blob: fix ValueFetcher value index lookup

Previously the ValueFetcher did not correctly translate the ValueID of a
blob.Handle to the index of the value in the block if the blob file was
rewritten. This commit fixes the bug. Unit tests covering this case will be
added in future commits.

Author: jbowens

sstable/blob/fetcher.go

@@ -291,8 +291,14 @@ func (cr *cachedReader) GetUnsafeValue(
 		cr.currentValueBlock.loaded = true
 	}
 
-	invariants.CheckBounds(int(valueID), cr.currentValueBlock.dec.bd.Rows())
-	v := cr.currentValueBlock.dec.values.Slice(cr.currentValueBlock.dec.values.Offsets(int(valueID)))
+	// Convert the ValueID to an index into the block's values. When a blob file
+	// is first constructed, the ValueID == the index. However when a blob file
+	// is rewritten, multiple blocks from the original blob file may be combined
+	// into the same physical block. To translate the ValueID to the
+	// apppropriate index, we need to add the 'virtual block' valueIDOffset.
+	valueIndex := int(valueID) + int(cr.currentValueBlock.valueIDOffset)
+	invariants.CheckBounds(valueIndex, cr.currentValueBlock.dec.bd.Rows())
+	v := cr.currentValueBlock.dec.values.Slice(cr.currentValueBlock.dec.values.Offsets(valueIndex))
 	return v, nil
 }