add rsynccmd entrypoint

This is a generic entrypoint into any supported rsync functionality.

If you want to implement a server, see github.com/gokrazy/rsync/rsyncd.

rsynccmd/rsynccmd.go

@@ -0,0 +1,59 @@
+// Package rsynccmd provides a command-like interface to gokrazy/rsync, which
+// contains a native Go rsync implementation that supports sending and receiving
+// files as client or server, compatible with the original tridge rsync (from
+// the samba project) or openrsync (used on OpenBSD and macOS 15+).
+//
+// This interface allows you to replace calls to an external rsync program, like:
+//
+//	rsync -a rsync://share01/dataset /tmp/dataset
+//
+// …with calls into Go code running in the same process (no dependency on an
+// external rsync program!):
+//
+//	cmd := rsynccmd.Command("rsync", "-a", "rsync://share01/dataset", "/tmp/dataset")
+//	cmd.Stdout = os.Stdout
+//	cmd.Stderr = os.Stderr
+//	if _, err := cmd.Run(context.Background()); err != nil {
+//	  return fmt.Errorf("%v: %v", cmd.Args, err)
+//	}
+package rsynccmd
+
+import (
+	"context"
+	"io"
+
+	"github.com/gokrazy/rsync/internal/maincmd"
+	"github.com/gokrazy/rsync/internal/rsyncstats"
+)
+
+// Cmd represents an rsync invocation being prepared or run.
+type Cmd struct {
+	Args   []string
+	Stdin  io.Reader
+	Stdout io.Writer
+	Stderr io.Writer
+}
+
+// Command returns the [Cmd] struct to execute rsync with the given arguments.
+//
+// The name parameter is ignored and only specified for symmetry with
+// os/exec.Command.
+func Command(name string, arg ...string) *Cmd {
+	return &Cmd{
+		Args: append([]string{name}, arg...),
+	}
+}
+
+// Result contains information about the transfer.
+type Result struct {
+	Stats *rsyncstats.TransferStats
+}
+
+// Run starts the specified rsync invocation.
+func (c *Cmd) Run(ctx context.Context) (*Result, error) {
+	stats, err := maincmd.Main(ctx, c.Args, c.Stdin, c.Stdout, c.Stderr, nil)
+	if err != nil {
+		return nil, err
+	}
+	return &Result{Stats: stats}, nil
+}