing runtimes like
> JRuby and TruffleRuby, it can have a tremendous impact on performance,
> since each frame has a much higher cost than on CRuby.
>
> I propose that we need a new form of `caller` that takes a block for
> processing each element.
>
> ```ruby
> def find_matching_frame(regex)
>   caller do |frame|
>     return frame if frame.file =~ regex
>   end
> end
> ```
>
> An alternative API would be to allow passing a query object as a keyword
> argument, avoiding the block dispatch by performing the match internally:
>
> ```ruby
> def find_matching_frame(regex)
>   caller(file: regex)
> end
> ```
>
> This API would provide a middle ground between explicitly specifying a
> maximum number of stack frames and asking for all frames. Most common,
> hot-path uses of `caller` could be replaced by these forms, reducing
> overhead on all Ruby implementations and drastically reducing it where
> stack traces are expensive.
>
>
>
> --
> https://bugs.ruby-lang.org/
>
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