Fix which command test isolation

Author: afurm

PR_DESCRIPTION.md

@@ -0,0 +1,13 @@
+# PR Description
+
+## Title
+Fix gem which command test isolation
+
+## Problem
+The `gem which` command reports the filesystem path for a requireable file; `test_execute_one_missing` verifies it prints the found path and errors on a missing file. The test fails when run in isolation because it relies on gem path/spec state set by other tests.
+
+## Fix
+Set `GEM_HOME`/`GEM_PATH` to `@gemhome`, call `Gem.use_paths(@gemhome)`, and reset the spec cache before the test runs. Restore the env and gem paths in an ensure block and reset the spec cache afterward.
+
+## Testing
+- `ruby -Ilib:test:bundler/lib test/rubygems/test_gem_commands_which_command.rb -n test_execute_one_missing`

test/rubygems/test_gem_commands_which_command.rb

@@ -38,7 +38,13 @@ def test_execute_directory
   end
 
   def test_execute_one_missing
-    # TODO: this test fails in isolation
+    original_gem_home = ENV["GEM_HOME"]
+    original_gem_path = ENV["GEM_PATH"]
+
+    ENV["GEM_HOME"] = @gemhome
+    ENV["GEM_PATH"] = @gemhome
+    Gem.use_paths(@gemhome)
+    Gem::Specification.reset
 
     util_foo_bar
 
@@ -53,6 +59,21 @@ def test_execute_one_missing
     assert_equal "#{@foo_bar.full_gem_path}/lib/foo_bar.rb\n", @ui.output
     assert_match(/Can.t find Ruby library file or shared library missinglib\n/,
                  @ui.error)
+  ensure
+    if original_gem_home
+      ENV["GEM_HOME"] = original_gem_home
+    else
+      ENV.delete("GEM_HOME")
+    end
+
+    if original_gem_path
+      ENV["GEM_PATH"] = original_gem_path
+    else
+      ENV.delete("GEM_PATH")
+    end
+
+    Gem.use_paths(@gemhome)
+    Gem::Specification.reset
   end
 
   def test_execute_missing