Curl::Easy#perform crashes on Ruby 2.6

[jbockler]

Since curb 1.3.0 every call to Curl::Easy#perform raises on Ruby 2.6. The gemspec advertises required_ruby_version >= 2.2, but the bug is invisible to curb's own CI because the test matrix starts at Ruby 2.7.

Discovered this while submitting a PR to webmock bumping the confirmed-curb-version range to 1.3.5. webmock's CI still includes Ruby 2.6 and the curb-adapter test suite calls easy.perform.

Reproduction

require 'curb'
Curl::Easy.new("file:///etc/hostname").perform

Result on Ruby 2.6

ArgumentError: cannot define finalizer for TrueClass
  lib/curl/multi.rb:278:in `[]='
  lib/curl/multi.rb:278:in `__register_idle_easy_reference'
  lib/curl/easy.rb:100:in `multi='
  lib/curl/easy.rb:177:in `perform'

Result on Ruby 2.7+

Works as expected.

Root cause

Curl::Easy#perform creates an implicit Curl::Multi, then assigns it via the Ruby setter.
Curl::Easy#multi= (lib/curl/easy.rb:94–102) calls
multi.__send__(:__register_idle_easy_reference, self), which does:

def __register_idle_easy_reference(easy)
  __idle_easy_references[easy] = true   # <-  raises on Ruby 2.6
  self
end

__idle_easy_references is an ObjectSpace::WeakMap. In Ruby ≤ 2.6, WeakMap calls ObjectSpace.define_finalizer on the key and the value. define_finalizer rejects:

1. Immediates (true, false, nil, Symbol, Integer)
2. Frozen objects

Ruby 2.7 reworked WeakMap to no longer require define_finalizer for either side (Bug #16035), which is why the issue is invisible on every tested CI version.

I tried fixing this with

__idle_easy_references[easy] = easy # instead of "true"

but that doesn't work for frozen Easy objects.

[taf2]

Thanks I have too decide if we keep supporting it but maybe can get a fix soon. 2.7 is maybe a good upgrade with small or few compatibility issues for 2.6…

[taf2]

@jbockler spent some time on this and was able to get a fix in master - however there are things in the ruby 2.6 environment that will be problematic going to see if we can get them resolved next... challenge was getting 2.6 to compile for me locally but now that that is solved the C fixes shouldn't be too long

[taf2]

ruby 2.6 should be good now as of 1.3.6

[jbockler]

Thanks a lot for looking into this!