[ruby-core:54618] [ruby-trunk - Feature #7688] Error hiding with rb_rescue() on Comparable#==, #coerce and others

Issue #7688 has been updated by Eregon (Benoit Daloze).


Hello,

I think this is really a bug: error hiding *is* harmful.
Anyway, is it OK to commit this to trunk now that 2.0 is released and in a separate branch?
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Feature #7688: Error hiding with rb_rescue() on Comparable#==, #coerce and others
https://bugs.ruby-lang.org/issues/7688#change-38952

Author: Eregon (Benoit Daloze)
Status: Open
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: next minor


Hello,

I believe error hiding is harmful because very dangerous
(it forgets errors which is likely unexpected) and hard to debug.

But I guess the compatibility is the main reason to keep these cases.

In the cases of Comparable#== and #coerce, I believe it is not worth to be compatible with this dangerous behavior
as it will at worse help to discover bugs in #<=> and #coerce methods which would raise an exception.

I doubt anyone rely on this and the #coerce spec (see #7645) itself makes me think this is unexpected behavior.
It would also simplify the spec, and no specific #coerce behavior would be needed to be defined as it would behave as a simple method call without edge cases.

So I think rb_rescue() or rb_rescue2(..., GenericErrorClass, ...) should be avoided if possible.
I analyzed these in the code base and it is used in a couple places:
* compar.c in cmp_equal(): this is the main offender in this regard with #coerce
* numeric.c in rb_num_coerce_{cmp,relop}() which call do_coerce(,,FALSE): This is the current subject of #7645.

* io.c in io_close(): to avoid raising if #close fails, which is likely less problematic,
  although it would be nicer to rescue only IO-related errors and warn when an exception is raised.
* range.c in range_init(): this is to provide a clearer error. I think it would be nice to show the original error as well.

Removing the general rescue in cmp_equal() revealed a couple bugs in RDoc due to this problem. I guess there are many others in the wild.

Can we please remove this anti-pattern?
I believe impact is only positive and that it should be done as soon as possible.

What do you think?


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