diff mercurial/utils/procutil.py @ 45048:8403cc54bc83

procutil: make mercurial.utils.procutil.stderr unbuffered

For most Mercurial code, it doesn’t make a difference, as the ui object flushes
stderr explicitly (after the change, we could get rid of the explicit flush).
One example where it makes a observable difference is mercurial.util.timed().
Without the patch, the time is not immediately shown on Python 3. With the
patch, it’s shown immediately on all Python versions and platforms.

--- a/mercurial/utils/procutil.py	Sun Jul 05 13:05:06 2020 +0200
+++ b/mercurial/utils/procutil.py	Sun Jul 05 13:09:22 2020 +0200
@@ -99,6 +99,18 @@
     else:
         stdout = os.fdopen(stdout.fileno(), 'wb', 1)
 
+# stderr should be unbuffered
+if pycompat.ispy3:
+    # On Python 3, buffered streams may expose an underlying raw stream. This is
+    # definitively the case for the streams initialized by the interpreter. If
+    # the attribute isn't present, the stream is already unbuffered or doesn't
+    # expose an underlying raw stream, in which case we use the stream as-is.
+    stderr = getattr(stderr, 'raw', stderr)
+elif pycompat.iswindows:
+    # On Windows, stderr is buffered at least when connected to a pipe.
+    stderr = os.fdopen(stderr.fileno(), 'wb', 0)
+# On other platforms, stderr is always unbuffered.
+
 
 findexe = platform.findexe
 _gethgcmd = platform.gethgcmd