[BUGFIX] TSDB: Head compaction race condition

There was a small time window where things can go wrong, if a query
starts after `WaitForPendingReadersInTimeRange` returns, but before
`minTime` is stored.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/head.go

@@ -1149,14 +1149,16 @@ func (h *Head) truncateMemory(mint int64) (err error) {
 		h.memTruncationCallBack()
 	}
 
+	// Set the minTime that new queries will see, before waiting for queries to finish.
+	h.minTime.Store(mint)
+	// Don't allow samples to be added before this time.
+	h.minValidTime.Store(mint)
+
 	// We wait for pending queries to end that overlap with this truncation.
 	if initialized {
 		h.WaitForPendingReadersInTimeRange(h.MinTime(), mint)
 	}
 
-	h.minTime.Store(mint)
-	h.minValidTime.Store(mint)
-
 	// Ensure that max time is at least as high as min time.
 	for h.MaxTime() < mint {
 		h.maxTime.CompareAndSwap(h.MaxTime(), mint)