Prevent extensions from building in parallel

Building an extension requires changing the path so commands executed
via a subprocess will work correctly.  In order to prevent parallel
extension builds from changing each other's directory a mutex is
required.

Fixes #607

History.txt

@@ -1,5 +1,12 @@
 # coding: UTF-8
 
+=== 2.0.7
+
+Bug fixes:
+
+* Extensions may now be built in parallel (therefore gems may be installed in
+  parallel).  Bug #606 by Hemant Kumar.
+
 === 2.0.6 / 2013-07-24
 
 Bug fixes:

lib/rubygems/installer.rb

@@ -9,6 +9,7 @@
 require 'rubygems/ext'
 require 'rubygems/user_interaction'
 require 'fileutils'
+require 'thread'
 
 ##
 # The installer installs the files contained in the .gem into the Gem.home.
@@ -31,6 +32,14 @@ class Gem::Installer
 
   ENV_PATHS = %w[/usr/bin/env /bin/env]
 
+  ##
+  # The builder shells-out to run various commands after changing the
+  # directory.  This means multiple installations cannot be allowed to build
+  # extensions in parallel as they may change each other's directories leading
+  # to broken extensions or failed installations.
+
+  CHDIR_MUTEX = Mutex.new # :nodoc:
+
   ##
   # Raised when there is an error while building extensions.
   #
@@ -675,11 +684,13 @@ def build_extensions
       begin
         FileUtils.mkdir_p dest_path
 
-        Dir.chdir extension_dir do
-          results = builder.build(extension, gem_dir, dest_path,
-                                  results, @build_args)
+        CHDIR_MUTEX.synchronize do
+          Dir.chdir extension_dir do
+            results = builder.build(extension, gem_dir, dest_path,
+                                    results, @build_args)
 
-          say results.join("\n") if Gem.configuration.really_verbose
+            say results.join("\n") if Gem.configuration.really_verbose
+          end
         end
       rescue
         extension_build_error(extension_dir, results.join("\n"), $@)