xio: Add Reader/Writer/... interfaces - io analogs that add support for contexts

In many cases IO needs to be able to be canceled. For example in WCFS
filesystem I need to cancel handling sysread(/head/watch) when FUSE
INTERRUPT request comes in [1,2,3]. The READ handler for /head/watch
inside WCFS is interally implemented via io.Pipe which does not provide
read/write cancellattion besides "destructive" close.

Standard Go answer for cancellations is via contexts. So as a first step
let's add corresponding interfaces - xio.Reader, xio.Writer etc - that
are io analogs that add support for contexts.

For compatibility with legacy code that work with only io.X (e.g.  only
with io.Reader), in spirit of [4], add BindCtx which binds xio.X
instance with context and converts it into io.X. Add WithCtx -
corresponding inverse operation that converts io.X back into xio.X and
for general io.X adds cancellation handling on a best-effort basis.

[1] wendelin.core@b17aeb8c
[2] wendelin.core@f05271b1
[3] wendelin.core@5ba816da
[4] https://github.com/golang/go/issues/20280
[5] https://github.com/golang/go/issues/16522

xio/xio.go

@@ -18,9 +18,259 @@
 // See https://www.nexedi.com/licensing for rationale and options.
 
 // Package xio provides addons to standard package io.
+//
+//	- Reader, Writer, ReadWriter, etc are io analogs that add support for contexts.
+//	- BindCtx*(X, ctx) converts xio.X into io.X that implicitly passes ctx
+//	  to xio.X and can be used in legacy code.
+//	- WithCtx*(X) converts io.X back into xio.X that accepts context.
+//	  It is the opposite operation for BindCtx, but for arbitrary io.X
+//	  returned xio.X handles context only on best-effort basis. In
+//	  particular IO cancellation is not reliably handled for os.File .
 package xio
 
