[ruby-core:53339] [CommonRuby - Feature #7895] Exception#backtrace_locations to go with Thread#backtrace_locations and Kernel#caller_locations

Issue #7895 has been updated by kallistec (Daniel DeLeo).


Hi, I work on Opscode's Chef, a server configuration framework. I have an interest in both parts of this topic. In Chef, we use set_backtrace but we also parse backtrace lines to find relevant code in our error messages. 

When we use set_backtrace, it is because we are catching an exception and then re-raising as either the same exception class or as a different exception class. Our goal here is to add context to exceptions that would otherwise be very difficult for the end user to understand.

We somewhat recently rewrote our error messaging to be more user friendly. To do this, we need to process backtraces and filter out framework code (leaving only user code) and then use regular expressions to find the relevant line numbers. Having structured backtrace data would be much better than what we're doing now.

For our use case, we need a way to set backtrace_locations from one exception to another. We currently only use set_backtrace immediately after creating an exception, so it would be fine if we could pass the needed data into the constructor, as long as there is a reasonable way to make the method signatures in current ruby compatible via monkey patching or #respond_to?
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Feature #7895: Exception#backtrace_locations to go with Thread#backtrace_locations and Kernel#caller_locations
https://bugs.ruby-lang.org/issues/7895#change-37536

Author: headius (Charles Nutter)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


Thread#backtrace_locations and Kernel#caller_locations were added in Ruby 2.0.0, but no equivalent method was added to Exception to get backtrace locations. The String format of Exception#backtrace elements makes it difficult to do any inspection of the actual files and lines it reports, so having backtrace_locations would be very useful.

In #7435 ko1 pointed out that Exception defines set_backtrace, which takes an array (presumably of Strings in the same format as Exception#backtrace) and that this would need to be addressed in order to support backtrace_locations. I propose that if you set_backtrace, you are already breaking the ability to get structured Location elements, so set_backtrace should cause backtrace_locations to return nil or an empty array (probably an empty array, so it can always be expected to be non-nil).

We could consider also adding set_backtrace_locations as a way to set Location elements into the exception.

An example script where backtrace_locations would be very useful is here (avoiding the need to regexp match to get file and line number): https://gist.github.com/headius/4999244


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