wcfs: Add FileSock FUSE utility

FileSock is bidirectional channel associated with opened file.

FileSock provides streaming write/read operations for filesystem server that
are correspondingly matched with read/write operations on filesystem user side.

WCFS will use FileSock to implement exchange over .wcfs/zhead and,
later, head/watch files.

Some preliminary history:

kirr/wendelin.core@b17aeb8c    X Change FileSock to use xio.Pipe which is io.Pipe + support for IO cancellation

wcfs/misc.go

@@ -24,6 +24,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"math"
 	"sync/atomic"
 	"syscall"
 
@@ -32,6 +33,8 @@ import (
 	"github.com/hanwen/go-fuse/v2/fuse"
 	"github.com/hanwen/go-fuse/v2/fuse/nodefs"
 	"github.com/pkg/errors"
+
+	"lab.nexedi.com/kirr/go123/xio"
 )
 
 // ---- FUSE ----
@@ -263,6 +266,164 @@ func mount(mntpt string, root nodefs.Node, opts *fuse.MountOptions) (*fuse.Serve
 }
 
 
+// FileSock is bidirectional channel associated with opened file.
+//
+// FileSock provides streaming write/read operations for filesystem server that
+// are correspondingly matched with read/write operations on filesystem user side.
+type FileSock struct {
+	file *skFile         // filesock's file peer
+	rx   *xio.PipeReader // socket reads from file here
+	tx   *xio.PipeWriter // socket writes to file here
+}
+
+// skFile is File peer of FileSock.
+//
+// skFile.Read  is connected with sk.Write.
+// skFile.Write is connected with sk.Read.
+//
+// skFile is always created with nonseekable & directio flags, to support
+// streaming semantics.
+type skFile struct {
+	nodefs.File
+
+	rx *xio.PipeReader // file reads from socket here
+	tx *xio.PipeWriter // file writes to socket here
+}
+
+// NewFileSock creates new file socket.
+//
+// After file socket is created, File return should be given to kernel for the
+// socket to be connected to an opened file.
+func NewFileSock() *FileSock {
+	sk := &FileSock{}
+	f  := &skFile{
+		File: nodefs.NewDefaultFile(),
+	}
+	sk.file = f
+
+	rx, tx := xio.Pipe()
+	sk.rx = rx
+	f .tx = tx
+
+	rx, tx = xio.Pipe()
+	f .rx = rx
+	sk.tx = tx
+
+	return sk
+}
+
+// File returns nodefs.File handle that is connected to the socket.
+//
+// The handle should be given to kernel as result of a file open, for that file
+// to be connected to the socket.
+func (sk *FileSock) File() nodefs.File {
+	// nonseekable & directio for opened file to have streaming semantic as
+	// if it was a socket. FOPEN_STREAM is used so that both read and write
+	// could be run simultaneously: git.kernel.org/linus/10dce8af3422
+	return &nodefs.WithFlags{
+		File:      sk.file,
+		FuseFlags: fuse.FOPEN_STREAM | fuse.FOPEN_NONSEEKABLE | fuse.FOPEN_DIRECT_IO,
+	}
+}
+
+
+// Write writes data to filesock.
+//
+// The data will be read by client reading from filesock's file.
+// Write semantic is that of xio.Writer.
+func (sk *FileSock) Write(ctx context.Context, data []byte) (n int, err error) {
+	return sk.tx.Write(ctx, data)
+}
+
+// Read implements nodefs.File and is paired with filesock.Write().
+func (f *skFile) Read(dest []byte, /*ignored*/off int64, fctx *fuse.Context) (fuse.ReadResult, fuse.Status) {
+	n, err := f.rx.Read(fctx, dest)
+	if n != 0 {
+		err = nil
+	}
+	if err == io.EOF {
+		n = 0 // read(2): "zero indicates end of file"
+		err = nil
+	}
+	if err != nil {
+		return nil, err2LogStatus(err)
+	}
+	return fuse.ReadResultData(dest[:n]), fuse.OK
+}
+
+
+// Read reads data from filesock.
+//
+// The data read will be that the client writes into filesock's file.
+// Read semantic is that of xio.Reader.
+func (sk *FileSock) Read(ctx context.Context, dest []byte) (n int, err error) {
+	return sk.rx.Read(ctx, dest)
+}
+
+// Write implements nodefs.File and is paired with filesock.Read()
+func (f *skFile) Write(data []byte, /*ignored*/off int64, fctx *fuse.Context) (uint32, fuse.Status) {
+	// cap data to 2GB (not 4GB not to overflow int on 32-bit platforms)
+	l := len(data)
+	if l > math.MaxInt32 {
+		l = math.MaxInt32
+		data = data[:l]
+	}
+
+	n, err := f.tx.Write(fctx, data)
+	if n != 0 {
+		err = nil
+	}
+	if err == io.ErrClosedPipe {
+		err = syscall.ECONNRESET
+	}
+	if err != nil {
+		return 0, err2LogStatus(err)
+	}
+	return uint32(n), fuse.OK
+}
+
+
+// CloseRead closes reading side of filesock.
+func (sk *FileSock) CloseRead() error {
+	return sk.rx.Close()
+}
+
+// CloseWrite closes writing side of filesock.
+func (sk *FileSock) CloseWrite() error {
+	return sk.tx.Close()
+}
+
+// Close closes filesock.
+//
+// it is semantically equivalent to CloseRead + CloseWrite.
+func (sk *FileSock) Close() error {
+	err := sk.CloseRead()
+	err2 := sk.CloseWrite()
+	if err == nil {
+		err = err2
+	}
+	return err
+}
+
+// Release implements nodefs.File to handle when last user reference to the file is gone.
+//
+// Note: it is not Flush, since Fush is called on close(file) and in general
+// multiple time (e.g. in case of duplicated file descriptor).
+func (f *skFile) Release() {
+	err := f.rx.Close()
+	err2 := f.tx.Close()
+	if err == nil {
+		err = err2
+	}
+
+	// the kernel ignores any error return from release.
+	// close on pipes always returns nil and can be called multiple times.
+	if err != nil {
+		panic(err)
+	}
+}
+
+
 // ---- make df happy (else it complains "function not supported") ----
 
 func (root *Root) StatFs() *fuse.StatfsOut {