-import "io"
+import (
+	"context"
+	"io"
+)
+
+// Reader is like io.Reader but additionally takes context for Read.
+type Reader interface {
+	Read(ctx context.Context, dst []byte) (n int, err error)
+}
+
+// Writer is like io.Writer but additionally takes context for Write.
+type Writer interface {
+	Write(ctx context.Context, src []byte) (n int, err error)
+}
+
+// ReadWriter combines Reader and Writer.
+type ReadWriter interface {
+	Reader
+	Writer
+}
+
+// ReadCloser combines Reader and io.Closer.
+type ReadCloser interface {
+	Reader
+	io.Closer
+}
+
+// WriteCloser combines Writer and io.Closer.
+type WriteCloser interface {
+	Writer
+	io.Closer
+}
+
+// ReadWriteCloser combines Reader, Writer and io.Closer.
+type ReadWriteCloser interface {
+	Reader
+	Writer
+	io.Closer
+}
+
+
+// BindCtx*(xio.X, ctx) -> io.X
+//
+// XXX better just BindCtx(x T, ctx) -> T with all x IO methods without ctx,
+// but that needs either generics, or support from reflect to preserve optional
+// methods: https://github.com/golang/go/issues/16522.
+
+
+// BindCtxR binds Reader r and ctx into io.Reader which passes ctx to r on every Read.
+func BindCtxR(r Reader, ctx context.Context) io.Reader {
+	// BindCtx(WithCtx(X), BG) = X
+	if ctx.Done() == nil {
+		switch s := r.(type) {
+		case *stubCtxR:   return s.r
+		case *stubCtxRW:  return s.rw
+		case *stubCtxRC:  return s.r
+		case *stubCtxRWC: return s.rw
+		}
+	}
+
+	return &bindCtxR{r, ctx}
+}
+type bindCtxR struct {r Reader; ctx context.Context}
+func (b *bindCtxR) Read(dst []byte) (int, error)	{ return b.r.Read(b.ctx, dst) }
+
+// BindCtxW binds Writer w and ctx into io.Writer which passes ctx to w on every Write.
+func BindCtxW(w Writer, ctx context.Context) io.Writer {
+	if ctx.Done() == nil {
+		switch s := w.(type) {
+		case *stubCtxW:   return s.w
+		case *stubCtxRW:  return s.rw
+		case *stubCtxWC:  return s.w
+		case *stubCtxRWC: return s.rw
+		}
+	}
+	return &bindCtxW{w, ctx}
+}
+type bindCtxW struct {w Writer; ctx context.Context}
+func (b *bindCtxW) Write(src []byte) (int, error)	{ return b.w.Write(b.ctx, src)	}
+
+// BindCtxRW binds ReadWriter rw and ctx into io.ReadWriter which passes ctx to
+// rw on every Read and Write.
+func BindCtxRW(rw ReadWriter, ctx context.Context) io.ReadWriter {
+	if ctx.Done() == nil {
+		switch s := rw.(type) {
+		case *stubCtxRW:  return s.rw
+		case *stubCtxRWC: return s.rw
+		}
+	}
+	return &bindCtxRW{rw, ctx}
+}
+type bindCtxRW struct {rw ReadWriter; ctx context.Context}
+func (b *bindCtxRW) Read (dst []byte) (int, error)	{ return b.rw.Read (b.ctx, dst) }
+func (b *bindCtxRW) Write(src []byte) (int, error)	{ return b.rw.Write(b.ctx, src) }
+
+// BindCtxRC binds ReadCloser r and ctx into io.ReadCloser which passes ctx to r on every Read.
+func BindCtxRC(r ReadCloser, ctx context.Context) io.ReadCloser {
+	if ctx.Done() == nil {
+		switch s := r.(type) {
+		case *stubCtxRC:  return s.r
+		case *stubCtxRWC: return s.rw
+		}
+	}
+	return &bindCtxRC{r, ctx}
+}
+type bindCtxRC struct {r ReadCloser; ctx context.Context}
+func (b *bindCtxRC) Read(dst []byte) (int, error)	{ return b.r.Read(b.ctx, dst)	}
+func (b *bindCtxRC) Close() error			{ return b.r.Close()		}
+
+// BindCtxWC binds WriteCloser w and ctx into io.WriteCloser which passes ctx to w on every Write.
+func BindCtxWC(w WriteCloser, ctx context.Context) io.WriteCloser {
+	if ctx.Done() == nil {
+		switch s := w.(type) {
+		case *stubCtxWC:  return s.w
+		case *stubCtxRWC: return s.rw
+		}
+	}
+	return &bindCtxWC{w, ctx}
+}
+type bindCtxWC struct {w WriteCloser; ctx context.Context}
+func (b *bindCtxWC) Write(src []byte) (int, error)	{ return b.w.Write(b.ctx, src)	}
+func (b *bindCtxWC) Close() error			{ return b.w.Close()		}
+
+// BindCtxRWC binds ReadWriteCloser rw and ctx into io.ReadWriteCloser
+// which passes ctx to rw on every Read and Write.
+func BindCtxRWC(rw ReadWriteCloser, ctx context.Context) io.ReadWriteCloser {
+	if ctx.Done() == nil {
+		switch s := rw.(type) {
+		case *stubCtxRWC: return s.rw
+		}
+	}
+	return &bindCtxRWC{rw, ctx}
+}
+type bindCtxRWC struct {rw ReadWriteCloser; ctx context.Context}
+func (b *bindCtxRWC) Read(dst []byte) (int, error)	{ return b.rw.Read(b.ctx, dst)	}
