Auto merge of #1904 - rubygems:seg-binstubs-backwards-compat, r=indirect

[Installer] Generate backwards-compatible binstubs

# Description:

Since older versions of RubyGems didn't include `Gem.activate_bin_path`, installing with the latest version would then make using the binstubs with an older RG version re-installed impossible.

Re-introduce backwards compatibility in those binstubs.

# Tasks:

- [ ] Describe the problem / feature
- [ ] Write tests
- [ ] Write code to solve the problem
- [ ] Get code review from coworkers / friends

I will abide by the [code of conduct](https://github.com/rubygems/rubygems/blob/master/CODE_OF_CONDUCT.md).

(cherry picked from commit 20bece5)

Author: homu

lib/rubygems/installer.rb

@@ -214,7 +214,7 @@ def check_executable_overwrite filename # :nodoc:
 
       ruby_executable = true
       existing = io.read.slice(%r{
-          ^(
+          ^\s*(
             gem \s |
             load \s Gem\.bin_path\( |
             load \s Gem\.activate_bin_path\(
@@ -701,6 +701,8 @@ def verify_gem_home(unpack = false) # :nodoc:
   # Return the text for an application file.
 
   def app_script_text(bin_file_name)
+    # note that the `load` lines cannot be indented, as old RG versions match
+    # against the beginning of the line
     return <<-TEXT
 #{shebang bin_file_name}
 #
@@ -723,7 +725,12 @@ def app_script_text(bin_file_name)
   end
 end
 
+if Gem.respond_to?(:activate_bin_path)
 load Gem.activate_bin_path('#{spec.name}', '#{bin_file_name}', version)
+else
+gem #{spec.name.dump}, version
+load Gem.bin_path(#{spec.name.dump}, #{bin_file_name.dump}, version)
+end
 TEXT
   end
 

test/rubygems/test_gem_installer.rb

@@ -62,7 +62,12 @@ def test_app_script_text
   end
 end
 
+if Gem.respond_to?(:activate_bin_path)
 load Gem.activate_bin_path('a', 'executable', version)
+else
+gem "a", version
+load Gem.bin_path("a", "executable", version)
+end
     EOF
 
     wrapper = @installer.app_script_text 'executable'