You could just do a pthread_sigmask() before and after the pthread_setspecific() to guarantee that no SIGPIPE will arrive on that thread in that time. I think it's pretty safe to assume that as long as you're not doing a pthread_[gs]etspecific() on that same pthread_key_t, it's safe.
Actually, thinking about this a bit more, that might not even be necessary. Is SIGPIPE-via-(read|write) synchronous or asynchronous? (I.e., is the SIGPIPE guaranteed to arrive during the offending system call?) I was thinking not, but maybe yes. I can't seem to find a straight answer. A lot of documents seem to confuse thread-directed and synchronous, when they're not quite the same thing. SIGALRM-via-alarm() is thread-directed but obviously asynchronous.
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