view tests/test-show-stack.t @ 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)

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > show =
  > EOF

  $ hg init repo0
  $ cd repo0

Empty repo / no checkout results in error

  $ hg show stack
  abort: stack view only available when there is a working directory
  [255]

Stack displays single draft changeset as root revision

  $ echo 0 > foo
  $ hg -q commit -A -m 'commit 0'
  $ hg show stack
    @  9f17 commit 0

Stack displays multiple draft changesets

  $ echo 1 > foo
  $ hg commit -m 'commit 1'
  $ echo 2 > foo
  $ hg commit -m 'commit 2'
  $ echo 3 > foo
  $ hg commit -m 'commit 3'
  $ echo 4 > foo
  $ hg commit -m 'commit 4'
  $ hg show stack
    @  2737 commit 4
    o  d1a6 commit 3
    o  128c commit 2
    o  181c commit 1
    o  9f17 commit 0

Public parent of draft base is displayed, separated from stack

  $ hg phase --public -r 0
  $ hg show stack
    @  2737 commit 4
    o  d1a6 commit 3
    o  128c commit 2
    o  181c commit 1
   /   (stack base)
  o  9f17 commit 0

  $ hg phase --public -r 1
  $ hg show stack
    @  2737 commit 4
    o  d1a6 commit 3
    o  128c commit 2
   /   (stack base)
  o  181c commit 1

Draft descendants are shown

  $ hg -q up 2
  $ hg show stack
    o  2737 commit 4
    o  d1a6 commit 3
    @  128c commit 2
   /   (stack base)
  o  181c commit 1

  $ hg -q up 3
  $ hg show stack
    o  2737 commit 4
    @  d1a6 commit 3
    o  128c commit 2
   /   (stack base)
  o  181c commit 1

working dir on public changeset should display special message

  $ hg -q up 1
  $ hg show stack
  (empty stack; working directory parent is a published changeset)

Branch point in descendants displayed at top of graph

  $ hg -q up 3
  $ echo b > foo
  $ hg commit -m 'commit 5 (new dag branch)'
  created new head
  $ hg -q up 2
  $ hg show stack
   \ /  (multiple children)
    |
    o  d1a6 commit 3
    @  128c commit 2
   /   (stack base)
  o  181c commit 1

  $ cd ..

Base is stopped at merges

  $ hg init merge-base
  $ cd merge-base
  $ echo 0 > foo
  $ hg -q commit -A -m initial
  $ echo h1 > foo
  $ hg commit -m 'head 1'
  $ hg -q up 0
  $ echo h2 > foo
  $ hg -q commit -m 'head 2'
  $ hg phase --public -r 0:tip
  $ hg -q up 1
  $ hg merge -t :local 2
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg commit -m 'merge heads'

TODO doesn't yet handle case where wdir is a draft merge

  $ hg show stack
    @  8ee9 merge heads
   /   (stack base)
  o  5947 head 1

  $ echo d1 > foo
  $ hg commit -m 'draft 1'
  $ echo d2 > foo
  $ hg commit -m 'draft 2'

  $ hg show stack
    @  430d draft 2
    o  787b draft 1
   /   (stack base)
  o  8ee9 merge heads

  $ cd ..

Now move on to stacks when there are more commits after the base branchpoint

  $ hg init public-rebase
  $ cd public-rebase
  $ echo 0 > foo
  $ hg -q commit -A -m 'base'
  $ hg phase --public -r .
  $ echo d1 > foo
  $ hg commit -m 'draft 1'
  $ echo d2 > foo
  $ hg commit -m 'draft 2'
  $ hg -q up 0
  $ echo 1 > foo
  $ hg commit -m 'new 1'
  created new head
  $ echo 2 > foo
  $ hg commit -m 'new 2'
  $ hg -q up 2

Newer draft heads don't impact output

  $ hg show stack
    @  eaff draft 2
    o  2b21 draft 1
   /   (stack base)
  o  b66b base

Newer public heads are rendered

  $ hg phase --public -r '::tip'

  $ hg show stack
    o  baa4 new 2
   /    (2 commits ahead)
  :
  :    (stack head)
  : @  eaff draft 2
  : o  2b21 draft 1
  :/   (stack base)
  o  b66b base

If rebase is available, we show a hint how to rebase to that head

  $ hg --config extensions.rebase= show stack
    o  baa4 new 2
   /    (2 commits ahead; hg rebase --source 2b21 --dest baa4)
  :
  :    (stack head)
  : @  eaff draft 2
  : o  2b21 draft 1
  :/   (stack base)
  o  b66b base

Similar tests but for multiple heads

  $ hg -q up 0
  $ echo h2 > foo
  $ hg -q commit -m 'new head 2'
  $ hg phase --public -r .
  $ hg -q up 2

  $ hg show stack
    o  baa4 new 2
   /    (2 commits ahead)
  : o  9a84 new head 2
  :/    (1 commits ahead)
  :
  :    (stack head)
  : @  eaff draft 2
  : o  2b21 draft 1
  :/   (stack base)
  o  b66b base

  $ hg --config extensions.rebase= show stack
    o  baa4 new 2
   /    (2 commits ahead; hg rebase --source 2b21 --dest baa4)
  : o  9a84 new head 2
  :/    (1 commits ahead; hg rebase --source 2b21 --dest 9a84)
  :
  :    (stack head)
  : @  eaff draft 2
  : o  2b21 draft 1
  :/   (stack base)
  o  b66b base