snaprecv: avoid double counting in msstw

We were previously adding the last open SST's DataSize into EstimatedDataSize()
even if that DataSize was already reflected in `msstw.sstSize`. This led to
double-counting and is now fixed.

Author: tbg

pkg/kv/kvserver/kvstorage/snaprecv/multi_sst_writer.go

@@ -321,6 +321,8 @@ func (msstw *MultiSSTWriter) finalizeSST(ctx context.Context, nextKey *storage.E
 	msstw.sstSize += int64(msstw.currSST.Meta.Size)
 	msstw.currSpan++
 	msstw.currSST.Close()
+	// Zero the SSTWriter to avoid double-counting in EstimatedDataSize.
+	msstw.currSST = storage.SSTWriter{}
 	return nil
 }
 

pkg/kv/kvserver/kvstorage/snaprecv/testdata/echotest/TestMultiSSTWriterInitSST_interesting=true

@@ -9,7 +9,7 @@ echo
 >> rangekeyset ["e"-"f")
 >> sstSize=4140 estDataSize=4140
 >> finishing msstw
->> sstSize=5092 estDataSize=5096
+>> sstSize=5092 estDataSize=5092
 >> sst0:
 rangedel: /Local/RangeID/100/r{""-/AbortSpan/"00000000-0000-0000-0000-000000000000"}
 rangedel: /Local/RangeID/100/r/AbortSpan/"{00000000-0000-0000-0000-000000000000"-6ba7b810-9dad-11d1-80b4-00c04fd430c8"}