Copy xid string to reduce memory usage in bulk loader (#4287)

In bulk loader, we read raw data from input file line by line. We
use substrings of line in various places in the bulk loader. All the
substring references are gone soon enough except the reference
to xid. Reference to xid is stored in the AssignUid map and hence,
the whole line (string) is alive througout the process. We make a
copy of xid now before storing it in the map.

dgraph/cmd/bulk/mapper.go

@@ -241,7 +241,16 @@ func (m *mapper) uid(xid string) uint64 {
 }
 
 func (m *mapper) lookupUid(xid string) uint64 {
-	uid, isNew := m.xids.AssignUid(xid)
+	// We create a copy of xid string here because it is stored in
+	// the map in AssignUid and going to be around throughout the process.
+	// We don't want to keep the whole line that we read from file alive.
+	// xid is a substring of the line that we read from the file and if
+	// xid is alive, the whole line is going to be alive and won't be GC'd.
+	// Also, checked that sb goes on the stack whereas sb.String() goes on
+	// heap. Note that the calls to the strings.Builder.* are inlined.
+	sb := strings.Builder{}
+	sb.WriteString(xid)
+	uid, isNew := m.xids.AssignUid(sb.String())
 	if !m.opt.StoreXids || !isNew {
 		return uid
 	}