+func (b *bindCtxRWC) Write(src []byte) (int, error)	{ return b.rw.Write(b.ctx, src)	}
+func (b *bindCtxRWC) Close() error			{ return b.rw.Close()		}
+
+
+// WithCtx*(io.X) -> xio.X that handles ctx on best-effort basis.
+//
+// FIXME for arbitrary io.X for now ctx is completely ignored.
+// TODO add support for cancellation if io.X provides working .Set{Read/Write}Deadline:
+// https://medium.com/@zombiezen/canceling-i-o-in-go-capn-proto-5ae8c09c5b29
+// https://github.com/golang/go/issues/20280
+
+// WithCtxR converts io.Reader r into Reader that accepts ctx.
+//
+// It returns original IO object if r was created via BindCtx*, but in general
+// returned Reader will handle context only on best-effort basis.
+func WithCtxR(r io.Reader) Reader {
+	// WithCtx(BindCtx(X)) = X
+	switch b := r.(type) {
+	case *bindCtxR:   return b.r
+	case *bindCtxRW:  return b.rw
+	case *bindCtxRC:  return b.r
+	case *bindCtxRWC: return b.rw
+	}
+
+	return &stubCtxR{r}
+}
+type stubCtxR struct {r io.Reader}
+func (s *stubCtxR) Read(ctx context.Context, dst []byte) (int, error)	{ return s.r.Read(dst) }
+
+// WithCtxW converts io.Writer w into Writer that accepts ctx.
+//
+// It returns original IO object if w was created via BindCtx*, but in general
+// returned Writer will handle context only on best-effort basis.
+func WithCtxW(w io.Writer) Writer {
+	switch b := w.(type) {
+	case *bindCtxW:   return b.w
+	case *bindCtxRW:  return b.rw
+	case *bindCtxWC:  return b.w
+	case *bindCtxRWC: return b.rw
+	}
+	return &stubCtxW{w}
+}
+type stubCtxW struct {w io.Writer}
+func (s *stubCtxW) Write(ctx context.Context, src []byte) (int, error)	{ return s.w.Write(src) }
+
+// WithCtxRW converts io.ReadWriter rw into ReadWriter that accepts ctx.
+//
+// It returns original IO object if rw was created via BindCtx*, but in general
+// returned ReadWriter will handle context only on best-effort basis.
+func WithCtxRW(rw io.ReadWriter) ReadWriter {
+	switch b := rw.(type) {
+	case *bindCtxRW:  return b.rw
+	case *bindCtxRWC: return b.rw
+	}
+	return &stubCtxRW{rw}
+}
+type stubCtxRW struct {rw io.ReadWriter}
+func (s *stubCtxRW) Read (ctx context.Context, dst []byte) (int, error)	{ return s.rw.Read (dst) }
+func (s *stubCtxRW) Write(ctx context.Context, src []byte) (int, error)	{ return s.rw.Write(src) }
+
+// WithCtxRC converts io.ReadCloser r into ReadCloser that accepts ctx.
+//
+// It returns original IO object if r was created via BindCtx*, but in general
+// returned ReadCloser will handle context only on best-effort basis.
+func WithCtxRC(r io.ReadCloser) ReadCloser {
+	switch b := r.(type) {
+	case *bindCtxRC:  return b.r
+	case *bindCtxRWC: return b.rw
+	}
+	return &stubCtxRC{r}
+}
+type stubCtxRC struct {r io.ReadCloser}
+func (s *stubCtxRC) Read (ctx context.Context, dst []byte) (int, error)	{ return s.r.Read(dst) }
+func (s *stubCtxRC) Close() error					{ return s.r.Close() }
+
+// WithCtxWC converts io.WriteCloser w into WriteCloser that accepts ctx.
+//
+// It returns original IO object if w was created via BindCtx*, but in general
+// returned WriteCloser will handle context only on best-effort basis.
+func WithCtxWC(w io.WriteCloser) WriteCloser {
+	switch b := w.(type) {
+	case *bindCtxWC:  return b.w
+	case *bindCtxRWC: return b.rw
+	}
+	return &stubCtxWC{w}
+}
+type stubCtxWC struct {w io.WriteCloser}
+func (s *stubCtxWC) Write(ctx context.Context, src []byte) (int, error)	{ return s.w.Write(src) }
+func (s *stubCtxWC) Close() error					{ return s.w.Close() }
+
+// WithCtxRWC converts io.ReadWriteCloser rw into ReadWriteCloser that accepts ctx.
+//
+// It returns original IO object if rw was created via BindCtx*, but in general
+// returned ReadWriteCloser will handle context only on best-effort basis.
+func WithCtxRWC(rw io.ReadWriteCloser) ReadWriteCloser {
+	switch b := rw.(type) {
+	case *bindCtxRWC: return b.rw
+	}
+	return &stubCtxRWC{rw}
+}
+type stubCtxRWC struct {rw io.ReadWriteCloser}
+func (s *stubCtxRWC) Read (ctx context.Context, dst []byte) (int, error){ return s.rw.Read (dst) }
+func (s *stubCtxRWC) Write(ctx context.Context, src []byte) (int, error){ return s.rw.Write(src) }
+func (s *stubCtxRWC) Close() error					{ return s.rw.Close() }
+
+
+// ----------------------------------------
+
 
