merged #8624 manually; fixes #8442; closes #8575

lib/pure/osproc.nim

@@ -884,9 +884,11 @@ elif not defined(useNimRtl):
         chck posix_spawn_file_actions_adddup2(fops, data.pStdin[readIdx], readIdx)
         chck posix_spawn_file_actions_addclose(fops, data.pStdout[readIdx])
         chck posix_spawn_file_actions_adddup2(fops, data.pStdout[writeIdx], writeIdx)
-        if (poStdErrToStdOut in data.options):
-          chck posix_spawn_file_actions_addclose(fops, data.pStderr[readIdx])
+        chck posix_spawn_file_actions_addclose(fops, data.pStderr[readIdx])
+        if poStdErrToStdOut in data.options:
           chck posix_spawn_file_actions_adddup2(fops, data.pStdout[writeIdx], 2)
+        else:
+          chck posix_spawn_file_actions_adddup2(fops, data.pStderr[writeIdx], 2)
 
       var res: cint
       if data.workingDir.len > 0:

tests/osproc/tstderr.nim

@@ -0,0 +1,32 @@
+discard """
+  output: '''--------------------------------------
+to stderr
+to stderr
+--------------------------------------
+'''
+"""
+import osproc, os, streams
+
+const filename = "ta_out".addFileExt(ExeExt)
+
+doAssert fileExists(getCurrentDir() / "tests" / "osproc" / filename)
+
+var p = startProcess(filename, getCurrentDir() / "tests" / "osproc",
+                     options={})
+
+try:
+  let stdoutStream = p.outputStream
+  let stderrStream = p.errorStream
+  var x = newStringOfCap(120)
+  var output = ""
+  while stderrStream.readLine(x.TaintedString):
+    output.add(x & "\n")
+
+  echo "--------------------------------------"
+  stdout.flushFile()
+  stderr.write output
+  stderr.flushFile()
+  echo "--------------------------------------"
+  stdout.flushFile()
+finally:
+  p.close()