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)
hgrc settings command line options final result
quiet verbo debug quiet verbo debug quiet verbo debug
0 False False False False False False -> False False False
1 True False False False False False -> True False False
2 False True False False False False -> False True False
3 True True False False False False -> False False False
4 False False True False False False -> False True True
5 True False True False False False -> False True True
6 False True True False False False -> False True True
7 True True True False False False -> False True True
8 False False False True False False -> True False False
9 True False False True False False -> True False False
10 False True False True False False -> True False False
11 True True False True False False -> True False False
12 False False True True False False -> True False False
13 True False True True False False -> True False False
14 False True True True False False -> True False False
15 True True True True False False -> True False False
16 False False False False True False -> False True False
17 True False False False True False -> False True False
18 False True False False True False -> False True False
19 True True False False True False -> False True False
20 False False True False True False -> False True False
21 True False True False True False -> False True False
22 False True True False True False -> False True False
23 True True True False True False -> False True False
24 False False False True True False -> False False False
25 True False False True True False -> False False False
26 False True False True True False -> False False False
27 True True False True True False -> False False False
28 False False True True True False -> False False False
29 True False True True True False -> False False False
30 False True True True True False -> False False False
31 True True True True True False -> False False False
32 False False False False False True -> False True True
33 True False False False False True -> False True True
34 False True False False False True -> False True True
35 True True False False False True -> False True True
36 False False True False False True -> False True True
37 True False True False False True -> False True True
38 False True True False False True -> False True True
39 True True True False False True -> False True True
40 False False False True False True -> False True True
41 True False False True False True -> False True True
42 False True False True False True -> False True True
43 True True False True False True -> False True True
44 False False True True False True -> False True True
45 True False True True False True -> False True True
46 False True True True False True -> False True True
47 True True True True False True -> False True True
48 False False False False True True -> False True True
49 True False False False True True -> False True True
50 False True False False True True -> False True True
51 True True False False True True -> False True True
52 False False True False True True -> False True True
53 True False True False True True -> False True True
54 False True True False True True -> False True True
55 True True True False True True -> False True True
56 False False False True True True -> False True True
57 True False False True True True -> False True True
58 False True False True True True -> False True True
59 True True False True True True -> False True True
60 False False True True True True -> False True True
61 True False True True True True -> False True True
62 False True True True True True -> False True True
63 True True True True True True -> False True True