Bug #17181: Hash with default block breaks after transforming values

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]
• Backport: 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONE

[crashtech (Carlos Silva)]

I found a problem with Hashes that use a default proc.

x = Hash.new { |h, k| h[k] = [] }
x[:a] << 1
x[:b] << 2

I expected that transforming the values as follows:

y = x.transform_values { |arr| arr.map(&:next) }

should create a new array, and I should still be able to push a new element into it as follows:

y[:c] << 3
y # => {:a=>[2], :b=>[3], :c=>[3]}

But the result is weird.

y[:c] << 3 # >> TypeError (callable object is expected)
y[:c] # => the default_proc Proc object
Y[:c].call # 'h' is nil, meaning that it doesn't have the reference to the hash anymore

[jeremyevans0 (Jeremy Evans)]

Thanks for the report. I can confirm the issue. It looks like rb_hash_transform_values is missing COPY_DEFAULT(result, hash);. I'll update the tests and submit a PR to fix this tonight.

[jeremyevans0 (Jeremy Evans)]

jeremyevans0 (Jeremy Evans) wrote in #note-1:

Thanks for the report. I can confirm the issue. It looks like rb_hash_transform_values is missing COPY_DEFAULT(result, hash);. I'll update the tests and submit a PR to fix this tonight.

Actually, it looks like the tests specify there shouldn't be a default in the returned object, they just only test for the default proc and not for the default itself. Still, the fix should be simple and I'll try to submit a PR for it tonight.

[jeremyevans0 (Jeremy Evans)]

PR: https://github.com/ruby/ruby/pull/3563

[sawa (Tsuyoshi Sawada)]

• Subject changed from Hash with default block breaks after transform values to Hash with default block breaks after transforming values
• Description updated (diff)

• Status changed from Open to Closed

Applied in changeset git|df14c758fc705c49c2aaf4c9276a8f7229438fbf.

---

Make hash returned by Hash#transform_values not have a default

This sets an explicit default of nil. There is probably a better
approach of removing the default.

Fixes [Bug #17181]

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN to 2.5: DONTNEED, 2.6: DONTNEED, 2.7: REQUIRED

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.5: DONTNEED, 2.6: DONTNEED, 2.7: REQUIRED to 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONE

ruby_2_7 665589cbdf7bf652067113dd1c0bc49012b990e0 merged revision(s) df14c758fc705c49c2aaf4c9276a8f7229438fbf.