go/zodb/zodbtools/dump: Don't use goto

Tomáš Peterka noticed that gotos in dump.go are not actually needed because
the same functionality could be achieved with defer in more clean and
structured way.

Do it.

This brings ~ 5% performance hit

	name        old time/op  new time/op  delta
	ZodbDump-4   148µs ± 1%   155µs ± 2%  +4.69%  (p=0.000 n=9+10)

because defer implementation is currently not great
(https://github.com/golang/go/issues/14939)

If we absolutely need those 5% back it could be worked around similar to
e.g. FileStorage.Load:

https://lab.nexedi.com/kirr/neo/blob/6faed528/go/zodb/storage/fs1/filestorage.go#L133
https://lab.nexedi.com/kirr/neo/blob/6faed528/go/zodb/storage/fs1/filestorage.go#L141

/suggested-by @katomaso

go/zodb/zodbtools/dump.go

@@ -81,7 +81,15 @@ var _LF = []byte{'\n'}
 
 
 // DumpData dumps one data record
-func (d *dumper) DumpData(datai *zodb.DataInfo) error {
+func (d *dumper) DumpData(datai *zodb.DataInfo) (err error) {
+	// XXX do we need this context ?
+	// see for rationale in similar place in DumpTxn
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("%v: %v", datai.Oid, err)
+		}
+	}()
+
 	buf := &d.buf
 	buf.Reset()
 
@@ -115,35 +123,33 @@ func (d *dumper) DumpData(datai *zodb.DataInfo) error {
 	}
 
 	// TODO use writev(buf, data, "\n") via net.Buffers (it is already available)
-	_, err := d.W.Write(buf.Bytes())
+	_, err = d.W.Write(buf.Bytes())
 	if err != nil {
-		goto out
+		return err
 	}
 
 	if data != nil {
 		_, err = d.W.Write(datai.Data)
 		if err != nil {
-			goto out
+			return err
 		}
 	}
 
 	_, err = d.W.Write(_LF)
-	if err != nil {
-		goto out
-	}
+	return err
+}
 
-out:
+// DumpTxn dumps one transaction record
+func (d *dumper) DumpTxn(ctx context.Context, txni *zodb.TxnInfo, dataIter zodb.IDataIterator) (err error) {
 	// XXX do we need this context ?
-	// see for rationale in similar place in DumpTxn
+	// rationale: dataIter.NextData() if error in db - will include db context
+	// if error is in writer - it will include its own context
+	defer func() {
 		if err != nil {
-		return fmt.Errorf("%v: %v", datai.Oid, err)
+			err = fmt.Errorf("%v: %v", txni.Tid, err)
 		}
+	}()
 
-	return nil
-}
-
-// DumpTxn dumps one transaction record
-func (d *dumper) DumpTxn(ctx context.Context, txni *zodb.TxnInfo, dataIter zodb.IDataIterator) error {
 	var datai *zodb.DataInfo
 
 	// LF in-between txn records
@@ -153,10 +159,10 @@ func (d *dumper) DumpTxn(ctx context.Context, txni *zodb.TxnInfo, dataIter zodb.
 		d.afterFirst = true
 	}
 
-	_, err := fmt.Fprintf(d.W, "%stxn %s %q\nuser %q\ndescription %q\nextension %q\n",
+	_, err = fmt.Fprintf(d.W, "%stxn %s %q\nuser %q\ndescription %q\nextension %q\n",
 			vskip, txni.Tid, string(txni.Status), txni.User, txni.Description, txni.Extension)
 	if err != nil {
-		goto out
+		return err
 	}
 
 	// data records
@@ -176,15 +182,7 @@ func (d *dumper) DumpTxn(ctx context.Context, txni *zodb.TxnInfo, dataIter zodb.
 		}
 	}
 
-out:
-	// XXX do we need this context ?
-	// rationale: dataIter.NextData() if error in db - will include db context
-	// if error is in writer - it will include its own context
-	if err != nil {
-		return fmt.Errorf("%v: %v", txni.Tid, err)
-	}
-
-	return nil
+	return err
 }
 
 // Dump dumps transaction records in between tidMin..tidMax