Oh, I agree it's just a warning, and I suspect few people look into warnings, but since it's not from an actual test, I'm not sure the overall build should be flagged.

The manual typeperf looks fine, but there's no way I could tell visually how the I/O is being done under the covers.  My bet is typeperf is probably issuing multiple output calls, one for the leading timestamp and then one for each value.  So regrtest just happens to interleave its own read in between the two.  The subsequent read would then get the trailing ",####" which the current code would parse normally as it only cares about the second field.  In which case it's not even affecting the load monitoring, other than the warning message.

I suspect it's happening on the Windows 10 buildbot as the machine is reasonably fast.  But to your earlier comment about things just working previously, I'm not sure there's that much value in adding code to deal with this case.

I'd probably just remove the warning to restore the earlier behavior.  The current code still ignores other issues like an actual read failure in read_output.  Or, if there's a way to generate the message without it being considered an overall test build warning, that would work too.