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writer_reader.go
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/
writer_reader.go
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package fileutil
import (
"fmt"
"io"
"os"
)
// WriterThenReader writes data to storage, then allows reading it. It is suitable for
// repeatedly processing large volumes of data.
type WriterThenReader interface {
io.Writer
// StartReading stops writing and returns a RewindableReader that will allow repeatedly
// reading the data and the total length of data. The WriterThenReader should be
// discarded; calls to Write() after StartReading() will fail.
StartReading() (RewindableReader, int64, error)
// Name returns a user-visible name for underlying storage. It may help debug some issues.
Name() string
}
// RewindableReader allows repeatedly reading the same stream.
type RewindableReader interface {
io.ReadSeeker
// Rewind allows sets RewindableReader to start re-reading the same data.
Rewind() error
// Name returns a user-visible name for underlying storage. It may help debug some issues.
Name() string
}
type fileWriterThenReader struct{ file *os.File }
func NewFileWriterThenReader(basename string) (WriterThenReader, error) {
file, err := os.CreateTemp("", basename)
if err != nil {
return nil, fmt.Errorf("creating temporary file: %w", err)
}
if err = os.Remove(file.Name()); err != nil {
return nil, fmt.Errorf("removing file %s from directory: %w", file.Name(), err)
}
return &fileWriterThenReader{file: file}, nil
}
func (f fileWriterThenReader) Write(p []byte) (int, error) {
return f.file.Write(p)
}
func (f *fileWriterThenReader) StartReading() (RewindableReader, int64, error) {
offset, err := f.file.Seek(0, io.SeekEnd)
if err != nil {
return nil, -1, err
}
_, err = f.file.Seek(0, io.SeekStart)
ret := &fileRewindableReader{file: f.file}
// Break future attempts to use f
f.file = nil
return ret, offset, err
}
func (f fileWriterThenReader) Name() string { return f.file.Name() }
type fileRewindableReader struct{ file *os.File }
func (f fileRewindableReader) Read(p []byte) (n int, err error) {
return f.file.Read(p)
}
func (f fileRewindableReader) Seek(offset int64, whence int) (int64, error) {
return f.file.Seek(offset, whence)
}
func (f fileRewindableReader) Rewind() error {
_, err := f.file.Seek(0, io.SeekStart)
return err
}
func (f fileRewindableReader) Name() string { return f.file.Name() }