Drain command output before waiting for the child

lib/pkg-config.rb

@@ -377,9 +377,10 @@ def run_command(*command_line)
                       out: output,
                       err: File::NULL)
           output.close
+          result = input.read
           _, status = Process.waitpid2(pid)
           return nil unless status.success?
-          input.read
+          result
         rescue SystemCallError
           nil
         end

test/test-pkg-config.rb

@@ -1,5 +1,6 @@
 require "mkmf"
 require "tempfile"
+require "timeout"
 
 require "pkg-config"
 
@@ -21,6 +22,18 @@ def setup
     @glib = PackageConfig.new("glib-2.0", options)
   end
 
+  def test_run_command_drains_output_before_waiting
+    output = Timeout.timeout(5) do
+      PackageConfig.__send__(
+        :run_command,
+        RbConfig.ruby,
+        "-e",
+        "STDOUT.write('x' * 131072)",
+      )
+    end
+    assert_equal(131072, output.bytesize)
+  end
+
   def only_pkg_config_version(major, minor)
     pkg_config_version = `#{@pkgconf} --version`.chomp
     current_major, current_minor = pkg_config_version.split(".").collect(&:to_i)