[ruby-core:98874] [Ruby master Bug#16967] Branch coverage duplicates branches inside ensure

Issue #16967 has been updated by jeremyevans0 (Jeremy Evans).


Here is a lightly tested workaround that merges the coverage:

```ruby
        Coverage.singleton_class.prepend(Module.new do
          def result
            res = super
            check_branch = true
            skip_2nd = lambda do |ary|
              ary = ary.dup
              ary.slice!(1)
              ary
            end
            res.values.each do |hash|
              if check_branch
                unless hash.is_a?(Hash) && hash[:branches]
                  return res
                end
                check_branch = false
              end
              unique_branches = {}
              branch_counters = {}
              new_branches = {}
              branches = hash[:branches]
              branches.each do |k, v|
                new_k = skip_2nd[k]
                if branch_values = unique_branches[new_k]
                  v.each do |k1, v1|
                    branch_counters[skip_2nd[k1]] += v1
                  end
                  branch_values.keys.each do |k1|
                    branch_values[k1] = branch_counters[skip_2nd[k1]]
                  end
                else
                  unique_branches[new_k] = new_branches[k] = v
                  v.each do |k1, v1|
                    branch_counters[skip_2nd[k1]] = v1
                  end
                end
              end
              hash[:branches] = new_branches
            end
            res
          end
        end)
      end
```

We'd probably want a more efficient version in core.  Also, I'm assuming the approach is sound, but I'm not sure about that.  It ignores the 2nd array element when merging, which I think is the unique id, relying solely on the line/column information to determine which branches are equivalent.

----------------------------------------
Bug #16967: Branch coverage duplicates branches inside ensure
https://bugs.ruby-lang.org/issues/16967#change-86246

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-06-05T21:26:28Z master ca15b7b8ee) [x86_64-openbsd6.7]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
The following file, which should have perfect coverage, is reported as only have 50% of the branches covered:

```ruby
def a
  yield
ensure
  p 1
  p 2 if $!
end
a{}
a{raise} rescue nil
```

The branches reported by branch coverage are:

```ruby
{[:if, 0, 5, 2, 5, 11]=>
  {[:then, 1, 5, 2, 5, 5]=>1, [:else, 2, 5, 2, 5, 11]=>0},
 [:if, 3, 5, 2, 5, 11]=>
  {[:then, 4, 5, 2, 5, 5]=>0, [:else, 5, 5, 2, 5, 11]=>1}}
```

Instead of reporting 2 branches for the `if`, 4 branches are reported:

1. exception raised, if condition true
2. exception raised, if condition false
3. exception not raised, if condition true
4. exception not raised, if condition false

In this example, it is impossible to cover branches 2 and 3, because the if condition is only true if an exception is raised.

Note that ensure blocks by themselves are not considered branches.  This code results in no branches reported by branch coverage:

```ruby
def a
  yield
ensure
  p $!
end
a{}
a{raise} rescue nil
```

Nested ensure usage duplicates all branches.  This code with 3 nested ensures generates 16 branches:

```ruby
def a
  yield
ensure
  begin
  ensure
    begin
    ensure
      p 1
      p 2 if $!
    end
  end
end
a{}
a{raise} rescue nil
```

I think this is a bug in the coverage library, and that it should not duplicate branches inside ensure.  This issue is not theoretical, it affects branch coverage testing in my libraries.



-- 
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