[ruby-core:75948] [Ruby trunk Bug#12480] Restarting Coverage does not capture additional coverage for already loaded files

Issue #12480 has been reported by Michael Grosser.

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Bug #12480: Restarting Coverage does not capture additional coverage for already loaded files
https://bugs.ruby-lang.org/issues/12480

* Author: Michael Grosser
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: 2.3.1
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
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I'm trying to combine coverage from before fork and after fork to make coverage reporting work in a forking test runner.
The problem I ran into is 2-fold:
 - A: when forking, previous coverage is lost
 - B: when restarting coverage, old files do not get added to

I could work around issue A by storing the old result and then merging it with the new result post fork, but issue B makes that impossible.

Please fix either A or B ... 

Reproduction steps for A:

~~~
# reproduce.rb
require 'coverage'

Coverage.start
require_relative 'test'
a

fork do
  b
  new = Coverage.result
  puts "NEW: #{new}"
end


# test.rb
def a
  1
end

def b
  1
end
~~~

NEW: {"/Users/mgrosser/Code/tools/forking_test_runner/test.rb"=>[0, 0, nil, nil, 0, 1, nil]}
-> missing coverage information for method `a`

Reproduction steps for B:
~~~
# reproduce.rb
require 'coverage'

Coverage.start
require_relative 'test'
a
old = Coverage.result
Coverage.start
b
new = Coverage.result
puts "OLD: #{old} -- NEW: #{new}"

# test.rb
def a
  1
end

def b
  1
end
~~~

OLD: {"test.rb"=>[1, 1, nil, nil, 1, 0, nil]} -- NEW: {"test.rb"=>[]}
-> missing coverage information for method `b`



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