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From "Bert Huijben" <b...@qqmail.nl>
Subject RE: thread safe
Date Mon, 03 Sep 2012 19:09:33 GMT


> -----Original Message-----
> From: Stefan K√ľng [mailto:tortoisesvn@gmail.com]
> Sent: maandag 3 september 2012 20:48
> To: Subversion Development
> Subject: thread safe
> 
> Hi,
> 
> It seems that the svn_config_t structure isn't thread safe, i.e. can't
> be shared among multiple threads.
> See here for a detailed report on what problems this causes:
> http://tortoisesvn.tigris.org/ds/viewMessage.do?dsForumId=757&dsMessa
> geId=3003152
> 
> Is this by design? I assumed that all svn APIs are thread safe for read
> access at least unless noted otherwise (example: the API
> svn_utf_initialize states its thread-unsafeness in the note section of
> the docs).

I always assume(d) that all functions that are not documented otherwise are
thread safe, but that they also assume that the data is not passed to
multiple functions in multiple threads at once. 

So mostly the same thing as what the C runtime or the .Net runtime promise:
Static functions are thread safe, but instances and functions on those are
not.

In my testing via SharpSvn (and the bug reports over the years) this mostly
proved safe. (There were a few issues fixed in early 1.6 development).


In SharpSvn I keep a config instance alive per SvnClient. And in AnkhSVN I
keep a small pool of recently used SvnClient instances at hand to avoid
re-reading the config files over and over again.

This keeps svn_client_context_t and everything below thread safe as an
SvnClient is documented to be only usable in one thread at a time.

> While we can work around that thread problem a little bit, that
> workaround has its own big problems:
> * each thread would need its own svn_config_t structure
>    --> for each thread the config file is read (open, read, close)
>    which results in multiple unnecessary disk accesses
>    --> when the config file is read multiple times, sometimes the read
>    fails (at least on Windows) because the file is opened already -
>    usually because virus scanners open and scan every file that a
>    process accesses, even when only for reading.

I think this is the proper way to fix this issue at your side: keep data
thread safe.

How are you handling the authentication cache, and other settings. I would
just keep it safe for future extension.

> Joel Jirak made a patch for subversion/libsvn_subr/config.c which would
> fix the problem (see the thread linked above). Would that be an
> acceptable solution?

The patch on
http://tortoisesvn.tigris.org/ds/getDSMessageAttachment.do/config.c.hack-fix
.patch?dsForumId=757&dsMessageId=3003152&dsAttachmentId=3606977&dsAttachment
Mime=application/octet-stream
creates a string in a very short living thread pool, in a static function
with a few callers that all have a pool available.

With a bit of cleanup it should be possible to apply this patch with a
proper scratch sub-pool and I don't see why such a patch wouldn't be
accepted.


But I don't think that this will be the solution to make the rest of
Subversion thread safe, or to guarantee that all other memory structures
will be thread safe in the future. (But it might make a read-once config
struct reusable in multiple threads)

	Bert 


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