diff --git a/mercurial/exewrapper.c b/mercurial/exewrapper.c
--- a/mercurial/exewrapper.c
+++ b/mercurial/exewrapper.c
@@ -7,6 +7,7 @@
  GNU General Public License version 2 or any later version.
 */
 
+#include <Python.h>
 #include <stdio.h>
 #include <tchar.h>
 #include <windows.h>
@@ -46,6 +47,10 @@ int _tmain(int argc, TCHAR *argv[])
 	void(__cdecl * Py_SetPythonHome)(TCHAR * home);
 	int(__cdecl * Py_Main)(int argc, TCHAR *argv[]);
 
+#if PY_MAJOR_VERSION >= 3
+	Py_LegacyWindowsStdioFlag = 1;
+#endif
+
 	if (GetModuleFileName(NULL, pyscript, _countof(pyscript)) == 0) {
 		err = "GetModuleFileName failed";
 		goto bail;
diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -2640,6 +2640,10 @@ class TestRunner(object):
             self._tmpbindir = self._bindir
             self._pythondir = os.path.join(self._installdir, b"lib", b"python")
 
+        # Force the use of hg.exe instead of relying on MSYS to recognize hg is
+        # a python script and feed it to python.exe.  Legacy stdio is force
+        # enabled by hg.exe, and this is a more realistic way to launch hg
+        # anyway.
         if os.name == 'nt' and not self._hgcommand.endswith(b'.exe'):
             self._hgcommand += b'.exe'