Display show_cmds's output in a pager when in TTY environment

This can:

- Make it easier to scroll up and down the commands list
- Avoid pushing up users' previous output
- Allow users to do basic search with `/<word>`

lib/irb/cmd/show_cmds.rb

@@ -2,6 +2,7 @@
 
 require "stringio"
 require_relative "nop"
+require_relative "../pager"
 
 module IRB
   # :stopdoc:
@@ -28,9 +29,9 @@ def execute(*args)
           output.puts
         end
 
-        puts output.string
-
-        nil
+        Pager.page do |io|
+          io.puts output.string
+        end
       end
     end
   end

lib/irb/pager.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module IRB
+  # The implementation of this class is borrowed from RDoc's lib/rdoc/ri/driver.rb.
+  # Please do NOT use this class directly outside of IRB.
+  class Pager
+    PAGE_COMMANDS = [ENV['RI_PAGER'], ENV['PAGER'], 'pager', 'less -r', 'more -r'].compact.uniq
+
+    class << self
+      def page
+        if STDIN.tty? && pager = setup_pager
+          begin
+            pid = pager.pid
+            yield pager
+          ensure
+            pager.close
+          end
+        else
+          yield $stdout
+        end
+      # When user presses Ctrl-C, IRB would raise `IRB::Abort`
+      # But since Pager is implemented by running paging commands like `less` in another process with `IO.popen`,
+      # the `IRB::Abort` exception only interrupts IRB's execution but doesn't affect the pager
+      # So to properly terminate the pager with Ctrl-C, we need to catch `IRB::Abort` and kill the pager process
+      rescue IRB::Abort
+        Process.kill("TERM", pid) if pid
+        nil
+      rescue Errno::EPIPE
+      end
+
+      private
+
+      def setup_pager
+        require 'shellwords'
+
+        PAGE_COMMANDS.each do |pager|
+          pager = Shellwords.split(pager)
+          next if pager.empty?
+
+          begin
+            io = IO.popen(pager, 'w')
+          rescue
+            next
+          end
+
+          if $? && $?.pid == io.pid && $?.exited? # pager didn't work
+            next
+          end
+
+          return io
+        end
+
+        nil
+      end
+    end
+  end
+end

test/irb/test_cmd.rb

@@ -688,6 +688,16 @@ def test_whereami_alias
 
 
   class ShowCmdsTest < CommandTestCase
+    def setup
+      STDIN.singleton_class.define_method :tty? do
+        false
+      end
+    end
+
+    def teardown
+      STDIN.singleton_class.remove_method :tty?
+    end
+
     def test_show_cmds
       out, err = execute_lines(
         "show_cmds\n"

test/irb/yamatanooroti/test_rendering.rb

@@ -252,6 +252,24 @@ def test_assignment_expression_truncate
     EOC
   end
 
+  def test_show_cmds_with_pager_can_quit_with_ctrl_c
+    write_irbrc <<~'LINES'
+      puts 'start IRB'
+    LINES
+    start_terminal(40, 80, %W{ruby -I#{@pwd}/lib #{@pwd}/exe/irb}, startup_message: 'start IRB')
+    write("show_cmds\n")
+    write("G") # move to the end of the screen
+    write("\C-c") # quit pager
+    write("'foo' + 'bar'\n") # eval something to make sure IRB resumes
+    close
+
+    screen = result.join("\n").sub(/\n*\z/, "\n")
+    # IRB::Abort should be rescued
+    assert_not_match(/IRB::Abort/, screen)
+    # IRB should resume
+    assert_match(/foobar/, screen)
+  end
+
   private
 
   def write_irbrc(content)