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