[GR-18264] Use 1 concurrent download in RubyGems to avoid races in Gem::Specification._all

* Same as in Ruby <= 2.5 where that logic used no threads.

Author: eregon

CHANGELOG.md

@@ -73,6 +73,7 @@ Bug fixes:
 * Fixed issue with installing latest bundler (#1880).
 * Fixed type conversion for `Numeric#step` `step` parameter.
 * Fixed `Kernel#Integer` conversion.
+* Avoid race conditions during `gem install` by using a single download thread.
 
 Compatibility:
 

lib/mri/rubygems/config_file.rb

@@ -44,7 +44,12 @@ class Gem::ConfigFile
   DEFAULT_BULK_THRESHOLD = 1000
   DEFAULT_VERBOSITY = true
   DEFAULT_UPDATE_SOURCES = true
-  DEFAULT_CONCURRENT_DOWNLOADS = 8
+  if defined?(::TruffleRuby)
+    # GR-18264: Gem::Specification._all does not seem thread safe and raises "nil spec! included in"
+    DEFAULT_CONCURRENT_DOWNLOADS = 1
+  else
+    DEFAULT_CONCURRENT_DOWNLOADS = 8
+  end
   DEFAULT_CERT_EXPIRATION_LENGTH_DAYS = 365
 
   ##

spec/truffle/rubygems/install_spec.rb

@@ -0,0 +1,17 @@
+# Copyright (c) 2020 Oracle and/or its affiliates. All rights reserved. This
+# code is released under a tri EPL/GPL/LGPL license. You can use it,
+# redistribute it and/or modify it under the terms of the:
+#
+# Eclipse Public License version 2.0, or
+# GNU General Public License version 2, or
+# GNU Lesser General Public License version 2.1.
+# OTHER DEALINGS IN THE SOFTWARE.
+
+require_relative '../../ruby/spec_helper'
+require 'rubygems'
+
+describe "Downloading gems with RubyGems" do
+  it "is thread-safe by using a single thread to fetch (GR-18264)" do
+    Gem.configuration.concurrent_downloads.should == 1
+  end
+end