FIX: preserve NUL bytes in callback exceptions

Use the Ruby string length when forwarding callback exception messages to V8 so embedded NUL bytes do not truncate the message or leave the context deadlocked.

Add regression coverage for same-thread and cross-thread callback exceptions with NUL-containing messages, and note the fix in the changelog.

Author: SamSaffron

CHANGELOG

@@ -2,6 +2,7 @@
   - Add `Context#perform_microtask_checkpoint` to synchronously drain the V8 microtask queue, useful for spec-compliant `dispatchEvent` sequencing inside Ruby callbacks
   - Fix native memory leaks in `Context#heap_snapshot`/`Context#write_heap_snapshot`; thanks to Pranjali Thakur from depthfirst.com
   - Fix large integral JavaScript numbers wrapping to negative Ruby integers; thanks to Pranjali Thakur from depthfirst.com
+  - Fix Ruby callback exceptions with embedded NUL bytes permanently deadlocking a context; thanks to Pranjali Thakur from depthfirst.com
 
 - 0.21.1 - 25-05-2026
   - Run `:single_threaded` V8 dispatches on a reusable mini_racer-owned native thread so V8 does not execute on Ruby-owned threads

ext/mini_racer_extension/mini_racer_extension.c

@@ -988,9 +988,9 @@ static void *rendezvous_callback(void *arg)
     ser_init0(&s);   // ruby exception pending
     w_byte(&s, 'e'); // send ruby error message to v8 thread
     r = rb_funcall(c->exception, rb_intern("to_s"), 0);
-    err = StringValueCStr(r);
+    err = StringValuePtr(r);
     if (err)
-        w(&s, err, strlen(err));
+        w(&s, err, RSTRING_LEN(r));
     goto out;
 }
 

test/mini_racer_test.rb

@@ -226,6 +226,91 @@ def test_attached_exceptions
     end
   end
 
+  def test_attached_exception_with_nul_message_does_not_deadlock_context
+    require "open3"
+    require "rbconfig"
+
+    script = <<~'RUBY'
+      $stdout.sync = true
+      $stderr.sync = true
+
+      Thread.new do
+        sleep 10
+        warn "child timed out"
+        exit!(99)
+      end
+
+      require "mini_racer"
+
+      NUL_PAYLOAD = "boom(\"a" + 0.chr + "b\")"
+
+      def assert_reusable(context, label)
+        thread = Thread.new { context.eval("40 + 2") }
+        unless thread.join(2)
+          warn "#{label} reuse eval blocked"
+          exit!(10)
+        end
+
+        result = thread.value
+        unless result == 42
+          warn "#{label} reuse eval returned #{result.inspect}"
+          exit!(11)
+        end
+      end
+
+      context = MiniRacer::Context.new
+      context.attach("boom", proc { |x| raise(ArgumentError, x) })
+      begin
+        context.eval(NUL_PAYLOAD)
+        warn "expected same-thread callback exception"
+        exit!(12)
+      rescue ArgumentError => e
+        unless e.message == "a\x00b"
+          warn "same-thread wrong exception message: #{e.message.inspect}"
+          exit!(13)
+        end
+      end
+      assert_reusable(context, "same-thread")
+
+      context = MiniRacer::Context.new
+      context.attach("boom", proc { |x| raise(ArgumentError, x) })
+      poison = Thread.new do
+        begin
+          context.eval(NUL_PAYLOAD)
+          ["no exception", nil]
+        rescue => e
+          [e.class.name, e.message]
+        end
+      end
+
+      unless poison.join(2)
+        warn "cross-thread poison eval blocked"
+        exit!(14)
+      end
+
+      unless poison.value == ["ArgumentError", "a\x00b"]
+        warn "cross-thread wrong exception: #{poison.value.inspect}"
+        exit!(15)
+      end
+      assert_reusable(context, "cross-thread")
+    RUBY
+
+    stdout, stderr, status = Open3.capture3(
+      RbConfig.ruby,
+      "-I#{File.expand_path("../lib", __dir__)}",
+      "-e",
+      script
+    )
+
+    assert status.success?, <<~MSG
+      NUL callback exception script failed with status #{status.exitstatus || "signal #{status.termsig}"}
+      stdout:
+      #{stdout}
+      stderr:
+      #{stderr}
+    MSG
+  end
+
   def test_attached_on_object
     context = MiniRacer::Context.new
     context.eval "var minion"