Harden alpha8 publication safety

test/opencode/sandbox_file_test.rb

@@ -11,6 +11,7 @@ require "marcel"  # SandboxFile#content_type uses Marcel::MimeType.for
 class Opencode::SandboxFileTest < Minitest::Test
   def setup
     @tmpdir = Dir.mktmpdir("opencode-rails-sandbox-file-test-")
+    @outside_dir = Dir.mktmpdir("opencode-rails-sandbox-file-outside-")
     @path = File.join(@tmpdir, "notes.md")
     File.write(@path, "# hello\nworld\n")
     # SandboxFile uses `start_with?` against this prefix to detect path
@@ -21,6 +22,7 @@ class Opencode::SandboxFileTest < Minitest::Test
 
   def teardown
     FileUtils.remove_entry(@tmpdir) if @tmpdir && File.exist?(@tmpdir)
+    FileUtils.remove_entry(@outside_dir) if @outside_dir && File.exist?(@outside_dir)
   end
 
   def test_basic_readers
@@ -59,4 +61,49 @@ class Opencode::SandboxFileTest < Minitest::Test
     assert_equal "notes.md", artifact.filename
     assert_equal "# hello\nworld\n", artifact.content
   end
+
+  def test_content_rejects_a_file_replaced_by_an_outside_symlink
+    file = Opencode::SandboxFile.new(
+      path: @path, sandbox_prefix: @sandbox_prefix, max_bytes: 10_000
+    )
+    outside = File.join(@outside_dir, "private.txt")
+    File.write(outside, "private")
+    assert file.safe?
+
+    File.unlink(@path)
+    File.symlink(outside, @path)
+
+    assert_raises(Opencode::SandboxFile::UnsafeFileError) { file.content }
+  end
+
+  def test_content_rejects_an_inode_that_grows_past_the_cap_while_reading
+    file = Opencode::SandboxFile.new(
+      path: @path, sandbox_prefix: @sandbox_prefix, max_bytes: 20
+    )
+    real_open = File.method(:open)
+    grow_on_read = lambda do |path, *args, **kwargs, &block|
+      real_open.call(path, *args, **kwargs) do |opened|
+        proxy = Object.new
+        proxy.define_singleton_method(:stat) { opened.stat }
+        proxy.define_singleton_method(:read) do |length = nil|
+          real_open.call(path, "ab") { |writer| writer.write("x" * 100) }
+          opened.read(length)
+        end
+        block.call(proxy)
+      end
+    end
+
+    File.stub(:open, grow_on_read) do
+      assert_raises(Opencode::SandboxFile::UnsafeFileError) { file.content }
+    end
+  end
+
+  def test_content_returns_binary_empty_string_for_a_zero_byte_file
+    File.write(@path, "")
+    file = Opencode::SandboxFile.new(
+      path: @path, sandbox_prefix: @sandbox_prefix, max_bytes: 20
+    )
+
+    assert_equal "".b, file.content
+  end
 end