X go/neo: Switch to NEO-kind SHA1

NEO/py sends all zeros for empty data. Without this fix tests fail:

    --- FAIL: TestLoad/py (0.74s)
        xtesting.go:297: load 0285cbacc06d3a4c:0000000000000007: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 0285cbacc06d3a4c:0000000000000007: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
        xtesting.go:297: load 0285cbad858bf2e6:0000000000000006: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 0285cbad858bf2e6:0000000000000006: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
        xtesting.go:315: load 7fffffffffffffff:0000000000000006: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000006: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
        xtesting.go:315: load 7fffffffffffffff:0000000000000007: returned err unexpected:
            have: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000007: data corrupt: checksum mismatch
            want: neo://1@127.0.0.1:25910: load 7fffffffffffffff:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c

go/neo/client.go

@@ -463,7 +463,7 @@ func (c *Client) Load(ctx context.Context, xid zodb.Xid) (buf *mem.Buf, serial z
 	buf = resp.Data
 
 	if !xsha1.Skip {
-		checksum := xsha1.Sum(buf.Data)
+		checksum := xsha1.NEOSum(buf.Data)
 		if checksum != resp.Checksum {
 			return nil, 0, fmt.Errorf("data corrupt: checksum mismatch")
 		}

go/neo/internal/xsha1/xsha1.go

-// Copyright (C) 2017-2018  Nexedi SA and Contributors.
+// Copyright (C) 2017-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -43,3 +43,14 @@ func Sum(b []byte) [sha1.Size]byte {
 
 	return [sha1.Size]byte{} // all 0
 }
+
+// NEOSum returns SHA1(b) computed by NEO rules.
+//
+// it is the same as regular SHA1, but returns all-zeros for empty b.
+// https://lab.nexedi.com/nexedi/neoppod/blob/c1c26894/neo/client/app.py#L464-468
+func NEOSum(b []byte) [sha1.Size]byte {
+	if len(b) == 0 {
+		return [sha1.Size]byte{} // all 0
+	}
+	return Sum(b)
+}

go/neo/storage/fs1/fs1.go

-// Copyright (C) 2016-2018  Nexedi SA and Contributors.
+// Copyright (C) 2016-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -85,7 +85,7 @@ func (f *Backend) Load(ctx context.Context, xid zodb.Xid) (*proto.AnswerObject,
 
 		Compression:	false,
 		Data:		buf,
-		Checksum:	xsha1.Sum(buf.Data),	// XXX computing every time
+		Checksum:	xsha1.NEOSum(buf.Data),	// XXX computing every time
 
 		// XXX .DataSerial
 	}, nil