[ruby/irb] Display `show_cmds`'s output in a pager when in TTY

environment
(https://github.com/ruby/irb/pull/647)

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>`

https://github.com/ruby/irb/commit/f94e8a66dd

2 files changed, 65 insertions, 3 deletions

diff --git a/lib/irb/cmd/show_cmds.rb b/lib/irb/cmd/show_cmds.rb
index 490561825e..1c9f1ea19a 100644
--- a/lib/irb/cmd/show_cmds.rb
+++ b/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 @@ module IRB
           output.puts
         end
 
-        puts output.string
-
-        nil
+        Pager.page do |io|
+          io.puts output.string
+        end
       end
     end
   end
diff --git a/lib/irb/pager.rb b/lib/irb/pager.rb
new file mode 100644
index 0000000000..16066cf273
--- /dev/null
+++ b/lib/irb/pager.rb
@@ -0,0 +1,61 @@
+# 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'], 'less', 'more'].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?
+
+          if pager.first == 'less' || pager.first == 'more'
+            pager << '-R' unless pager.include?('-R')
+          end
+
+          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