Bug #22269: `Coverage.line_stub` clobbers already-collected coverage data

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 4.1.0dev (2026-08-27T01:47:31Z master 5ee95de080) +PRISM [arm64-darwin25]
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[mugikurash (Shuta Mugikura)]

Coverage.line_stub compiles the file with compile_file only to read the ISeq's trace_points. Since https://bugs.ruby-lang.org/issues/22018 compile_file registers the file into the coverage table, so calling line_stub while Coverage is running resets the file's already-collected counters.

This also reproduces on 3.4 and 4.0, so I would like it backported.

Reproduction:

require "coverage"

f = File.expand_path("m.rb")
File.write(f, "def m\n  :ok\nend\n")

Coverage.start
require f
m
p Coverage.peek_result[f]

Coverage.line_stub(f)
p Coverage.peek_result[f]

m
p Coverage.result[f]

Expected:

[1, 1, nil]
[1, 1, nil]
[1, 2, nil]

Actual:

[1, 1, nil]
[0, 0, nil]
[0, 0, nil]

[mugikurash (Shuta Mugikura)]

Patch: https://github.com/ruby/ruby/pull/18533