dircache: factor DirMove code out of backends into dircache

Before this change there was lots of duplicated code in all the
dircache using backends to support DirMove.

This change factors this code into the dircache library.

lib/dircache/dircache.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"path"
 	"strings"
 	"sync"
 
@@ -368,3 +369,66 @@ func (dc *DirCache) ResetRoot() {
 	// Put the root directory in
 	dc.Put("", dc.rootID)
 }
+
+// DirMove prepares to move the directory (srcDC, srcRoot, srcRemote)
+// into the directory (dc, dstRoot, dstRemote)
+//
+// It does all the checking, creates intermediate directories and
+// returns leafs and IDs ready for the move.
+//
+// This returns
+//
+// - srcID - ID of the source directory
+// - srcDirectoryID - ID of the parent of the source directory
+// - srcLeaf - leaf name of the source directory
+// - dstDirectoryID - ID of the parent of the destination directory
+// - dstLeaf - leaf name of the destination directory
+//
+// These should be used to do the actual move then
+// srcDC.FlushDir(srcRemote) should be called.
+func (dc *DirCache) DirMove(
+	ctx context.Context, srcDC *DirCache, srcRoot, srcRemote, dstRoot, dstRemote string) (srcID, srcDirectoryID, srcLeaf, dstDirectoryID, dstLeaf string, err error) {
+	var (
+		dstDC   = dc
+		srcPath = path.Join(srcRoot, srcRemote)
+		dstPath = path.Join(dstRoot, dstRemote)
+	)
+
+	// Refuse to move to or from the root
+	if srcPath == "" || dstPath == "" {
+		// fs.Debugf(src, "DirMove error: Can't move root")
+		err = errors.New("can't move root directory")
+		return
+	}
+
+	// Find ID of dst parent, creating subdirs if necessary
+	dstLeaf, dstDirectoryID, err = dstDC.FindPath(ctx, dstRemote, true)
+	if err != nil {
+		return
+	}
+
+	// Check destination does not exist
+	_, err = dstDC.FindDir(ctx, dstRemote, false)
+	if err == fs.ErrorDirNotFound {
+		// OK
+	} else if err != nil {
+		return
+	} else {
+		err = fs.ErrorDirExists
+		return
+	}
+
+	// Find ID of src parent
+	srcLeaf, srcDirectoryID, err = srcDC.FindPath(ctx, srcRemote, false)
+	if err != nil {
+		return
+	}
+
+	// Find ID of src
+	srcID, err = srcDC.FindDir(ctx, srcRemote, false)
+	if err != nil {
+		return
+	}
+
+	return
+}