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91 lines
2.6 KiB
91 lines
2.6 KiB
package pgproto3
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import (
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"io"
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"github.com/jackc/pgx/v5/internal/iobufpool"
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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 only valid until the next call to Next.
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//
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// This is roughly equivalent to a bufio.Reader that only uses Peek and Discard to never copy bytes.
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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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minBufSize int
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}
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// newChunkReader creates and returns a new chunkReader for r with default configuration. If minBufSize is <= 0 it uses
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// a default value.
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func newChunkReader(r io.Reader, minBufSize int) *chunkReader {
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if minBufSize <= 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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//
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// In addition, testing has found no benefit of any larger buffer.
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minBufSize = 8192
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}
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return &chunkReader{
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r: r,
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minBufSize: minBufSize,
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buf: iobufpool.Get(minBufSize),
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}
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}
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// Next returns buf filled with the next n bytes. buf is only valid until next call of Next. If an error occurs, buf
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// will be nil.
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func (r *chunkReader) Next(n int) (buf []byte, err error) {
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// Reset the buffer if it is empty
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if r.rp == r.wp {
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if len(*r.buf) != r.minBufSize {
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iobufpool.Put(r.buf)
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r.buf = iobufpool.Get(r.minBufSize)
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}
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r.rp = 0
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r.wp = 0
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}
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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 : 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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// buf is smaller than requested number of bytes
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if len(*r.buf) < n {
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bigBuf := iobufpool.Get(n)
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r.wp = copy((*bigBuf), (*r.buf)[r.rp:r.wp])
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r.rp = 0
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iobufpool.Put(r.buf)
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r.buf = bigBuf
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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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r.wp = copy((*r.buf), (*r.buf)[r.rp:r.wp])
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r.rp = 0
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}
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// Read at least the required number of bytes from the underlying io.Reader
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readBytesCount, err := io.ReadAtLeast(r.r, (*r.buf)[r.wp:], minReadCount)
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r.wp += readBytesCount
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// fmt.Println("read", n)
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if 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 : 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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