procutil: avoid using os.fork() to implement runbgcommand
We ran into the following deadlock:
- some command creates an ssh peer, then raises without explicitly
closing the peer (hg id + extension in our case)
- dispatch catches the exception, calls ui.log('commandfinish', ..)
(the sshpeer is still not closed), which calls logtoprocess, which
calls procutil.runbgcommand.
- in the child of runbgcommand's fork(), between the fork and the
exec, the opening of file descriptors triggers a gc which runs the
destructor for sshpeer, which waits on ssh's stderr being closed,
which never happens since ssh's stderr is held open by the parent of
the fork where said destructor hasn't run
Remotefilelog appears to have a hack around this deadlock as well.
I don't know if there's more subtlety to it, because even though the
problem is determistic, it is very fragile, so I didn't manage to
reduce it.
I can imagine three ways of tackling this problem:
1. don't run any python between fork and exec in runbgcommand
2. make the finalizer harmless after the fork
3. close the peer without relying on gc behavior
This commit goes with 1, as forking without exec'ing is tricky in
general in a language with gc finalizers. And maybe it's better in the
presence of rust threads. A future commit will try 2 or 3.
Performance wise: at low memory usage, it's an improvement. At higher
memory usage, it's about 2x faster than before when ensurestart=True,
but 2x slower when ensurestart=False. Not sure if that matters. The
reason for that last bit is that the subprocess.Popen always waits for
the execve to finish, and at high memory usage, execve is slow because
it deallocates the large page table. Numbers and script:
before after
mem=1.0GB, ensurestart=True 52.1ms 26.0ms
mem=1.0GB, ensurestart=False 14.7ms 26.0ms
mem=0.5GB, ensurestart=True 23.2ms 11.2ms
mem=0.5GB, ensurestart=False 6.2ms 11.3ms
mem=0.2GB, ensurestart=True 15.7ms 7.4ms
mem=0.2GB, ensurestart=False 4.3ms 8.1ms
mem=0.0GB, ensurestart=True 2.3ms 0.7ms
mem=0.0GB, ensurestart=False 0.8ms 0.8ms
import time
for memsize in [1_000_000_000, 500_000_000, 250_000_000, 0]:
mem = 'a' * memsize
for ensurestart in [True, False]:
now = time.time()
n = 100
for i in range(n):
procutil.runbgcommand([b'true'], {}, ensurestart=ensurestart)
after = time.time()
ms = (after - now) / float(n) * 1000
print(f'mem={memsize / 1e9:.1f}GB, ensurestart={ensurestart} -> {ms:.1f}ms')
Differential Revision: https://phab.mercurial-scm.org/D9019
#!/usr/bin/env python3
"""usage: %s DOC ...
where DOC is the name of a document
"""
from __future__ import absolute_import
import os
import sys
import textwrap
try:
import msvcrt
msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
msvcrt.setmode(sys.stderr.fileno(), os.O_BINARY)
except ImportError:
pass
# This script is executed during installs and may not have C extensions
# available. Relax C module requirements.
os.environ['HGMODULEPOLICY'] = 'allow'
# import from the live mercurial repo
sys.path.insert(0, "..")
