[ruby-core:114596] [Ruby master Bug#19857] Eval coverage is reset after each `eval`.

Issue #19857 has been updated by ioquatix (Samuel Williams).

Backport changed from 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN to 3.0: DONTNEED, 3.1: DONTNEED, 3.2: REQUIRED

> Currently only the coverage of the last call to Kernel#eval is kept.

This is not true. Coverage is stored in a global map of path -> coverage. Every attempt is made to use the same coverage for a given path. If you load or eval the same path multiple times, the same coverage will be reused. The bug is because `iseq_set_sequence` sets the coverage to zero for executable lines, for the lines it covers. So, it will:

1. Load the current shared coverage for a given path.
2. Based on the eval line number, update that coverage.
3. For each executable instruction, assign to the `coverage[instruction_line + eval_line_number]`.

So, it won't keep the last result, it just resets some or all of the coverage, depending on how the `eval` call is invoked. In theory, the same problem can apply to `require` and `load`, but would be, I expect, a little more involved to reproduce.

----------------------------------------
Bug #19857: Eval coverage is reset after each `eval`.
https://bugs.ruby-lang.org/issues/19857#change-104411

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: DONTNEED, 3.1: DONTNEED, 3.2: REQUIRED
----------------------------------------
It seems like `eval` based coverage is reset every time eval is invoked.

```ruby
#!/usr/bin/env ruby

require 'coverage'

def measure(flag)
  c = Class.new
  c.class_eval(<<~RUBY, "foo.rb", 1)
    def foo(flag)
      if flag
        puts "foo"
      else
        puts "bar"
      end
    end
  RUBY
  
  return c.new.foo(flag)
end

Coverage.start(lines: true, eval: true)

# Depending on the order of these two operations, different computation is calculated, because the evaluation of the code is considered different, even if the content/path is the same.
measure(false)
measure(true)

p Coverage.result
```

Further investigation is required.



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