[rubygems/rubygems] Let gem exec raise an error in ambiguous cases

When `gem exec foo` is run, and "foo" is a gem that has multiple
executables, none of them named "foo", raise an error explaining the
situation and telling user to be more specific.

Currently the first command in the executables array is run, but this
may come as surprising sometimes, so better raise an error.

ruby/rubygems@acda5d8f6e

Author: deivid-rodriguez

lib/rubygems/commands/exec_command.rb

@@ -195,7 +195,7 @@ def load!
     argv = ARGV.clone
     ARGV.replace options[:args]
 
-    exe = executable = options[:executable]
+    executable = options[:executable]
 
     contains_executable = Gem.loaded_specs.values.select do |spec|
       spec.executables.include?(executable)
@@ -206,13 +206,22 @@ def load!
     end
 
     if contains_executable.empty?
-      if (spec = Gem.loaded_specs[executable]) && (exe = spec.executable)
-        contains_executable << spec
-      else
+      spec = Gem.loaded_specs[executable]
+
+      if spec.nil? || spec.executables.empty?
         alert_error "Failed to load executable `#{executable}`," \
               " are you sure the gem `#{options[:gem_name]}` contains it?"
         terminate_interaction 1
       end
+
+      if spec.executables.size > 1
+        alert_error "Ambiguous which executable from gem `#{executable}` should be run: " \
+              "the options are #{spec.executables}, specify one via COMMAND, and use `-g` and `-v` to specify gem and version"
+        terminate_interaction 1
+      end
+
+      contains_executable << spec
+      executable = spec.executable
     end
 
     if contains_executable.size > 1
@@ -223,8 +232,8 @@ def load!
     end
 
     old_exe = $0
-    $0 = exe
-    load Gem.activate_bin_path(contains_executable.first.name, exe, ">= 0.a")
+    $0 = executable
+    load Gem.activate_bin_path(contains_executable.first.name, executable, ">= 0.a")
   ensure
     $0 = old_exe if old_exe
     ARGV.replace argv

test/rubygems/test_gem_commands_exec_command.rb

@@ -370,8 +370,11 @@ def test_gem_with_multiple_executables_no_match
     util_clear_gems
 
     use_ui @ui do
-      @cmd.invoke "a:2"
-      assert_equal "a-2 foo\n", @ui.output
+      e = assert_raise Gem::MockGemUi::TermError do
+        @cmd.invoke "a:2"
+      end
+      assert_equal 1, e.exit_code
+      assert_equal "ERROR:  Ambiguous which executable from gem `a` should be run: the options are [\"foo\", \"bar\"], specify one via COMMAND, and use `-g` and `-v` to specify gem and version\n", @ui.error
     end
   end