from mercurial import demandimport
demandimport.enable()
from mercurial import (
commands,
encoding,
extensions,
fancyopts,
help,
minirst,
pycompat,
ui as uimod,
)
from mercurial.i18n import (
gettext,
_,
)
from mercurial.utils import stringutil
table = commands.table
globalopts = commands.globalopts
helptable = help.helptable
loaddoc = help.loaddoc
def get_desc(docstr):
if not docstr:
return b"", b""
# sanitize
docstr = docstr.strip(b"\n")
docstr = docstr.rstrip()
shortdesc = docstr.splitlines()[0].strip()
i = docstr.find(b"\n")
if i != -1:
desc = docstr[i + 2 :]
else:
desc = shortdesc
desc = textwrap.dedent(desc.decode('latin1')).encode('latin1')
return (shortdesc, desc)
def get_opts(opts):
for opt in opts:
if len(opt) == 5:
shortopt, longopt, default, desc, optlabel = opt
else:
shortopt, longopt, default, desc = opt
optlabel = _(b"VALUE")
allopts = []
if shortopt:
allopts.append(b"-%s" % shortopt)
if longopt:
allopts.append(b"--%s" % longopt)
if isinstance(default, list):
allopts[-1] += b" <%s[+]>" % optlabel
elif (default is not None) and not isinstance(default, bool):
allopts[-1] += b" <%s>" % optlabel
if b'\n' in desc:
# only remove line breaks and indentation
desc = b' '.join(l.lstrip() for l in desc.split(b'\n'))
if isinstance(default, fancyopts.customopt):
default = default.getdefaultvalue()
if default:
default = stringutil.forcebytestr(default)
desc += _(b" (default: %s)") % default
yield (b", ".join(allopts), desc)
def get_cmd(cmd, cmdtable):
d = {}
attr = cmdtable[cmd]
cmds = cmd.lstrip(b"^").split(b"|")
d[b'cmd'] = cmds[0]
d[b'aliases'] = cmd.split(b"|")[1:]
d[b'desc'] = get_desc(gettext(pycompat.getdoc(attr[0])))
d[b'opts'] = list(get_opts(attr[1]))
s = b'hg ' + cmds[0]
if len(attr) > 2:
if not attr[2].startswith(b'hg'):
s += b' ' + attr[2]
else:
s = attr[2]
d[b'synopsis'] = s.strip()
return d
def showdoc(ui):
# print options
ui.write(minirst.section(_(b"Options")))
multioccur = False
for optstr, desc in get_opts(globalopts):
ui.write(b"%s\n %s\n\n" % (optstr, desc))
if optstr.endswith(b"[+]>"):
multioccur = True
if multioccur:
ui.write(_(b"\n[+] marked option can be specified multiple times\n"))
ui.write(b"\n")
# print cmds
ui.write(minirst.section(_(b"Commands")))
commandprinter(ui, table, minirst.subsection, minirst.subsubsection)
# print help topics
# The config help topic is included in the hgrc.5 man page.
helpprinter(ui, helptable, minirst.section, exclude=[b'config'])
ui.write(minirst.section(_(b"Extensions")))
ui.write(
_(
b"This section contains help for extensions that are "
b"distributed together with Mercurial. Help for other "
b"extensions is available in the help system."
)
)
ui.write(
(
b"\n\n"
b".. contents::\n"
b" :class: htmlonly\n"
b" :local:\n"
b" :depth: 1\n\n"
)
)
for extensionname in sorted(allextensionnames()):
mod = extensions.load(ui, extensionname, None)
ui.write(minirst.subsection(extensionname))
ui.write(b"%s\n\n" % gettext(pycompat.getdoc(mod)))
cmdtable = getattr(mod, 'cmdtable', None)
if cmdtable:
ui.write(minirst.subsubsection(_(b'Commands')))
commandprinter(
ui,
cmdtable,
minirst.subsubsubsection,
minirst.subsubsubsubsection,
)
def showtopic(ui, topic):
extrahelptable = [
([b"common"], b'', loaddoc(b'common'), help.TOPIC_CATEGORY_MISC),
([b"hg.1"], b'', loaddoc(b'hg.1'), help.TOPIC_CATEGORY_CONFIG),
([b"hg-ssh.8"], b'', loaddoc(b'hg-ssh.8'), help.TOPIC_CATEGORY_CONFIG),
(
[b"hgignore.5"],
b'',
loaddoc(b'hgignore.5'),
help.TOPIC_CATEGORY_CONFIG,
),
([b"hgrc.5"], b'', loaddoc(b'hgrc.5'), help.TOPIC_CATEGORY_CONFIG),
(
[b"hgignore.5.gendoc"],
b'',
loaddoc(b'hgignore'),
help.TOPIC_CATEGORY_CONFIG,
),
(
[b"hgrc.5.gendoc"],
b'',
loaddoc(b'config'),
help.TOPIC_CATEGORY_CONFIG,
),
]
helpprinter(ui, helptable + extrahelptable, None, include=[topic])
def helpprinter(ui, helptable, sectionfunc, include=[], exclude=[]):
for h in helptable:
names, sec, doc = h[0:3]
if exclude and names[0] in exclude:
continue
if include and names[0] not in include:
continue
for name in names:
ui.write(b".. _%s:\n" % name)
ui.write(b"\n")
if sectionfunc:
ui.write(sectionfunc(sec))
if callable(doc):
doc = doc(ui)
ui.write(doc)
ui.write(b"\n")
def commandprinter(ui, cmdtable, sectionfunc, subsectionfunc):
"""Render restructuredtext describing a list of commands and their
documentations, grouped by command category.
