Fix subprocess.communicate("") raising IOError: closed stream (#78)

`subprocess.communicate("")` would raise `IOError: closed stream` because the code closed stdin immediately when `input.empty?`, but then later tried to use the closed stdin file descriptor in `IO.select`

This only occurs with empty strings (`""`), not with `nil` or non-empty strings.

We've fixed that by excluding `@stdin` from `wait_w` when `@stdin.closed?`

### Backward compatibility

This fix maintains full backward compatibility:
- `nil` input behaviour unchanged
- Non-empty string input behaviour unchanged  
- Only empty string input is fixed (was previously broken)

Author: qaisjp

lib/subprocess.rb

@@ -436,13 +436,15 @@ def communicate(input=nil, timeout_s=nil)
       # the input depending on how many bytes were written
       input = input.dup.force_encoding('BINARY') unless input.nil?
 
-      @stdin.close if (input.nil? || input.empty?) && !@stdin.nil?
+      # Close stdin immediately if input is nil or empty
+      @stdin.close if @stdin && (input.nil? || input.empty?)
 
       timeout_at = Time.now + timeout_s if timeout_s
 
       self.class.catching_sigchld(pid) do |global_read, self_read|
         wait_r = [@stdout, @stderr, self_read, global_read].compact
-        wait_w = [input && @stdin].compact
+        wait_w = @stdin&.closed? ? [] : [input && @stdin].compact
+
         done = false
         while !done
           # If the process has exited, we want to drain any remaining output before returning

test/test_empty_stdin.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+require 'rubygems'
+gem 'minitest'
+require 'minitest/autorun'
+require 'subprocess'
+
+describe Subprocess do
+  describe "communicate with empty string input" do
+    # Bug report: subprocess.communicate("") doesn't properly handle stdin,
+    # causing it to close incorrectly and result in a broken pipe.
+    it "should not raise IOError when passing empty string" do
+      # Before the fix, this would raise: IOError: closed stream
+      Subprocess.check_call(['cat'],
+                           stdin: Subprocess::PIPE,
+                           stdout: Subprocess::PIPE) do |p|
+        stdout, stderr = p.communicate("")
+        assert_equal("", stdout, "Empty input should produce empty output")
+        assert_equal("", stderr, "No errors expected")
+      end
+    end
+
+    it "should work correctly with non-empty string input" do
+      test_input = "hello world"
+      Subprocess.check_call(['cat'],
+                           stdin: Subprocess::PIPE,
+                           stdout: Subprocess::PIPE) do |p|
+        stdout, stderr = p.communicate(test_input)
+        assert_equal(test_input, stdout, "Input should be echoed back")
+        assert_equal("", stderr, "No errors expected")
+      end
+    end
+
+    it "should work correctly with nil input" do
+      Subprocess.check_call(['cat'],
+                           stdin: Subprocess::PIPE,
+                           stdout: Subprocess::PIPE) do |p|
+        stdout, stderr = p.communicate(nil)
+        assert_equal("", stdout, "Nil input should produce empty output")
+        assert_equal("", stderr, "No errors expected")
+      end
+    end
+
+    it "should handle already closed stdin gracefully" do
+      # Edge case: what if stdin is already closed?
+      p = Subprocess.popen(['cat'], stdin: Subprocess::PIPE, stdout: Subprocess::PIPE)
+      p.stdin.close
+      stdout, stderr = p.communicate("")
+      assert_equal("", stdout)
+      assert_equal("", stderr)
+      p.wait
+    end
+  end
+end