[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

Here's a retelling of this bug report as a silly fantasy saga:

https://engineering.mongodb.com/post/the-saga-of-concurrent-dns-in-python-and-the-defeat-of-the-wicked-mutex-troll/

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

Related to #26406, a fix for NetBSD and OpenBSD.

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Ned Deily]

Ned Deily added the comment:

I've committed a revised version of the "try 2" patch for release in 2.7.12, 
3.5.2, and 3.6.0.  The revisions were to better follow the somewhat arcane 
conventions of Apple's availability macros, in particular, to take into account 
the value of MACOSX_DEPLOYMENT_TARGET.  I didn't have an opportunity to test 
building it on an actual 10.4 (or 10.3 :) system but I did test with various 
settings of MACOSX_DEPLOYMENT_TARGET.  Thanks very much, Jesse, for the 
research and the patch and to everyone else for the reviews and comments.

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Roundup Robot]

Roundup Robot added the comment:

New changeset 58ebfa7c1361 by Ned Deily in branch '2.7':
Issue #25924: Avoid unnecessary serialization of getaddrinfo(3) calls on OS X
https://hg.python.org/cpython/rev/58ebfa7c1361

New changeset 86ddb4d747f8 by Ned Deily in branch '3.5':
Issue #25924: Avoid unnecessary serialization of getaddrinfo(3) calls on OS X
https://hg.python.org/cpython/rev/86ddb4d747f8

New changeset caca2b354773 by Ned Deily in branch 'default':
Issue #25924: merge with 3.5
https://hg.python.org/cpython/rev/caca2b354773

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

I think we have consensus for "try 2". I'm not a core dev, would one of you 
please merge 25924-getaddrinfo-is-thread-safe-2.patch? Thanks!

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

Martin, here's a third patch, "try 3", which does a runtime version check 
instead of compile-time. It's a bit complex, compared to "try 2", which did a 
compile-time check instead.

I do NOT think this extra complexity is worth it to support Pythons that are 
built on 10.5+ and running on Mac OS 10.4 and older. 10.4 is long obsolete.

I propose merging "try 2" with the compile-time check. If that patch is 
accepted, I'll submit a patch for Python 2.7 also.

Thank you!

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Added file: 
http://bugs.python.org/file41878/25924-getaddrinfo-is-thread-safe-3.patch

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Ned Deily]

Ned Deily added the comment:

I agree that the approach in "try 2" is fine and the runtime check in "try 3" 
is overkill.  While it is possible to do so, we've never really supported 
building on an OS X release n for release m, where m < n, without using the m 
SDK on n (and in particular for m=10.4) in which case the "try 2" test should 
work correctly.  Ronald, what do you think?

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Martin Panter]

Martin Panter added the comment:

I am happy to defer to Ned who probably knows a lot more than me about the OS X 
situation.

FTR it was the code added in 2006 by r45660 (Issue 1471925) that motivated my 
concern. Since that was added by Ronald, it would be good to hear his opinion :)

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

Thanks Martin. The MAC_OS_X_VERSION_10_5 macro ensures that Python is still 
compatible with Mac OS before version 10.5: if it's built on 10.4 or older, it 
locks around getaddrinfo (since it's not thread-safe there), and on 10.5 and 
later it doesn't lock.

I've built on a Mac OS 10.4 virtual machine and verified we'll still use the 
lock on that OS.

I think this is good enough, without a runtime check. We distribute prebuilt 
binaries for Mac OS 10.5+:

https://www.python.org/downloads/release/python-2711/

So those prebuilt Pythons should no longer lock around getaddrinfo. 

People who still use older Mac OSes (more than 12 years old!) will have to 
build Python themselves in order to get recent versions of Python, since we 
don't distribute binaries for them any more. Since they're building Python on 
old Mac OS, the MAC_OS_X_VERSION_10_5 macro will be undefined and they'll keep 
using the getaddrinfo lock.

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Yury Selivanov]

Yury Selivanov added the comment:

Hi Jesse, could you please update your patch with a detailed comment 
summarizing your research?  Also it would be great if you can provide a 
separate patch for 2.7.

