1 files changed, 91 insertions, 0 deletions
diff --git a/lib/irb/pager.rb b/lib/irb/pager.rb
new file mode 100644
index 0000000000..3391b32c66
--- /dev/null
+++ b/lib/irb/pager.rb
@@ -0,0 +1,91 @@
+# 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_content(content, **options)
+        if content_exceeds_screen_height?(content)
+          page(**options) do |io|
+            io.puts content
+          end
+        else
+          $stdout.puts content
+        end
+      end
+
+      def page(retain_content: false)
+        if should_page? && pager = setup_pager(retain_content: retain_content)
+          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 should_page?
+        IRB.conf[:USE_PAGER] && STDIN.tty? && (ENV.key?("TERM") && ENV["TERM"] != "dumb")
+      end
+
+      def content_exceeds_screen_height?(content)
+        screen_height, screen_width = begin
+          Reline.get_screen_size
+        rescue Errno::EINVAL
+          [24, 80]
+        end
+
+        pageable_height = screen_height - 3 # leave some space for previous and the current prompt
+
+        # If the content has more lines than the pageable height
+        content.lines.count > pageable_height ||
+          # Or if the content is a few long lines
+          pageable_height * screen_width < Reline::Unicode.calculate_width(content, true)
+      end
+
+      def setup_pager(retain_content:)
+        require 'shellwords'
+
+        PAGE_COMMANDS.each do |pager_cmd|
+          cmd = Shellwords.split(pager_cmd)
+          next if cmd.empty?
+
+          if cmd.first == 'less'
+            cmd << '-R' unless cmd.include?('-R')
+            cmd << '-X' if retain_content && !cmd.include?('-X')
+          end
+
+          begin
+            io = IO.popen(cmd, 'w')
+          rescue
+            next
+          end
+
+          if $? && $?.pid == io.pid && $?.exited? # pager didn't work
+            next
+          end
+
+          return io
+        end
+
+        nil
+      end
+    end
+  end
+end