Bugfix/double newlines in stderr (#5426)

lib/system.nim

@@ -2743,6 +2743,7 @@ when not defined(JS): #and not defined(nimscript):
       # we use binary mode on Windows:
       c_setmode(c_fileno(stdin), O_BINARY)
       c_setmode(c_fileno(stdout), O_BINARY)
+      c_setmode(c_fileno(stderr), O_BINARY)
 
     when defined(endb):
       proc endbStep()

tests/osproc/ta.nim

@@ -1,3 +0,0 @@
-import strutils
-let x = stdin.readLine()
-echo x.parseInt + 5

tests/osproc/ta_in.nim

@@ -0,0 +1,5 @@
+# This file is prefixed with an "a", because other tests
+# depend on it and it must be compiled first.
+import strutils
+let x = stdin.readLine()
+echo x.parseInt + 5

tests/osproc/ta_out.nim

@@ -0,0 +1,16 @@
+# This file is prefixed with an "a", because other tests
+# depend on it and it must be compiled first.
+stdout.writeLine("to stdout")
+stdout.flushFile()
+stdout.writeLine("to stdout")
+stdout.flushFile()
+
+stderr.writeLine("to stderr")
+stderr.flushFile()
+stderr.writeLine("to stderr")
+stderr.flushFile()
+
+stdout.writeLine("to stdout")
+stdout.flushFile()
+stdout.writeLine("to stdout")
+stdout.flushFile()

tests/osproc/tstdin.nim

@@ -4,7 +4,7 @@ discard """
 """
 import osproc, os, streams
 
-const filename = when defined(Windows): "ta.exe" else: "ta"
+const filename = when defined(Windows): "ta_in.exe" else: "ta_in"
 
 doAssert fileExists(getCurrentDir() / "tests" / "osproc" / filename)
 

tests/osproc/tstdout.nim

@@ -0,0 +1,29 @@
+discard """
+  output: '''--------------------------------------
+to stdout
+to stdout
+to stderr
+to stderr
+to stdout
+to stdout
+--------------------------------------
+'''
+"""
+import osproc, os, streams
+
+const filename = when defined(Windows): "ta_out.exe" else: "ta_out"
+
+doAssert fileExists(getCurrentDir() / "tests" / "osproc" / filename)
+
+var p = startProcess(filename, getCurrentDir() / "tests" / "osproc",
+                     options={poStdErrToStdOut})
+
+let outputStream = p.outputStream
+var x = newStringOfCap(120)
+var output = ""
+while outputStream.readLine(x.TaintedString):
+  output.add(x & "\n")
+
+echo "--------------------------------------"
+stdout.write output
+echo "--------------------------------------"