 // CountedReader is an io.Reader that count total bytes read.
 type CountedReader struct {

xio/xio_test.go

+// Copyright (C) 2019  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+package xio
+
+import (
+	"context"
+	"testing"
+)
+
+// xIO is test Reader/Writer/Closer/...
+type xIO struct{}
+
+func (_ *xIO) Read(ctx context.Context, dst []byte) (int, error) {
+	for i := range dst {
+		dst[i] = 0
+	}
+	return len(dst), nil
+}
+
+func (_ *xIO) Write(ctx context.Context, src []byte) (int, error) {
+	return len(src), nil
+}
+
+func (_ *xIO) Close() error {
+	return nil
+}
+
+// tIO is test io.Reader/io.Writer/...
+type tIO struct{}
+
+func (_ *tIO) Read(dst []byte) (int, error) {
+	for i := range dst {
+		dst[i] = 0
+	}
+	return len(dst), nil
+}
+
+func (_ *tIO) Write(src []byte) (int, error) {
+	return len(src), nil
+}
+
+func (_ *tIO) Close() error {
+	return nil
+}
+
+
+// ok1 asserts that v is true.
+func ok1(v bool) {
+	if !v {
+		panic("not ok")
+	}
+}
+
+// Verify xio.X <-> io.X conversion
+func TestConvert(t *testing.T) {
+	x := new(xIO)
+	i := new(tIO)
+	bg := context.Background()
+
+	// WithCtx(BindCtx(X)) = X
+	ok1( WithCtxR(BindCtxR(x, bg)) == x )
+
+	ok1( WithCtxW(BindCtxW(x, bg)) == x )
+
+	ok1( WithCtxR (BindCtxRW(x, bg)) == x )
+	ok1( WithCtxW (BindCtxRW(x, bg)) == x )
+	ok1( WithCtxRW(BindCtxRW(x, bg)) == x )
+
+	ok1( WithCtxR (BindCtxRC(x, bg)) == x )
+	ok1( WithCtxRC(BindCtxRC(x, bg)) == x )
+
+	ok1( WithCtxW (BindCtxWC(x, bg)) == x )
+	ok1( WithCtxWC(BindCtxWC(x, bg)) == x )
+
+	ok1( WithCtxR  (BindCtxRWC(x, bg)) == x )
+	ok1( WithCtxW  (BindCtxRWC(x, bg)) == x )
+	ok1( WithCtxRW (BindCtxRWC(x, bg)) == x )
+	ok1( WithCtxRC (BindCtxRWC(x, bg)) == x )
+	ok1( WithCtxWC (BindCtxRWC(x, bg)) == x )
+	ok1( WithCtxRWC(BindCtxRWC(x, bg)) == x )
+
+
+	// BindCtx(WithCtx(X), bg) = X
+	ok1( BindCtxR(WithCtxR(i), bg) == i )
+
+	ok1( BindCtxW(WithCtxW(i), bg) == i )
+
+	ok1( BindCtxR (WithCtxRW(i), bg) == i )
+	ok1( BindCtxW (WithCtxRW(i), bg) == i )
+	ok1( BindCtxRW(WithCtxRW(i), bg) == i )
+
+	ok1( BindCtxR (WithCtxRC(i), bg) == i )
+	ok1( BindCtxRC(WithCtxRC(i), bg) == i )
+
+	ok1( BindCtxW (WithCtxWC(i), bg) == i )
+	ok1( BindCtxWC(WithCtxWC(i), bg) == i )
+
+	ok1( BindCtxR  (WithCtxRWC(i), bg) == i )
+	ok1( BindCtxW  (WithCtxRWC(i), bg) == i )
+	ok1( BindCtxRW (WithCtxRWC(i), bg) == i )
+	ok1( BindCtxRC (WithCtxRWC(i), bg) == i )
+	ok1( BindCtxWC (WithCtxRWC(i), bg) == i )
+	ok1( BindCtxRWC(WithCtxRWC(i), bg) == i )
+}