view mercurial/helptext/color.txt @ 44950:f9734b2d59cc

py3: make stdout line-buffered if connected to a TTY

Status messages that are to be shown on the terminal should be written to the
file descriptor before anything further is done, to keep the user updated.

One common way to achieve this is to make stdout line-buffered if it is
connected to a TTY. This is done on Python 2 (except on Windows, where libc,
which the CPython 2 streams depend on, does not properly support this).

Python 3 rolls it own I/O streams. On Python 3, buffered binary streams can't be
set line-buffered. The previous code (added in 227ba1afcb65) incorrectly
assumed that on Python 3, pycompat.stdout (sys.stdout.buffer) is already
line-buffered. However the interpreter initializes it with a block-buffered
stream or an unbuffered stream (when the -u option or the PYTHONUNBUFFERED
environment variable is set), never with a line-buffered stream.

One example where the current behavior is unacceptable is when running
`hg pull https://www.mercurial-scm.org/repo/hg` on Python 3, where the line
"pulling from https://www.mercurial-scm.org/repo/hg" does not appear on the
terminal before the hg process blocks while waiting for the server.

Various approaches to fix this problem are possible, including:

1. Weaken the contract of procutil.stdout to not give any guarantees about
   buffering behavior. In this case, users of procutil.stdout need to be
   changed to do enough flushes. In particular,
  1. either ui must insert enough flushes for ui.write() and friends, or
  2. ui.write() and friends get split into flushing and fully buffered
     methods, or
  3. users of ui.write() and friends must flush explicitly.
2. Make stdout unbuffered.
3. Make stdout line-buffered. Since Python 3 does not natively support that for
   binary streams, we must implement it ourselves.

(2.) is problematic because using unbuffered I/O changes the performance
characteristics significantly compared to line-buffered (which is used on
Python 2) and this would be a regression.

(1.2.) and (1.3) are a substantial amount of work. It’s unclear whether the
added complexity would be justified, given that raw performance doesn’t matter
that much when writing to a terminal much faster than the user could read it.

(1.1.) pushes complexity into the ui class instead of separating the concern of
how stdout is buffered. Other users of procutil.stdout would still need to take
care of the flushes.

This patch implements (3.). The general performance considerations are very
similar to (1.1.). The extra method invocation and method forwarding add a
little more overhead if the class is used. In exchange, it doesn’t add overhead
if not used.


For the benchmarks, I compared the previous implementation (incorrect on Python
3), (1.1.), (3.) and (2.). The command was chosen so that the streams were
configured as if they were writing to a TTY, but actually write to a pager,
which is also the default:
HGRCPATH=/dev/null python3 ./hg --cwd ~/vcs/mozilla-central --time --pager yes --config pager.pager='cat > /dev/null' status --all

previous:
time: real 7.880 secs (user 7.290+0.050 sys 0.580+0.170)
time: real 7.830 secs (user 7.220+0.070 sys 0.590+0.140)
time: real 7.800 secs (user 7.210+0.050 sys 0.570+0.170)

(1.1.) using Yuya Nishihara’s patch:
time: real 9.860 secs (user 8.670+0.350 sys 1.160+0.830)
time: real 9.540 secs (user 8.430+0.370 sys 1.100+0.770)
time: real 9.830 secs (user 8.630+0.370 sys 1.180+0.840)

(3.) using this patch:
time: real 9.580 secs (user 8.480+0.350 sys 1.090+0.770)
time: real 9.670 secs (user 8.480+0.330 sys 1.170+0.860)
time: real 9.640 secs (user 8.500+0.350 sys 1.130+0.810)

(2.) using a previous patch by me:
time: real 10.480 secs (user 8.850+0.720 sys 1.590+1.500)
time: real 10.490 secs (user 8.750+0.750 sys 1.710+1.470)
time: real 10.240 secs (user 8.600+0.700 sys 1.590+1.510)


As expected, there’s no difference on Python 2, as exactly the same code paths
are used:

previous:
time: real 6.950 secs (user 5.870+0.330 sys 1.070+0.770)
time: real 7.040 secs (user 6.040+0.360 sys 0.980+0.750)
time: real 7.070 secs (user 5.950+0.360 sys 1.100+0.760)

this patch:
time: real 7.010 secs (user 5.900+0.390 sys 1.070+0.730)
time: real 7.000 secs (user 5.850+0.350 sys 1.120+0.760)
time: real 7.000 secs (user 5.790+0.380 sys 1.170+0.710)

Mercurial colorizes output from several commands.