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

Second patch with a comment about how we know Mac OS 10.5+'s getaddrinfo is 
thread-safe. I'll wait for this to be merged before submitting another for 
Python 2.7.

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Added file: 
http://bugs.python.org/file41784/25924-getaddrinfo-is-thread-safe-2.patch

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

Great, how do we ensure this gets merged soon?

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Guido van Rossum]

Guido van Rossum added the comment:

Find an active developer who cares -- e.g. Martin Panter, Serhiy, Yury.

On Tue, Feb 2, 2016 at 5:46 AM, A. Jesse Jiryu Davis
 wrote:
>
> A. Jesse Jiryu Davis added the comment:
>
> Great, how do we ensure this gets merged soon?
>
> --
>
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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Martin Panter]

Martin Panter added the comment:

Hello Jesse. Maybe Yury might be more qualified here than me to handle OS X 
stuff (correct me if I’m wrong). I mainly just work with Linux.

But reading around socketmodule.c, it sounds like we want to be able to build 
it on newer OS versions, but still have it compatible with older versions. If 
so, how does that MAC_OS_X_VERSION_10_5 macro work? Would some run-time check 
also be needed?

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

I've created a Mac OS 10.4 virtual machine and reproduced the getaddrinfo 
concurrency bug there using the attached h_resolv.c. The man page on that OS 
version indeed includes the "not thread-safe" warning.

The same test passes on my Mac OS 10.10 system.

I am convinced that getaddrinfo is thread-safe on Mac OS 10.5+, and I'm 
attaching a patch to disable the lock on these systems.

--
keywords: +patch
Added file: 
http://bugs.python.org/file41755/25924-getaddrinfo-is-thread-safe.patch

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Guido van Rossum]

Guido van Rossum added the comment:

Thanks for the thorough work! Hopefully we can apply this fix to 3.5, 3.6 and 
even 2.7.

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

An Apple Developer Relations engineer tells me it's "reasonable to assume that 
getaddrinfo() is thread safe" on OS X 10.5 and later. (He mentioned that iOS 
inherited the OS X 10.5 DNS architecture, so Apple phones, watches, TVs, 
hairdryers, etc. can all do concurrent DNS too.)

Before OS X 10.5 the DNS system was a mix of new Mac OS X features, 
mDNSResponder and Open Directory, along with historical lookupd from NeXT and 
libresolv from an old BSD. In 10.5 the system was cleaned up to depend on OS 
X's mDNSResponder consistently; in the process getaddrinfo became thread-safe.

He cites as further evidence of its thread safety:

* The "bugs" section of getaddrinfo's man page was removed in 10.5
* 10.5 is the first OS X to be UNIX '03 certified, which includes the 
requirement that getaddrinfo be thread-safe
* The system itself uses getaddrinfo extensively as if it's thread-safe
* He hasn't seen reports of thread-safety problems with getaddrinfo

He explained how the code works. In the latest version:

http://www.opensource.apple.com/source/Libinfo/Libinfo-476/lookup.subproj/libinfo.c

getaddrinfo calls down to libinfo’s "mdns" module:

http://www.opensource.apple.com/source/Libinfo/Libinfo-476/lookup.subproj/mdns_module.c

"mdns" uses the DNS-SD API declared in dns_sd.h, which he says is "well known 
to be thread safe." The DNS-SD API is part of the mDNSResponder project:

http://www.opensource.apple.com/source/mDNSResponder/mDNSResponder-576.30.4/

The key function is DNSServiceQueryRecord:

http://www.opensource.apple.com/source/mDNSResponder/mDNSResponder-576.30.4/mDNSShared/dnssd_clientstub.c

My contact concludes, "As you can see, it does an IPC over to the mDNSResponder 
process, at which point thread safety is assured."

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

Related to #1288833, when FreeBSD 5.3's getaddrinfo was declared thread-safe.

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

Changes by A. Jesse Jiryu Davis :


Added file: http://bugs.python.org/file41597/d_mach

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

Changes by A. Jesse Jiryu Davis :


Added file: http://bugs.python.org/file41596/h_resolv.c

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

NetBSD has a concurrency test for getaddrinfo, so I tried it on Mac to see if 
getaddrinfo is thread-safe here, too. It appears that it is thread-safe.

