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blog for the day

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Joey Hess 2012-08-09 16:02:16 -04:00
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Probably won't be doing any big coding on the git-annex assistant in the
upcoming week, as I'll be traveling and/or slightly ill enough that I can't
fully get into [flow](http://en.wikipedia.org/wiki/Flow_(psychology)).
---
There was a new Yesod release this week, which required minor changes to
make the webapp build with it. I managed to keep the old version of Yesod
also supported, and plan to keep that working so it can be built with the
version of Yesod available in, eg, Linux distributions. TBD how much pain
that will involve going forward.
---
I'm mulling over how to support stopping/pausing transfers. The problem
is that if the assistant is running a transfer in one thread, and the
webapp is used to cancel it, killing that thread won't necessarily stop the
transfer, because, at least in Haskell's thread model, killing a thread
does not kill processes started by the thread (like rsync).
So one option is to have the transfer thread run a separate git-annex
process, which will run the actual transfer. And killing that process will
stop the transfer nicely. However, using a separate git-annex process means
a little startup overhead for each file transferred (I don't know if it'd
be enough to matter). Also, there's the problem that git-annex is sometimes
not installed in PATH (wish I understood why cabal does that), which
makes it kind of hard for it to run itself. (It can't simply fork, sadly.
See past horrible pain with forking and threads.)
The other option is to change the API for git-annex remotes, so that
their `storeKey` and `retrieveKeyFile` methods return a pid of the program
that they run. When they *do* run a program.. not all remotes do. This
seems like it'd make the code in the remotes hairier, and it is also asking
for bugs, when a remote's implementation changes. Or I could go
lower-level, and make every place in the utility libraries that forks a
process record its pid in a per-thread MVar. Still seems to be asking for
bugs.
Oh well, at least git-annex is already crash-safe, so once I figure out
how to kill a transfer process, I can kill it safely. :)