[ python-Bugs-1467080 ] Many functions in socket module are not thread safe

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Bugs item #1467080, was opened at 2006-04-08 18:09
Message generated for change (Comment added) made by sobomax
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Category: None
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Maxim Sobolev (sobomax)
Assigned to: Nobody/Anonymous (nobody)
Summary: Many functions in socket module are not thread safe

Initial Comment:
The socket module make a great effort to be thread-safe, but misses one 
big point - it uses single per-instance buffer to hold resolved 
sockaddr_xxx structures. Therefore, on SMP system it is possible that 
several threads calling functions that perform address resolution in 
parallel will stomp on each other resulting in incorrect or invalid address 
to be used in each case.

For example, two threads calling sendto() in parallel can result in packets 
to be sent to incorrect addresses - packets from thread one from time to 
time will be sent to address requested by thread two and vice versa.

Another, smaller issue is that the call to getnameinfo() is not protected 
with netdb mutex on systems that don't have thread-safe resolver.

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>Comment By: Maxim Sobolev (sobomax)
Date: 2006-04-24 15:31

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OK, it looks like the only way to get this bug fixed is to reimplement patch to 
use stack instead of heap, so that here we go. Attached please find new version 
of the patch which allocates address buffer on stack.

-Maxim

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Comment By: Maxim Sobolev (sobomax)
Date: 2006-04-17 16:05

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> The big win is in simplification of the code.

What do you call "big simplification"? Several malloc/free 
calls and appropriate NULL checks?

Aside from stack usage issues the big loss here is 
portability - we either need to use some fixed length 
buffer with the size sufficient to hold any type of address 
(ugly!) or use sockaddr_storage, which may not exist on all 
platforms supported by the python.

-Maxim

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Comment By: Martin v. Löwis (loewis)
Date: 2006-04-17 15:51

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The big win is in simplification of the code. Also, we are
not talking about 10k here. On Linux, sock_addr_t is 128
bytes. Take a look at set_error: it allocates 100 bytes for
an error buffer. Or sock_repr: 512 bytes for a buffer. Or
socket_gethostname: 1024 bytes. Or socket_gethostbyname_ex:
16384 bytes. socket_getaddrinfo: 30 bytes. os_init: 100 bytes

Or, looking at other modules: symtable.c:check_unoptimized:
300 bytes. Py_GetVersion: 250 bytes. PySys_SetArgv:
2*MAXPATHLEN+1 (on Linux, this is 8193 bytes). I could go on.

Don't worry about stack consumption. Or, if you do, analyse
the Python source code, and fix the big offenders first. 

Premature optimization is the root of all evil.

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Comment By: Maxim Sobolev (sobomax)
Date: 2006-04-17 15:10

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> The address space available to each thread typically 
doesn't
> depend on the number of threads. Instead, the stack size 
is
> pre-determined, so it's vice versa: the number of threads
> supported depends on that stack-size, which (currently)
isn't tunable.

Yes, but my point is that on low-end and/or embedded system 
the system can be configured with as low stack per thread 
as possible (since with for example 100 threads, every 
extra 10K of stack per thread means 1M of extra memory, 
which in the absence of paging needs to be wired down) and 
if only one thread needs this stack 990K of it will be 
effectively wasted. And since getaddrinfo()-family already 
uses heap for its results I don't see any big win in 
avoiding extra malloc()/free() call. Expecially considering 
that we are dealing with i/o here, so that system call 
overhead will be much more than that anyway, even if the 
program calls those functions heavily.

-Maxim

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Comment By: Martin v. Löwis (loewis)
Date: 2006-04-17 14:22

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The argument of "hundreds of threads" is a red herring. The
address space available to each thread typically doesn't
depend on the number of threads. Instead, the stack size is
pre-determined, so it's vice versa: the number of threads
supported depends on that stack-size, which (currently)
isn't tunable. Also, stack space is typically a scarce
resource only for recursive functions. For leave functions,
it doesn't matter, unless a single function consumes the
majority of the stack (which certainly wouldn't be the case
here).

Allocation on the stack is cheap, and over-allocation
doesn't hurt. Please change the patch to use automatic
variables. It will become simpler and more maintainable that
way (in addition, it should also become minimally faster).

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Comment By: Maxim Sobolev (sobomax)
Date: 2006-04-17 13:26

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> Why is it necessary to allocate the memory dynamically?
> Couldn't the caller provide the memory on the stack (i.e.
> through a local variable)?

Local variable is not very good IMHO since in threading 
environment with hundreds of threads running at the same 
time stack can be a scarce resource. Another issue is that 
the caller doesn't know the actual size it has to allocate 
until the resolution has been done, therefore it would need 
to overallocate in most cases.

-Maxim

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Comment By: Martin v. Löwis (loewis)
Date: 2006-04-10 22:22

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Why is it necessary to allocate the memory dynamically?
Couldn't the caller provide the memory on the stack (i.e.
through a local variable)?

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Comment By: Maxim Sobolev (sobomax)
Date: 2006-04-10 17:08

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Sorry, for some reason the patch did not go through first time. See attached.

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Comment By: Martin v. Löwis (loewis)
Date: 2006-04-09 14:18

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I wonder why the sock_addr member is there in the socket
object in the first place. AFAICT, there is no need for it;
it was introduced in r4509.

Would you like to work on a patch?

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