From d1890c4fd2bf22dd7aab92be43e1a937e1a9cad9 Mon Sep 17 00:00:00 2001
From: Arpit Jain <arpitjain099@gmail.com>
Date: Sat, 6 Jun 2026 16:28:25 +0900
Subject: [PATCH] Include runtime stderr in error messages

5cce03a stopped folding stderr into stdout to keep Bun's .env loading
noise out of the parsed output (#130). A side effect was that when the
runtime exits non-zero, the diagnostic it writes to stderr (parse errors,
stack traces) is dropped, so the raised ExecJS::RuntimeError carries no
useful detail (#142).

Capture stderr separately by redirecting the child's stderr to a temp file
(a file, not a pipe, so there is no risk of a deadlock between draining
stdout and stderr), and include it in the error only on the failure path.
The success path still reads stdout exactly as before, so #130 stays fixed.

exec_runtime_error takes an optional stderr argument and is backwards
compatible, so the Windows branch (which uses its own redirect) is
unaffected. The two IO.popen branches that 5cce03a changed (default and
JRuby) are the ones updated here.

Adds a regression test that a non-zero runtime exit surfaces its stderr.

Fixes #142

Signed-off-by: Arpit Jain <arpitjain099@gmail.com>
---
 lib/execjs/external_runtime.rb | 48 ++++++++++++++++++++++++----------
 test/test_execjs.rb            | 12 +++++++++
 2 files changed, 46 insertions(+), 14 deletions(-)

diff --git a/lib/execjs/external_runtime.rb b/lib/execjs/external_runtime.rb
index a0325f4..47b50b9 100644
--- a/lib/execjs/external_runtime.rb
+++ b/lib/execjs/external_runtime.rb
@@ -194,36 +194,56 @@ def shell_escape(*args)
         require 'shellwords'
 
         def exec_runtime(filename)
-          command = "#{Shellwords.join(binary.split(' ') << filename)}"
-          io = IO.popen(command, **@popen_options)
-          output = io.read
-          io.close
+          stderr_path = Dir::Tmpname.create(['execjs', 'stderr']) {}
+          begin
+            command = "#{Shellwords.join(binary.split(' ') << filename)}"
+            io = IO.popen(command, err: stderr_path, **@popen_options)
+            output = io.read
+            io.close
+            status = $?
+            stderr = File.file?(stderr_path) ? File.read(stderr_path) : ""
+          ensure
+            File.unlink(stderr_path) if stderr_path && File.exist?(stderr_path)
+          end
 
-          if $?.success?
+          if status.success?
             output
           else
-            raise exec_runtime_error(output)
+            raise exec_runtime_error(output, stderr)
           end
         end
       else
         def exec_runtime(filename)
-          io = IO.popen(binary.split(' ') << filename, **@popen_options)
-          output = io.read
-          io.close
+          stderr_path = Dir::Tmpname.create(['execjs', 'stderr']) {}
+          begin
+            io = IO.popen(binary.split(' ') << filename, err: stderr_path, **@popen_options)
+            output = io.read
+            io.close
+            status = $?
+            stderr = File.file?(stderr_path) ? File.read(stderr_path) : ""
+          ensure
+            File.unlink(stderr_path) if stderr_path && File.exist?(stderr_path)
+          end
 
-          if $?.success?
+          if status.success?
             output
           else
-            raise exec_runtime_error(output)
+            raise exec_runtime_error(output, stderr)
           end
         end
       end
       # Internally exposed for Context.
       public :exec_runtime
 
-      def exec_runtime_error(output)
-        error = RuntimeError.new(output)
-        lines = output.split("\n")
+      def exec_runtime_error(output, stderr = nil)
+        message = output.to_s
+        unless stderr.nil? || stderr.empty?
+          extra = stderr.dup.force_encoding(message.encoding)
+          extra = extra.scrub if extra.respond_to?(:scrub) && !extra.valid_encoding?
+          message = message.empty? ? extra : "#{message}\n#{extra}"
+        end
+        error = RuntimeError.new(message)
+        lines = message.split("\n")
         lineno = lines[0][/:(\d+)$/, 1] if lines[0]
         lineno ||= 1
         error.set_backtrace(["(execjs):#{lineno}"] + caller)
diff --git a/test/test_execjs.rb b/test/test_execjs.rb
index f2a5e23..64b51cb 100644
--- a/test/test_execjs.rb
+++ b/test/test_execjs.rb
@@ -67,6 +67,18 @@ def test_call_with_env_file
     end
   end
 
+  def test_runtime_error_includes_stderr
+    # Regression for #142: when the runtime exits non-zero it writes its
+    # diagnostic to stderr (here, a parse error). 5cce03a stopped capturing
+    # stderr, so the raised error lost that trace; it should be surfaced again.
+    # The diagnostic text is Node-style, so guard to Node-family runtimes.
+    skip unless ExecJS.runtime.name.to_s =~ /Node|Bun|Deno/i
+    err = assert_raises(ExecJS::RuntimeError) do
+      ExecJS.exec("?? not valid javascript ??")
+    end
+    assert err.message =~ /SyntaxError/, err.message
+  end
+
   def test_call_with_this
     # Known bug: https://github.com/cowboyd/therubyrhino/issues/39
     skip if ExecJS.runtime.is_a?(ExecJS::RubyRhinoRuntime)