zfs: implement ZFSSend

zfs/fork_reader.go

@@ -0,0 +1,72 @@
+package zfs
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"os"
+	"os/exec"
+	"sync"
+)
+
+// A ForkReader is an io.Reader for a forked process's stdout.
+// It Wait()s for the process to exit and - if it exits with error - returns this exit error
+// on subsequent Read()s.
+type ForkReader struct {
+	cancelFunc    context.CancelFunc
+	cmd           *exec.Cmd
+	stdout        io.Reader
+	waitErr       error
+	exitWaitGroup sync.WaitGroup
+}
+
+func NewForkReader(command string, args ...string) (r *ForkReader, err error) {
+
+	r = &ForkReader{}
+
+	var ctx context.Context
+	ctx, r.cancelFunc = context.WithCancel(context.Background())
+
+	cmd := exec.CommandContext(ctx, command, args...)
+
+	stderr := bytes.NewBuffer(make([]byte, 0, 1024))
+	cmd.Stderr = stderr
+
+	if r.stdout, err = cmd.StdoutPipe(); err != nil {
+		return
+	}
+
+	if err = cmd.Start(); err != nil {
+		return
+	}
+	r.exitWaitGroup.Add(1)
+
+	go func() {
+		defer r.exitWaitGroup.Done()
+		os.Stderr.WriteString("waiting")
+		if err := cmd.Wait(); err != nil {
+			os.Stderr.WriteString(err.Error())
+			r.waitErr = ZFSError{
+				Stderr:  stderr.Bytes(),
+				WaitErr: err,
+			}
+			return
+		}
+		os.Stderr.WriteString("exited")
+	}()
+	return
+}
+
+func (r *ForkReader) Read(buf []byte) (n int, err error) {
+	if r.waitErr != nil {
+		return 0, r.waitErr
+	}
+	if n, err = r.stdout.Read(buf); err == io.EOF {
+		// the command has exited but we need to wait for Wait()ing goroutine to finish
+		r.exitWaitGroup.Wait()
+		if r.waitErr != nil {
+			err = r.waitErr
+		}
+	}
+	return
+}