Args:
ui: UI object to write the output to
cmdtable: a dict that maps a string of the command name plus its aliases
(separated with pipes) to a 3-tuple of (the command's function, a list
of its option descriptions, and a string summarizing available
options). Example, with aliases added for demonstration purposes:
'phase|alias1|alias2': (
<function phase at 0x7f0816b05e60>,
[ ('p', 'public', False, 'set changeset phase to public'),
...,
('r', 'rev', [], 'target revision', 'REV')],
'[-p|-d|-s] [-f] [-r] [REV...]'
)
sectionfunc: minirst function to format command category headers
subsectionfunc: minirst function to format command headers
"""
h = {}
for c, attr in cmdtable.items():
f = c.split(b"|")[0]
f = f.lstrip(b"^")
h[f] = c
cmds = h.keys()
def helpcategory(cmd):
"""Given a canonical command name from `cmds` (above), retrieve its
help category. If helpcategory is None, default to CATEGORY_NONE.
"""
fullname = h[cmd]
details = cmdtable[fullname]
helpcategory = details[0].helpcategory
return helpcategory or help.registrar.command.CATEGORY_NONE
cmdsbycategory = {category: [] for category in help.CATEGORY_ORDER}
for cmd in cmds:
# If a command category wasn't registered, the command won't get
# rendered below, so we raise an AssertionError.
if helpcategory(cmd) not in cmdsbycategory:
raise AssertionError(
"The following command did not register its (category) in "
"help.CATEGORY_ORDER: %s (%s)" % (cmd, helpcategory(cmd))
)
cmdsbycategory[helpcategory(cmd)].append(cmd)
# Print the help for each command. We present the commands grouped by
# category, and we use help.CATEGORY_ORDER as a guide for a helpful order
# in which to present the categories.
for category in help.CATEGORY_ORDER:
categorycmds = cmdsbycategory[category]
if not categorycmds:
# Skip empty categories
continue
# Print a section header for the category.
# For now, the category header is at the same level as the headers for
# the commands in the category; this is fixed in the next commit.
ui.write(sectionfunc(help.CATEGORY_NAMES[category]))
# Print each command in the category
for f in sorted(categorycmds):
if f.startswith(b"debug"):
continue
d = get_cmd(h[f], cmdtable)
ui.write(subsectionfunc(d[b'cmd']))
# short description
ui.write(d[b'desc'][0])
# synopsis
ui.write(b"::\n\n")
synopsislines = d[b'synopsis'].splitlines()
for line in synopsislines:
# some commands (such as rebase) have a multi-line
# synopsis
ui.write(b" %s\n" % line)
ui.write(b'\n')
# description
ui.write(b"%s\n\n" % d[b'desc'][1])
# options
opt_output = list(d[b'opts'])
if opt_output:
opts_len = max([len(line[0]) for line in opt_output])
ui.write(_(b"Options:\n\n"))
multioccur = False
for optstr, desc in opt_output:
if desc:
s = b"%-*s %s" % (opts_len, optstr, desc)
else:
s = optstr
ui.write(b"%s\n" % s)
if optstr.endswith(b"[+]>"):
multioccur = True
if multioccur:
ui.write(
_(
b"\n[+] marked option can be specified"
b" multiple times\n"
)
)
ui.write(b"\n")
# aliases
if d[b'aliases']:
# Note the empty comment, this is required to separate this
# (which should be a blockquote) from any preceding things (such
# as a definition list).
ui.write(
_(b"..\n\n aliases: %s\n\n") % b" ".join(d[b'aliases'])
)
def allextensionnames():
return set(extensions.enabled().keys()) | set(extensions.disabled().keys())
if __name__ == "__main__":
doc = b'hg.1.gendoc'
if len(sys.argv) > 1:
doc = encoding.strtolocal(sys.argv[1])
ui = uimod.ui.load()
# Trigger extensions to load. This is disabled by default because it uses
# the current user's configuration, which is often not what is wanted.
if encoding.environ.get(b'GENDOC_LOAD_CONFIGURED_EXTENSIONS', b'0') != b'0':
extensions.loadall(ui)
if doc == b'hg.1.gendoc':
showdoc(ui)
else:
showtopic(ui, encoding.strtolocal(sys.argv[1]))