bpo-31009: Fix support.fd_count() on Windows (#2862)

* bpo-31009: Fix support.fd_count() on Windows

On Windows, test.support.fd_count() now calls
msvcrt.CrtSetReportMode() to not kill the process nor log any error
on stderr on os.dup(fd) if the file descriptor is invalid.

* Fix for release mode

Lib/test/support/__init__.py

@@ -2649,12 +2649,6 @@ def disable_faulthandler():
             faulthandler.enable(file=fd, all_threads=True)
 
 
-try:
-    MAXFD = os.sysconf("SC_OPEN_MAX")
-except Exception:
-    MAXFD = 256
-
-
 def fd_count():
     """Count the number of open file descriptors.
     """
@@ -2665,6 +2659,31 @@ def fd_count():
         except FileNotFoundError:
             pass
 
+    old_modes = None
+    if sys.platform == 'win32':
+        # bpo-25306, bpo-31009: Call CrtSetReportMode() to not kill the process
+        # on invalid file descriptor if Python is compiled in debug mode
+        try:
+            import msvcrt
+            msvcrt.CrtSetReportMode
+        except (AttributeError, ImportError):
+            # no msvcrt or a release build
+            pass
+        else:
+            old_modes = {}
+            for report_type in (msvcrt.CRT_WARN,
+                                msvcrt.CRT_ERROR,
+                                msvcrt.CRT_ASSERT):
+                old_modes[report_type] = msvcrt.CrtSetReportMode(report_type, 0)
+
+    MAXFD = 256
+    if hasattr(os, 'sysconf'):
+        try:
+            MAXFD = os.sysconf("SC_OPEN_MAX")
+        except OSError:
+            pass
+
+    try:
         count = 0
         for fd in range(MAXFD):
             try:
@@ -2677,4 +2696,11 @@ def fd_count():
             else:
                 os.close(fd2)
                 count += 1
+    finally:
+        if old_modes is not None:
+            for report_type in (msvcrt.CRT_WARN,
+                                msvcrt.CRT_ERROR,
+                                msvcrt.CRT_ASSERT):
+                msvcrt.CrtSetReportMode(report_type, old_modes[report_type])
+
     return count

Lib/test/test_regrtest.py

@@ -835,22 +835,10 @@ class ArgsTestCase(BaseTestCase):
             import os
             import unittest
 
-            # Issue #25306: Disable popups and logs to stderr on assertion
-            # failures in MSCRT
-            try:
-                import msvcrt
-                msvcrt.CrtSetReportMode
-            except (ImportError, AttributeError):
-                # no Windows, o release build
-                pass
-            else:
-                for m in [msvcrt.CRT_WARN, msvcrt.CRT_ERROR, msvcrt.CRT_ASSERT]:
-                    msvcrt.CrtSetReportMode(m, 0)
-
             class FDLeakTest(unittest.TestCase):
                 def test_leak(self):
                     fd = os.open(__file__, os.O_RDONLY)
-                    # bug: never cloes the file descriptor
+                    # bug: never close the file descriptor
         """)
         self.check_leak(code, 'file descriptors')