Make all interpreters opt-in for FFI.

Rename `backend.rb` to `implementation.rb`.

ruby/BUILD.bazel

@@ -191,6 +191,9 @@ conformance_test(
 
 conformance_test(
     name = "conformance_test_ffi",
+    env = {
+        "PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION": "ffi",
+    },
     failure_list = "//conformance:failure_list_ruby.txt",
     target_compatible_with = select({
         ":ruby_ffi": [],
@@ -213,6 +216,9 @@ conformance_test(
 
 conformance_test(
     name = "conformance_test_jruby_ffi",
+    env = {
+        "PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION": "ffi",
+    },
     failure_list = "//conformance:failure_list_jruby_ffi.txt",
     target_compatible_with = select({
         ":jruby_ffi": [],

ruby/Rakefile

@@ -2,12 +2,16 @@ require "rubygems"
 require "rubygems/package_task"
 require "rake/extensiontask" unless RUBY_PLATFORM == "java"
 require "rake/testtask"
+
+# FFI and FFI-Compiler are developer dependencies everywhere but only
+# dependencies for JRuby right now.
 begin
   require 'ffi'
   require "ffi-compiler/compile_task"
-  USE_FFI = true
+  FFI_AVAILABLE = true
 rescue LoadError
-  USE_FFI = false
+  warn "LoadError in Rakefile while loading FFI: #{$!}. FFI-related tasks unavailable. To enable, install `ffi` and `ffi-compiler` gems."
+  FFI_AVAILABLE = false
 end
 
 spec = Gem::Specification.load("google-protobuf.gemspec")
@@ -54,7 +58,7 @@ end
 
 genproto_output = []
 
-if USE_FFI
+if FFI_AVAILABLE
   common_excluded_source = [
       :convert, :defs, :map, :message, :protobuf, :repeated_field, :wrap_memcpy
   ]
@@ -233,7 +237,7 @@ else
   end
 end
 
-task :compile => ["ffi-compiler:default"] if USE_FFI
+task :compile => ["ffi-compiler:default"] if FFI_AVAILABLE
 
 task :genproto => genproto_output
 

ruby/lib/google/protobuf.rb

@@ -41,26 +41,12 @@ class ParseError < Error; end
     class TypeError < ::TypeError; end
 
     PREFER_FFI = case ENV['PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION']
-                 when nil, ""
-                   if RUBY_PLATFORM == "java"
-                     # When unspecified, PREFER_FFI is autodetected based on
-                     # available gems for JRuby. For CRuby, users must opt-in.
-                     begin
-                       require 'ffi'
-                       require 'ffi-compiler/loader'
-                       true
-                     rescue LoadError
-                       false
-                     end
-                   else
-                     false
-                   end
+                 when nil, "", /^native$/i
+                   false
                  when /^ffi$/i
                    true
-                 when /^native$/i
-                   false
                  else
-                   warn "Unexpected value `#{ENV['PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION']}` for environment variable `PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION`. Should be either \"FFI\", \"NATIVE\". Omit for autodetection."
+                   warn "Unexpected value `#{ENV['PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION']}` for environment variable `PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION`. Should be either \"FFI\", \"NATIVE\"."
                    false
                  end
 
@@ -85,10 +71,8 @@ def self.decode_json(klass, json, options = {})
         require 'google/protobuf_ffi'
         :FFI
       rescue LoadError
-        if $DEBUG
-          warn "Caught exception `#{$!.message}` while loading FFI implementation of google/protobuf."
-          warn "Falling back to native implementation."
-        end
+        warn "Caught exception `#{$!.message}` while loading FFI implementation of google/protobuf."
+        warn "Falling back to native implementation."
         require 'google/protobuf_native'
         :NATIVE
       end

ruby/tests/BUILD.bazel

@@ -15,8 +15,8 @@ filegroup(
 )
 
 ruby_test(
-    name = "backend",
-    srcs = ["backend.rb"],
+    name = "implementation",
+    srcs = ["implementation.rb"],
     deps = [
         "//ruby:protobuf",
         "@protobuf_bundle//:test-unit",

ruby/tests/implementation.rb

@@ -0,0 +1,37 @@
+require 'google/protobuf'
+require 'test/unit'
+
+class BackendTest < Test::Unit::TestCase
+  # Verifies the implementation of Protobuf is the preferred one.
+  # See protobuf.rb for the logic that defines PREFER_FFI.
+  def test_prefer_ffi_aligns_with_implementation
+    expected = Google::Protobuf::PREFER_FFI ? :FFI : :NATIVE
+    assert_equal expected, Google::Protobuf::IMPLEMENTATION
+  end
+
+  def test_prefer_ffi
+    unless ENV['PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION'] =~ /ffi/i
+      omit"FFI implementation requires environment variable PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION=FFI to activate."
+    end
+    assert_equal true, Google::Protobuf::PREFER_FFI
+  end
+  def test_ffi_implementation
+    unless ENV['PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION'] =~ /ffi/i
+      omit "FFI implementation requires environment variable PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION=FFI to activate."
+    end
+    assert_equal :FFI, Google::Protobuf::IMPLEMENTATION
+  end
+
+  def test_prefer_native
+    if ENV.include?('PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION') and ENV['PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION'] !~ /native/i
+      omit"Native implementation requires omitting environment variable PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION or setting it to `NATIVE` to activate."
+    end
+    assert_equal false, Google::Protobuf::PREFER_FFI
+  end
+  def test_native_implementation
+    if ENV.include?('PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION') and ENV['PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION'] !~ /native/i
+      omit"Native implementation requires omitting environment variable PROTOCOL_BUFFERS_RUBY_IMPLEMENTATION or setting it to `NATIVE` to activate."
+    end
+    assert_equal :NATIVE, Google::Protobuf::IMPLEMENTATION
+  end
+end