Add io.Pipe compatible interface

The ring buffer can be used as a compatible, but *asynchronous* replacement of `io.Pipe`. That means that Reads and Writes will go to the ring buffer. Writes will complete as long as the data fits within the ring buffer. Reads will attempt to satisfy reads with data from the ring buffer. Only if the ring buffer is empty will the read block. In the common case, where the Read and Write side can run concurrently, it is safe to replace `io.Pipe()` with `(*Ringbuffer).Pipe()`.

Author: klauspost

README.md

@@ -104,3 +104,42 @@ func readWebsite(url string) io.ReadCloser {
 return ring.ReadCloser()
 }
 ```
+
+# io.Pipe replacement
+
+The ring buffer can be used as a compatible, but *asynchronous* replacement of `io.Pipe`.
+
+That means that Reads and Writes will go to the ring buffer.
+Writes will complete as long as the data fits within the ring buffer.
+
+Reads will attempt to satisfy reads with data from the ring buffer.
+The read will only block if the ring buffer is empty.
+
+In the common case, where the Read and Write side can run concurrently, 
+it is safe to replace `io.Pipe()` with `(*Ringbuffer).Pipe()`.
+
+Compare the following to the [io.Pipe example](https://pkg.go.dev/io#example-Pipe): 
+
+```go
+func main() {
+// Create pipe from a 4KB ring buffer.
+r, w := ringbuffer.New(4 << 10).Pipe()
+
+go func() {
+fmt.Fprint(w, "some io.Reader stream to be read\n")
+w.Close()
+}()
+
+if _, err := io.Copy(os.Stdout, r); err != nil {
+log.Fatal(err)
+}
+}
+```
+
+When creating the pipe, the ring buffer is internally switched to blocking mode.
+
+Error reporting on Close and CloseWithError functions is similar to `io.Pipe`.
+
+It is possible to use the original ring buffer alongside the pipe functions. 
+So for example it is possible to "seed" the ring buffer with data, 
+so reads can complete at once.

example_test.go

@@ -1,6 +1,11 @@
 package ringbuffer
 
-import "fmt"
+import (
+"fmt"
+"io"
+"log"
+"os"
+)
 
 func ExampleRingBuffer() {
 rb := New(1024)
@@ -15,3 +20,20 @@ func ExampleRingBuffer() {
 // 1020
 // abcd
 }
+
+func ExampleRingBuffer_Pipe() {
+// Create pipe from a 4KB ring buffer.
+r, w := New(4 << 10).Pipe()
+
+go func() {
+fmt.Fprint(w, "some io.Reader stream to be read\n")
+w.Close()
+}()
+
+if _, err := io.Copy(os.Stdout, r); err != nil {
+log.Fatal(err)
+}
+
+// Output:
+// some io.Reader stream to be read
+}

pipe.go

@@ -0,0 +1,92 @@
+// Copyright 2019 smallnest. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package ringbuffer
+
+import "io"
+
+// Pipe creates an asynchronous in-memory pipe compatible with io.Pipe
+// It can be used to connect code expecting an [io.Reader]
+// with code expecting an [io.Writer].
+//
+// Reads and Writes will go to the ring buffer.
+// Writes will complete as long as the data fits within the ring buffer.
+// Reads will attempt to satisfy reads with data from the ring buffer.
+// Only if the ring buffer is empty will the read block.
+//
+// It is safe (and intended) to call Read and Write in parallel with each other or with Close.
+func (r *RingBuffer) Pipe() (*PipeReader, *PipeWriter) {
+r.SetBlocking(true)
+pr := PipeReader{pipe: r}
+return &pr, &PipeWriter{pipe: r}
+}
+
+// A PipeReader is the read half of a pipe.
+type PipeReader struct{ pipe *RingBuffer }
+
+// Read implements the standard Read interface:
+// it reads data from the pipe, blocking until a writer
+// arrives or the write end is closed.
+// If the write end is closed with an error, that error is
+// returned as err; otherwise err is io.EOF.
+func (r *PipeReader) Read(data []byte) (n int, err error) {
+return r.pipe.Read(data)
+}
+
+// Close closes the reader; subsequent writes to the
+// write half of the pipe will return the error [io.ErrClosedPipe].
+func (r *PipeReader) Close() error {
+r.pipe.setErr(io.ErrClosedPipe, false)
+return nil
+}
+
+// CloseWithError closes the reader; subsequent writes
+// to the write half of the pipe will return the error err.
+//
+// CloseWithError never overwrites the previous error if it exists
+// and always returns nil.
+func (r *PipeReader) CloseWithError(err error) error {
+if err == nil {
+return r.Close()
+}
+r.pipe.setErr(err, false)
+return nil
+}
+
+// A PipeWriter is the write half of a pipe.
+type PipeWriter struct{ pipe *RingBuffer }
+
+// Write implements the standard Write interface:
+// it writes data to the pipe.
+// The Write will block until all data has been written to the ring buffer.
+// If the read end is closed with an error, that err is
+// returned as err; otherwise err is [io.ErrClosedPipe].
+func (w *PipeWriter) Write(data []byte) (n int, err error) {
+if n, err = w.pipe.Write(data); err == ErrWriteOnClosed {
+// Replace error.
+err = io.ErrClosedPipe
+}
+return n, err
+}
+
+// Close closes the writer; subsequent reads from the
+// read half of the pipe will return no bytes and EOF.
+func (w *PipeWriter) Close() error {
+w.pipe.setErr(io.EOF, false)
+return nil
+}
+
+// CloseWithError closes the writer; subsequent reads from the
+// read half of the pipe will return no bytes and the error err,
+// or EOF if err is nil.
+//
+// CloseWithError never overwrites the previous error if it exists
+// and always returns nil.
+func (w *PipeWriter) CloseWithError(err error) error {
+if err == nil {
+return w.Close()
+}
+w.pipe.setErr(err, false)
+return nil
+}