I copied NetBSD's getaddrinfo concurrency test "h_resolv.c" and the test's data 
file "d_mach" from src/tests/lib/libpthread in the NetBSD 7 source:

http://cvsweb.netbsd.org/bsdweb.cgi/src/tests/lib/libpthread/

I've also attached these files to this ticket.*

The test program h_resolv.c compiles fine on Mac OS X 10.10 with clang 7, and 
the test passes using the same parameters that the NetBSD test uses, resolving 
five host names on five threads:

./h_resolv -d -n 5 -h 5 d_mach

There's even evidence that getaddrinfo is actually concurrent on my Mac: 
resolving 100 hostnames on 1 thread takes about 6 seconds, but using 100 
threads take only 3 seconds. To test concurrency I ran this:

sudo dscacheutil -flushcache
sudo killall -HUP mDNSResponder
time h_resolv -d -h 100 -n 1 d_mach

sudo dscacheutil -flushcache
sudo killall -HUP mDNSResponder
time h_resolv -d -h 1 -n 100 d_mach

Those sudo commands flush the Mac's DNS cache between runs.

* I removed one unresolvable domain, "cnftp.bjpu.edu.cn", from the test file 
d_mach. That domain took 15 seconds to time out and made the rest of the timing 
irrelevant.

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[A. Jesse Jiryu Davis]

A. Jesse Jiryu Davis added the comment:

In Apple's Libinfo version 222.4.12 (corresponding to the last OS X 10.4 
release), the man page says getaddrinfo isn't thread-safe:

http://www.opensource.apple.com/source/Libinfo/Libinfo-222.4.12/lookup.subproj/getaddrinfo.3

And here's its source:

http://www.opensource.apple.com/source/Libinfo/Libinfo-222.4.12/lookup.subproj/getaddrinfo.c

Glancing at the source naïvely, I might see the data race: getaddrinfo calls 
gai_lookupd, which reads and writes the global static variable "gai_proc". I 
can't see what will go wrong as a result of the race, but it sure LOOKS bad.

In the next release, version 278 (OS X 10.5.0), the thread-safety warning is 
gone from the man page:

http://www.opensource.apple.com/source/Libinfo/Libinfo-278/lookup.subproj/getaddrinfo.3

And getaddrinfo is largely rewritten:

http://www.opensource.apple.com/source/Libinfo/Libinfo-278/lookup.subproj/getaddrinfo.c

It calls a new function, "ds_getaddrinfo". But ds_getaddrinfo still accesses 
the global static variable "gai_proc"; I wonder why this is considered 
thread-safe now?

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[issue25924] investigate if getaddrinfo(3) on OSX is thread-safe

[Ronald Oussoren]

New submission from Ronald Oussoren:

https://emptysqua.re/blog/getaddrinfo-deadlock/ claims that getaddrinfo may 
deadlock when using threads and fork on (amongst others) OSX due to using a 
global lock.

That lock is used when getaddrinfo is believed to be not thread safe, see the 
relevant code below (from the blog post):

/* On systems on which getaddrinfo() is believed to not be thread-safe,
   (this includes the getaddrinfo emulation) protect access with a lock. */
#if defined(WITH_THREAD) && (defined(__APPLE__) || \
(defined(__FreeBSD__) && __FreeBSD_version+0 < 503000) || \
defined(__OpenBSD__) || defined(__NetBSD__) || \
defined(__VMS) || !defined(HAVE_GETADDRINFO))
#define USE_GETADDRINFO_LOCK
#endif

I think it is worthwhile to investigate whether or not getaddrinfo on OSX is 
really not thread safe. 

Some source code for OSX can be found at the link below, I haven't checked yet 
which OSX release this corresponds to:

http://www.opensource.apple.com/source/Libinfo/Libinfo-278/lookup.subproj/getaddrinfo.c

--
assignee: ronaldoussoren
components: Macintosh
messages: 256838
nosy: ned.deily, ronaldoussoren
priority: low
severity: normal
status: open
title: investigate if getaddrinfo(3) on OSX is thread-safe
type: enhancement
versions: Python 2.7, Python 3.5, Python 3.6

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