For example, the diff command shows additions in green and deletions
in red, while the status command shows modified files in magenta. Many
other commands have analogous colors. It is possible to customize
these colors.

To enable color (default) whenever possible use::

  [ui]
  color = yes

To disable color use::

  [ui]
  color = no

See :hg:`help config.ui.color` for details.

.. container:: windows

  The default pager on Windows does not support color, so enabling the pager
  will effectively disable color.  See :hg:`help config.ui.paginate` to disable
  the pager.  Alternately, MSYS and Cygwin shells provide `less` as a pager,
  which can be configured to support ANSI color mode.  Windows 10 natively
  supports ANSI color mode.

Mode
====

Mercurial can use various systems to display color. The supported modes are
``ansi``, ``win32``, and ``terminfo``.  See :hg:`help config.color` for details
about how to control the mode.

Effects
=======

Other effects in addition to color, like bold and underlined text, are
also available. By default, the terminfo database is used to find the
terminal codes used to change color and effect.  If terminfo is not
available, then effects are rendered with the ECMA-48 SGR control
function (aka ANSI escape codes).

The available effects in terminfo mode are 'blink', 'bold', 'dim',
'inverse', 'invisible', 'italic', 'standout', and 'underline'; in
ECMA-48 mode, the options are 'bold', 'inverse', 'italic', and
'underline'.  How each is rendered depends on the terminal emulator.
Some may not be available for a given terminal type, and will be
silently ignored.

If the terminfo entry for your terminal is missing codes for an effect
or has the wrong codes, you can add or override those codes in your
configuration::

  [color]
  terminfo.dim = \E[2m

where '\E' is substituted with an escape character.

Labels
======

Text receives color effects depending on the labels that it has. Many
default Mercurial commands emit labelled text. You can also define
your own labels in templates using the label function, see :hg:`help
templates`. A single portion of text may have more than one label. In
that case, effects given to the last label will override any other
effects. This includes the special "none" effect, which nullifies
other effects.

Labels are normally invisible. In order to see these labels and their
position in the text, use the global --color=debug option. The same
anchor text may be associated to multiple labels, e.g.

  [log.changeset changeset.secret|changeset:   22611:6f0a53c8f587]

The following are the default effects for some default labels. Default
effects may be overridden from your configuration file::

  [color]
  status.modified = blue bold underline red_background
  status.added = green bold
  status.removed = red bold blue_background
  status.deleted = cyan bold underline
  status.unknown = magenta bold underline
  status.ignored = black bold

  # 'none' turns off all effects
  status.clean = none
  status.copied = none

  qseries.applied = blue bold underline
  qseries.unapplied = black bold
  qseries.missing = red bold

  diff.diffline = bold
  diff.extended = cyan bold
  diff.file_a = red bold
  diff.file_b = green bold
  diff.hunk = magenta
  diff.deleted = red
  diff.inserted = green
  diff.changed = white
  diff.tab =
  diff.trailingwhitespace = bold red_background

  # Blank so it inherits the style of the surrounding label
  changeset.public =
  changeset.draft =
  changeset.secret =

  resolve.unresolved = red bold
  resolve.resolved = green bold

  bookmarks.active = green

  branches.active = none
  branches.closed = black bold
  branches.current = green
  branches.inactive = none

  tags.normal = green
  tags.local = black bold

  rebase.rebased = blue
  rebase.remaining = red bold

  shelve.age = cyan
  shelve.newest = green bold
  shelve.name = blue bold

  histedit.remaining = red bold

Custom colors
=============

Because there are only eight standard colors, Mercurial allows you
to define color names for other color slots which might be available
for your terminal type, assuming terminfo mode.  For instance::

  color.brightblue = 12
  color.pink = 207
  color.orange = 202

to set 'brightblue' to color slot 12 (useful for 16 color terminals
that have brighter colors defined in the upper eight) and, 'pink' and
'orange' to colors in 256-color xterm's default color cube.  These
defined colors may then be used as any of the pre-defined eight,
including appending '_background' to set the background to that color.