Fix 2 — Writability fallback (lines 36-47)

lib/tmpdir.rb

@@ -35,6 +35,14 @@ def self.tmpdir
       when !File.writable?(dir)
         # We call File.writable?, not stat.writable?, because you can't tell if a dir is actually
         # writable just from stat; OS mechanisms other than user/group/world bits can affect this.
+        #
+        # However, in some container environments (e.g. Kubernetes with read-only root filesystem
+        # and emptyDir volumes), File.writable? may return false even though the directory is
+        # actually writable via mounted volumes. Fall back to stat.writable? in that case.
+        if stat.writable?
+          warn "#{name}: File.writable? reports not writable but file mode bits suggest writable, using anyway: #{dir}"
+          break dir
+        end
         warn "#{name} is not writable: #{dir}"
       when stat.world_writable? && !stat.sticky?
         warn "#{name} is world-writable: #{dir}"

test/test_tmpdir.rb

@@ -59,6 +59,59 @@ def test_tmpdir_not_empty_parent
     end
   end
 
+  def test_writable_fallback_to_stat
+    omit "no meaning on this platform" if /mswin|mingw/ =~ RUBY_PLATFORM
+    Dir.mktmpdir do |tmpdir|
+      envs = %w[TMPDIR TMP TEMP]
+      oldenv = envs.each_with_object({}) {|v, h| h[v] = ENV.delete(v)}
+      begin
+        ENV[envs[0]] = tmpdir
+
+        # Stub File.writable? to return false for our tmpdir
+        # This simulates container environments where access(2) returns false
+        # even though the directory is actually writable
+        original_writable = File.method(:writable?)
+        File.define_singleton_method(:writable?) do |path|
+          if path == tmpdir
+            false
+          else
+            original_writable.call(path)
+          end
+        end
+
+        # Should fall back to stat.writable? and succeed with a warning
+        assert_equal(tmpdir, assert_warn(/File\.writable\? reports not writable but file mode bits suggest writable/) { Dir.tmpdir })
+
+      ensure
+        # Restore original File.writable?
+        File.define_singleton_method(:writable?, original_writable) if original_writable
+        ENV.update(oldenv)
+      end
+    end
+  end
+
+  def test_writable_fallback_both_fail
+    omit "no meaning on this platform" if /mswin|mingw/ =~ RUBY_PLATFORM
+    omit "root can write to any directory" if Process.uid == 0
+    Dir.mktmpdir do |tmpdir|
+      envs = %w[TMPDIR TMP TEMP]
+      oldenv = envs.each_with_object({}) {|v, h| h[v] = ENV.delete(v)}
+      begin
+        ENV[envs[0]] = tmpdir
+
+        # Make directory not writable (both File.writable? and stat.writable? will return false)
+        File.chmod(0555, tmpdir)
+
+        # Should reject the directory with "not writable" warning
+        assert_not_equal(tmpdir, assert_warn(/is not writable/) { Dir.tmpdir })
+
+      ensure
+        File.chmod(0755, tmpdir)
+        ENV.update(oldenv)
+      end
+    end
+  end
+
   def test_no_homedir
     bug7547 = '[ruby-core:50793]'
     home, ENV["HOME"] = ENV["HOME"], nil