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save.go
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save.go
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/*
* umoci: Umoci Modifies Open Containers' Images
* Copyright (C) 2016, 2017, 2018 SUSE LLC.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package generate
import (
"encoding/json"
"io"
"github.com/pkg/errors"
)
// fakeBuffer implements the io.Writer interface but just counts the number of
// bytes "written" to it.
type fakeBuffer struct {
n int64
}
// Write just counts the number of bytes requested to be written.
func (fb *fakeBuffer) Write(p []byte) (int, error) {
size := len(p)
fb.n += int64(size)
return size, nil
}
// WriteTo outputs a JSON-marshalled version of the current state of the
// generator. It is not guaranteed that the generator will produce the same
// output given the same state, so it's recommended to only call this function
// once. The JSON is not pretty-printed.
func (g *Generator) WriteTo(w io.Writer) (n int64, err error) {
// We need to return the number of bytes written, which json.NewEncoder
// won't give us. So we have to cheat a little to get the answer.
var fb fakeBuffer
w = io.MultiWriter(w, &fb)
if err := json.NewEncoder(w).Encode(g.image); err != nil {
return fb.n, errors.Wrap(err, "encode image")
}
return fb.n, nil
}