How to debug a unit test with debug gem

Way 1

Build the Ruby.

$ cd ~/git/ruby/ruby

$ ./configure ...

$ make -j$(nproc)

Run a unit test with RUN_OPTS=-rdebug/start.

$ make test-all V=1 TESTS="-v test/mkmf/test_flags.rb -n TestMkmfFlags#test_valid_warnflags" RUN_OPTS=-rdebug/start
...
(rdbg) b TestMkmfFlags#test_valid_warnflags    # break command
Unknown constant name: "TestMkmfFlags"
#0  BP - Method (pending)  TestMkmfFlags#test_valid_warnflags
(rdbg) c    # continue command
...

Way 2

Prepare Ruby and the debug gem.

$ mkdir ~/git/ruby

$ git clone https://github.com/ruby/ruby.git

$ git clone https://github.com/ruby/debug.git

Then compile the debug gem. The debug.so is built.

$ cd ~/git/ruby/debug

$ bundle install

$ bundle exec rake compile

$ ls -l lib/debug/debug.so 
-rwxr-xr-x. 1 jaruga jaruga 114768 May 25 15:26 lib/debug/debug.so*

Build the Ruby.

$ cd ~/git/ruby/ruby

$ ./configure ...

$ make -j$(nproc)

Here is an example of running a unit test.

$ make test-all V=1 TESTS="-v test/mkmf/test_flags.rb -n TestMkmfFlags#test_valid_warnflags"

Modify the unit test file to debug with "debug" gem like this.

$ git diff
diff --git a/test/mkmf/test_flags.rb b/test/mkmf/test_flags.rb
index aedf06b610..6da6e3bfb1 100644
--- a/test/mkmf/test_flags.rb
+++ b/test/mkmf/test_flags.rb
@@ -3,6 +3,9 @@
 
 class TestMkmfFlags < TestMkmf
   def test_valid_warnflags
+    require_relative '/home/jaruga/git/ruby/debug/lib/debug'
+    binding.break
+
     val = $extmk
     warnflags = $warnflags
     makefile = mkmf do

Then you can load the debug gem in the test case.

$ make test-all V=1 TESTS="-v test/mkmf/test_flags.rb -n TestMkmfFlags#test_valid_warnflags"
exec ./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems "./test/runner.rb" --ruby="./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems" --excludes-dir=./test/excludes --name=!/memory_leak/  -v test/mkmf/test_flags.rb -n TestMkmfFlags#test_valid_warnflags
Run options: 
  --seed=34032
  "--ruby=./miniruby -I./lib -I. -I.ext/common  ./tool/runruby.rb --extout=.ext  -- --disable-gems"
  --excludes-dir=./test/excludes
  -v
  -n
  TestMkmfFlags#test_valid_warnflags

# Running tests:

[1/0] TestMkmfFlags#test_valid_warnflags[2, 11] in ~/var/git/ruby/ruby/test/mkmf/test_flags.rb
     2| require_relative 'base'
     4| class TestMkmfFlags < TestMkmf
     5|   def test_valid_warnflags
     6|     require_relative '/home/jaruga/git/ruby/debug/lib/debug'
=>   7|     binding.break
     8| 
     9|     val = $extmk
    11|     makefile = mkmf do
=>#0	TestMkmfFlags#test_valid_warnflags at ~/var/git/ruby/ruby/test/mkmf/test_flags.rb:7
  #1	Test::Unit::TestCase#run_test(name=:test_valid_warnflags) at ~/var/git/ruby/ruby/tool/lib/test/unit/testcase.rb:200
  # and 24 frames (use `bt' command for all frames)