[Boost-build] concurrent build processes both over samba share and local influence each other

Subject: [Boost-build] concurrent build processes both over samba share and local influence each other
From: Moritz Hassert (mhassert_at_[hidden])
Date: 2010-12-10 06:31:17

Hi all,

from time to time I encounter a strange effect when using boost-build over a 
samba share:

I share my local disk with a windows machine over samba.

I connect to the windows box remotely, go to the source tree shared from my 
linux box and start a bjam build using mingw. Because I don't do that very 
often lots of files have changed and bjam has to rebuild almost the whole 
project.

While that build is still running on the remote windows box I start another 
build on my local linux box using native gcc. This build should only update 
one or two files I changed a few minutes ago. But sometimes when the windows 
build is still running in parallel bjam decides to rebuild almost everything 
in the project.

Note that both build processes should be _reading_ the same source files but 
should be _writing_ only to their different build directory subtrees. So it 
seems very strange that they influence each other in any way.

If I wait for the windows build to finish before I start the linux build, 
everything is fine. So apparently the windows build doesn't change any file 
timestamps.

Anyone any ideas why this is happening?

I can think of two possible explananations:

a) Could it be that bjam considers locked *cpp/*h files as being modified? That 
makes sense for read locked files as bjam isn't allowed to look into them for 
further dependencies and therefore cannot guarantee nothing changed. But 
write-locked files should be no problem.
Maybe bjam/gcc/samba create unnecessary strict locks on windows?
Or bjam opens source files in rw mode?

b) bjam creates some kind of lock files or other temporary files in the source 
tree outside the build subtree that clash when several concurrent bjam 
processes are running.

Note:
The Problem could be all samba/gcc 's fault. Non the less I hope to find 
someone with a similar experience on this list as I hope more people here do 
cross platform builds.

Greetings
Moritz

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