procutil: assign pseudo file object if sys.stdout/stderr is missing

This basically simulates the Python 2 behavior. If libc stdio were used,
these file objects would be available and raise EBADF. There is subtle
difference between py2 and py3, but I think py3 behavior (i.e. exit 255)
is more correct.

"if" conditions are adjust so that they look similar to
dispatch.initstdio().

mercurial/utils/procutil.py

@@ -131,17 +131,25 b' def _make_write_all(stream):'
 
 
 if pycompat.ispy3:
-    # Python 3 implements its own I/O streams.
+    # Python 3 implements its own I/O streams. Unlike stdio of C library,
+    # sys.stdin/stdout/stderr may be None if underlying fd is closed.
+
     # TODO: .buffer might not exist if std streams were replaced; we'll need
     # a silly wrapper to make a bytes stream backed by a unicode one.
 
-    # sys.stdin can be None
-    if sys.stdin:
+    if sys.stdin is None:
+        stdin = BadFile()
+    else:
         stdin = sys.stdin.buffer
+    if sys.stdout is None:
+        stdout = BadFile()
     else:
-        stdin = BadFile()
-    stdout = _make_write_all(sys.stdout.buffer)
-    stderr = _make_write_all(sys.stderr.buffer)
+        stdout = _make_write_all(sys.stdout.buffer)
+    if sys.stderr is None:
+        stderr = BadFile()
+    else:
+        stderr = _make_write_all(sys.stderr.buffer)
+
     if pycompat.iswindows:
         # Work around Windows bugs.
         stdout = platform.winstdout(stdout)

tests/test-basic.t

@@ -49,6 +49,31 b' Writes to stdio succeed and fail appropr'
   [255]
 #endif
 
+On Python 3, stdio may be None:
+
+  $ hg debuguiprompt --config ui.interactive=true 0<&-
+   abort: Bad file descriptor
+  [255]
+  $ hg version -q 0<&-
+  Mercurial Distributed SCM * (glob)
+
+#if py3
+  $ hg version -q 1>&-
+  abort: Bad file descriptor
+  [255]
+#else
+  $ hg version -q 1>&-
+#endif
+  $ hg unknown -q 1>&-
+  hg: unknown command 'unknown'
+  (did you mean debugknown?)
+  [255]
+
+  $ hg version -q 2>&-
+  Mercurial Distributed SCM * (glob)
+  $ hg unknown -q 2>&-
+  [255]
+
   $ hg commit -m test
 
 This command is ancient: