On 07/20/2010 03:11 PM, Eric Blake wrote: > On 07/20/2010 03:05 PM, Paul Eggert wrote: >> Staring at the code some more I see another race condition >> that could explain the problem. Suppose the parent, just >> after the last fork, races ahead of the child and jumps >> ahead to the "read at_token" cleanup loop. The parent then >> executes the last "read at_token" cleanup at a point >> where the second-to-the-last child has already output >> its token, but before the second-to-the-last-child has >> closed the fifo. The "read at_token" will then return 0 (because >> it sees end-of-file), but the parent incorrectly thinks >> that it has seen a token and then closes down the fifo >> before the last child gets a chance to write its token. > > Hmm, maybe we should do more validation in the parent - not just read > from the fifo, but validate that we actually read a token. That way, we > can ignore EOF (on the grounds that it may be a temporary condition, and > that more children remain to be run) and only advance when the token we > read back is non-empty.
But for that to work, the token has to be more than a newline (since read strips newlines). So we're back to the QoI issue of whether all shells output a newline as part of their echo (or maybe we switch over to printf - so far, all shells decent enough to support parallel tests also have a working printf(1) builtin). -- Eric Blake ebl...@redhat.com +1-801-349-2682 Libvirt virtualization library http://libvirt.org
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