[Bug 214964] pmcstat -R with an empty sample file waits indefinitely

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214964

            Bug ID: 214964
           Summary: pmcstat -R with an empty sample file waits
                    indefinitely
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: emaste at freebsd.org

I would like to get an automated 1s sample of profiling data and output sampled
calltraces with something like:

timeout 1 pmcstat -P unhalted-cycles -t <pid> -O sample.out
pmcstat -R sample.out -G /dev/stdout

However, if sample.out is empty (because the process is sleeping, say), the
second pmcstat waits indefinitely for data to appear in the sample.out file.

% sleep 3600 &
[1] 52998
% timeout 1 pmcstat -P unhalted-cycles -t 52998 -O sample.out
% pmcstat -R sample.out -G /dev/stdout            
load: 4.81  cmd: pmcstat 54160 [kqread] 1.19r 0.00u 0.00s 0% 2960k

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