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tailscale/util/jsonutil/unmarshal.go

90 lines
2.6 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
// Package jsonutil provides utilities to improve JSON performance.
// It includes an Unmarshal wrapper that amortizes allocated garbage over subsequent runs
// and a Bytes type to reduce allocations when unmarshalling a non-hex-encoded string into a []byte.
package jsonutil
import (
"bytes"
"encoding/json"
"sync"
)
// decoder is a re-usable json decoder.
type decoder struct {
dec *json.Decoder
r *bytes.Reader
}
var readerPool = sync.Pool{
New: func() any {
return bytes.NewReader(nil)
},
}
var decoderPool = sync.Pool{
New: func() any {
var d decoder
d.r = readerPool.Get().(*bytes.Reader)
d.dec = json.NewDecoder(d.r)
return &d
},
}
// Unmarshal is similar to encoding/json.Unmarshal.
// There are three major differences:
//
// On error, encoding/json.Unmarshal zeros v.
// This Unmarshal may leave partial data in v.
// Always check the error before using v!
// (Future improvements may remove this bug.)
//
// The errors they return don't always match perfectly.
// If you do error matching more precise than err != nil,
// don't use this Unmarshal.
//
// This Unmarshal allocates considerably less memory.
func Unmarshal(b []byte, v any) error {
d := decoderPool.Get().(*decoder)
d.r.Reset(b)
off := d.dec.InputOffset()
err := d.dec.Decode(v)
d.r.Reset(nil) // don't keep a reference to b
// In case of error, report the offset in this byte slice,
// instead of in the totality of all bytes this decoder has processed.
// It is not possible to make all errors match json.Unmarshal exactly,
// but we can at least try.
switch jsonerr := err.(type) {
case *json.SyntaxError:
jsonerr.Offset -= off
case *json.UnmarshalTypeError:
jsonerr.Offset -= off
case nil:
// json.Unmarshal fails if there's any extra junk in the input.
// json.Decoder does not; see https://github.com/golang/go/issues/36225.
// We need to check for anything left over in the buffer.
if d.dec.More() {
// TODO: Provide a better error message.
// Unfortunately, we can't set the msg field.
// The offset doesn't perfectly match json:
// Ours is at the end of the valid data,
// and theirs is at the beginning of the extra data after whitespace.
// Close enough, though.
err = &json.SyntaxError{Offset: d.dec.InputOffset() - off}
// TODO: zero v. This is hard; see encoding/json.indirect.
}
}
if err == nil {
decoderPool.Put(d)
} else {
// There might be junk left in the decoder's buffer.
// There's no way to flush it, no Reset method.
// Abandoned the decoder but reuse the reader.
readerPool.Put(d.r)
}
return err
}