dispatch: force \n for newlines on sys.std* streams (BC)

dispatch: force \n for newlines on sys.std* streams (BC)

The sys.std* streams behave differently on Python 3. On Python 3,
these streams are an io.TextIOWrapper that wraps a binary buffer
stored on a .buffer attribute. These TextIOWrapper instances
normalize \n to os.linesep by default. On Windows, this means
that \n is normalized to \r\n. So functions like print() which
have an implicit end='\n' will actually emit \r\n for line endings.

While most parts of Mercurial go through the ui.write() layer to
print output, some code - notably in extensions and hooks - can use
print(). If this code was using print() or otherwise writing to
sys.std* on Windows, Mercurial would emit \r\n.

In reality, pretty much everything on Windows reacts to \n just fine.
Mercurial itself doesn't emit \r\n when going through the ui layer.
Changing the sys.std* streams to not normalize line endings sounds
like a scary change. But I think it is safe. It also makes Mercurial
on Python 3 behave similarly to Python 2, which did not perform \r\n
normalization in print() by default.

.. bc:: sys.{stdout, stderr, stdin} now use \n line endings on Python 3

Differential Revision: https://phab.mercurial-scm.org/D8339

--- a/mercurial/dispatch.py	Sun Mar 29 11:58:50 2020 -0700
+++ b/mercurial/dispatch.py	Sun Mar 29 13:06:59 2020 -0700
@@ -10,6 +10,7 @@
 import difflib
 import errno
 import getopt
+import io
 import os
 import pdb
 import re
@@ -144,7 +145,50 @@
 if pycompat.ispy3:
 
     def initstdio():
-        pass
+        # stdio streams on Python 3 are io.TextIOWrapper instances proxying another
+        # buffer. These streams will normalize \n to \r\n by default. Mercurial's
+        # preferred mechanism for writing output (ui.write()) uses io.BufferedWriter
+        # instances, which write to the underlying stdio file descriptor in binary
+        # mode. ui.write() uses \n for line endings and no line ending normalization
+        # is attempted through this interface. This "just works," even if the system
+        # preferred line ending is not \n.
+        #
+        # But some parts of Mercurial (e.g. hooks) can still send data to sys.stdout
+        # and sys.stderr. They will inherit the line ending normalization settings,
+        # potentially causing e.g. \r\n to be emitted. Since emitting \n should
+        # "just work," here we change the sys.* streams to disable line ending
+        # normalization, ensuring compatibility with our ui type.
+
+        # write_through is new in Python 3.7.
+        kwargs = {
+            "newline": "\n",
+            "line_buffering": sys.stdout.line_buffering,
+        }
+        if util.safehasattr(sys.stdout, "write_through"):
+            kwargs["write_through"] = sys.stdout.write_through
+        sys.stdout = io.TextIOWrapper(
+            sys.stdout.buffer, sys.stdout.encoding, sys.stdout.errors, **kwargs
+        )
+
+        kwargs = {
+            "newline": "\n",
+            "line_buffering": sys.stderr.line_buffering,
+        }
+        if util.safehasattr(sys.stderr, "write_through"):
+            kwargs["write_through"] = sys.stderr.write_through
+        sys.stderr = io.TextIOWrapper(
+            sys.stderr.buffer, sys.stderr.encoding, sys.stderr.errors, **kwargs
+        )
+
+        # No write_through on read-only stream.
+        sys.stdin = io.TextIOWrapper(
+            sys.stdin.buffer,
+            sys.stdin.encoding,
+            sys.stdin.errors,
+            # None is universal newlines mode.
+            newline=None,
+            line_buffering=sys.stdin.line_buffering,
+        )
 
     def _silencestdio():
         for fp in (sys.stdout, sys.stderr):