go/zodb/fs1tools: Dumper += DumpEndOK

Some dumpers might want to print something at the end of their dump.
We will need this functionality for Verify (see next patch).

go/zodb/storage/fs1/fs1tools/dump.go

-// Copyright (C) 2017  Nexedi SA and Contributors.
+// Copyright (C) 2017-2021  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -51,6 +51,9 @@ type Dumper interface {
 	//
 	// If dumper return io.EOF the whole dumping process finishes.
 	DumpTxn(buf *xfmt.Buffer, it *fs1.Iter) error
+
+	// DumpEndOK is called at the end of successfull dump.
+	DumpEndOK(buf *xfmt.Buffer) error
 }
 
 // Dump dumps content of a FileStorage file @ path.
@@ -101,7 +104,7 @@ func Dump(w io.Writer, path string, dir fs1.IterDir, d Dumper) (err error) {
 		err = it.NextTxn(fs1.LoadAll)
 		if err != nil {
 			if err == io.EOF {
-				err = nil
+				break
 			}
 			return err
 		}
@@ -109,7 +112,7 @@ func Dump(w io.Writer, path string, dir fs1.IterDir, d Dumper) (err error) {
 		err = d.DumpTxn(buf, it)
 		if err != nil {
 			if err == io.EOF {
-				err = nil
+				break
 			}
 			return err
 		}
@@ -119,6 +122,12 @@ func Dump(w io.Writer, path string, dir fs1.IterDir, d Dumper) (err error) {
 			return err
 		}
 	}
+
+	err = d.DumpEndOK(buf)
+	if err != nil {
+		return err
+	}
+	return nil
 }
 
 // ----------------------------------------
@@ -198,6 +207,10 @@ func (d *DumperFsDump) DumpTxn(buf *xfmt.Buffer, it *fs1.Iter) error {
 	}
 }
 
+func (d *DumperFsDump) DumpEndOK(buf *xfmt.Buffer) error {
+	return nil
+}
+
 // DumperFsDumpVerbose implements a very verbose dumper with output identical
 // to fsdump.Dumper in zodb/py originally written by Jeremy Hylton:
 //
@@ -281,6 +294,10 @@ func (d *DumperFsDumpVerbose) dumpData(buf *xfmt.Buffer, it *fs1.Iter) error {
 	return nil
 }
 
+func (d *DumperFsDumpVerbose) DumpEndOK(buf *xfmt.Buffer) error {
+	return nil
+}
+
 const dumpSummary = "dump database transactions"
 
 func dumpUsage(w io.Writer) {
@@ -383,6 +400,10 @@ func (d *DumperFsTail) DumpTxn(buf *xfmt.Buffer, it *fs1.Iter) error {
 	return nil
 }
 
+func (d *DumperFsTail) DumpEndOK(buf *xfmt.Buffer) error {
+	return nil
+}
+
 const tailSummary = "dump last few transactions of a database"
 const ntxnDefault = 10