[ruby-core:107188] [Ruby master Feature#16663] Add block or filtered forms of Kernel#caller to allow early bail-out

["jeremyevans0 (Jeremy Evans)" <noreply@ruby-lang.org>]

Issue #16663 has been updated by jeremyevans0 (Jeremy Evans).


headius (Charles Nutter) wrote in #note-23:
> I did a quick implementation for JRuby in https://github.com/jruby/jruby/pull/7014. It passes all tests except the `to_enum` form, which might be tricky to support. When the Enumerator is actually run, we are at least two frames deeper (`to_a` and `each`) which throws off the "top" frame in the resulting iteration.

I added the `to_enum` tests to make sure that using it doesn't crash the interpreter, not because it should ever be used.  So I don't think it's an issue.  I'm fine modifying the tests so they pass on both CRuby's and JRuby's implementation.

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Feature #16663: Add block or filtered forms of Kernel#caller to allow early bail-out
https://bugs.ruby-lang.org/issues/16663#change-96048

* 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.



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