Add script to fix shebangs when using --enable-load-relative

* See ruby/setup-ruby#98 (comment)

Author: eregon

Diff for: .github/workflows/build.yml

@@ -44,6 +44,9 @@ jobs:
       run: ruby-install --prefix ~/build_prefix/${{ matrix.ruby }} --no-install-deps -j4 ${{ matrix.ruby }} -- --enable-shared --enable-rpath --enable-load-relative --disable-install-doc
       env:
         CPPFLAGS: "-DENABLE_PATH_CHECK=0" # https://github.com/actions/virtual-environments/issues/267
+    - name: Fix the RubyGems line when using load-relative
+      run: ~/build_prefix/${{ matrix.ruby }}/bin/ruby --disable-gems fix-rubygems-line.rb
+      if: startsWith(matrix.ruby, 'ruby-')
     - name: Create archive
       run: tar czf ${{ matrix.ruby }}-${{ matrix.os }}.tar.gz -C ~/build_prefix ${{ matrix.ruby }}
     - name: Install Bundler if needed

Diff for: fix-rubygems-line.rb

@@ -0,0 +1,39 @@
+require 'rbconfig'
+
+bindir = RbConfig::CONFIG["bindir"]
+
+FIRST_LINE = "#!/bin/sh\n"
+RUBY_SHEBANG = %r{^#!/usr/bin/env ruby$}
+RUBYGEMS_LINE = /This file was generated by RubyGems/
+
+Dir.glob("#{bindir}/*") do |file|
+  exe = "bin/#{File.basename(file)}"
+
+  if File.binread(file, FIRST_LINE.bytesize) == FIRST_LINE
+    puts "\nFound load-relative prolog in #{exe}"
+    contents = File.binread(file)
+    rubygems_line = contents.lines.index { |line| RUBYGEMS_LINE =~ line }
+
+    if !rubygems_line
+      puts "No RubyGems line in #{exe}, skipping it"
+    elsif rubygems_line == 2
+      # RubyGems expects RUBYGEMS_LINE to match the 3rd line
+      # https://github.com/rubygems/rubygems/blob/6d7fe84753/lib/rubygems/installer.rb#L220
+      # Otherwise, it will consider the executable to be conflicting and ask whether to override,
+      # and that results in an error when STDIN is not interactive
+    else
+      puts "The RubyGems line in #{exe} is not the 3rd line (but line #{rubygems_line+1}), fixing it"
+
+      index = contents =~ RUBY_SHEBANG
+      raise "Could not find ruby shebang in:\n#{contents}" unless index
+      contents = contents[index..-1]
+
+      rubygems_line = contents.lines.index { |line| RUBYGEMS_LINE =~ line }
+      unless rubygems_line == 2
+        raise "The RubyGems line is still not 3rd in #{exe}:\n#{contents}"
+      end
+
+      File.binwrite(file, contents)
+    end
+  end
+end