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105 lines
3.0 KiB
105 lines
3.0 KiB
// Package chunkreader provides an io.Reader wrapper that minimizes IO reads and memory allocations.
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package chunkreader
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import (
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"io"
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)
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// ChunkReader is a io.Reader wrapper that minimizes IO reads and memory allocations. It allocates memory in chunks and
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// will read as much as will fit in the current buffer in a single call regardless of how large a read is actually
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// requested. The memory returned via Next is owned by the caller. This avoids the need for an additional copy.
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//
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// The downside of this approach is that a large buffer can be pinned in memory even if only a small slice is
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// referenced. For example, an entire 4096 byte block could be pinned in memory by even a 1 byte slice. In these rare
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// cases it would be advantageous to copy the bytes to another slice.
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type ChunkReader struct {
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r io.Reader
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buf []byte
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rp, wp int // buf read position and write position
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config Config
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}
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// Config contains configuration parameters for ChunkReader.
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type Config struct {
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MinBufLen int // Minimum buffer length
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}
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// New creates and returns a new ChunkReader for r with default configuration.
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func New(r io.Reader) *ChunkReader {
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cr, err := NewConfig(r, Config{})
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if err != nil {
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panic("default config can't be bad")
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}
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return cr
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}
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// NewConfig creates and a new ChunkReader for r configured by config.
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func NewConfig(r io.Reader, config Config) (*ChunkReader, error) {
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if config.MinBufLen == 0 {
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// By historical reasons Postgres currently has 8KB send buffer inside,
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// so here we want to have at least the same size buffer.
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// @see https://github.com/postgres/postgres/blob/249d64999615802752940e017ee5166e726bc7cd/src/backend/libpq/pqcomm.c#L134
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// @see https://www.postgresql.org/message-id/0cdc5485-cb3c-5e16-4a46-e3b2f7a41322%40ya.ru
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config.MinBufLen = 8192
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}
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return &ChunkReader{
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r: r,
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buf: make([]byte, config.MinBufLen),
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config: config,
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}, nil
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}
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// Next returns buf filled with the next n bytes. The caller gains ownership of buf. It is not necessary to make a copy
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// of buf. If an error occurs, buf will be nil.
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func (r *ChunkReader) Next(n int) (buf []byte, err error) {
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// n bytes already in buf
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if (r.wp - r.rp) >= n {
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buf = r.buf[r.rp : r.rp+n]
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r.rp += n
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return buf, err
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}
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// available space in buf is less than n
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if len(r.buf) < n {
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r.copyBufContents(r.newBuf(n))
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}
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// buf is large enough, but need to shift filled area to start to make enough contiguous space
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minReadCount := n - (r.wp - r.rp)
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if (len(r.buf) - r.wp) < minReadCount {
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newBuf := r.newBuf(n)
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r.copyBufContents(newBuf)
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}
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if err := r.appendAtLeast(minReadCount); err != nil {
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return nil, err
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}
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buf = r.buf[r.rp : r.rp+n]
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r.rp += n
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return buf, nil
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}
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func (r *ChunkReader) appendAtLeast(fillLen int) error {
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n, err := io.ReadAtLeast(r.r, r.buf[r.wp:], fillLen)
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r.wp += n
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return err
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}
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func (r *ChunkReader) newBuf(size int) []byte {
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if size < r.config.MinBufLen {
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size = r.config.MinBufLen
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}
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return make([]byte, size)
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}
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func (r *ChunkReader) copyBufContents(dest []byte) {
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r.wp = copy(dest, r.buf[r.rp:r.wp])
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r.rp = 0
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r.buf = dest
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}
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