add basic S3-Compatible Object Storage support

we have now an interface for filesystem backeds, this make easy to add
new filesystem backends

vfs/vfs.go

@@ -0,0 +1,100 @@
+package vfs
+
+import (
+	"errors"
+	"os"
+	"runtime"
+	"time"
+
+	"github.com/drakkan/sftpgo/logger"
+	"github.com/eikenb/pipeat"
+	"github.com/pkg/sftp"
+)
+
+// Fs defines the interface for filesystems backends
+type Fs interface {
+	Name() string
+	ConnectionID() string
+	Stat(name string) (os.FileInfo, error)
+	Lstat(name string) (os.FileInfo, error)
+	Open(name string) (*os.File, *pipeat.PipeReaderAt, func(), error)
+	Create(name string, flag int) (*os.File, *pipeat.PipeWriterAt, func(), error)
+	Rename(source, target string) error
+	Remove(name string, isDir bool) error
+	Mkdir(name string) error
+	Symlink(source, target string) error
+	Chown(name string, uid int, gid int) error
+	Chmod(name string, mode os.FileMode) error
+	Chtimes(name string, atime, mtime time.Time) error
+	ReadDir(dirname string) ([]os.FileInfo, error)
+	IsUploadResumeSupported() bool
+	IsAtomicUploadSupported() bool
+	CheckRootPath(rootPath, username string, uid int, gid int) bool
+	ResolvePath(sftpPath, rootPath string) (string, error)
+	IsNotExist(err error) bool
+	IsPermission(err error) bool
+	ScanDirContents(dirPath string) (int, int64, error)
+	GetAtomicUploadPath(name string) string
+	GetRelativePath(name, rootPath string) string
+	Join(elem ...string) string
+}
+
+// IsDirectory checks if a path exists and is a directory
+func IsDirectory(fs Fs, path string) (bool, error) {
+	fileInfo, err := fs.Stat(path)
+	if err != nil {
+		return false, err
+	}
+	return fileInfo.IsDir(), err
+}
+
+// GetSFTPError returns an sftp error from a filesystem error
+func GetSFTPError(fs Fs, err error) error {
+	if fs.IsNotExist(err) {
+		return sftp.ErrSSHFxNoSuchFile
+	} else if fs.IsPermission(err) {
+		return sftp.ErrSSHFxPermissionDenied
+	} else if err != nil {
+		return sftp.ErrSSHFxFailure
+	}
+	return nil
+}
+
+// IsLocalOsFs returns true if fs is the local filesystem implementation
+func IsLocalOsFs(fs Fs) bool {
+	return fs.Name() == osFsName
+}
+
+// ValidateS3FsConfig returns nil if the specified s3 config is valid, otherwise an error
+func ValidateS3FsConfig(config *S3FsConfig) error {
+	if len(config.Bucket) == 0 {
+		return errors.New("bucket cannot be empty")
+	}
+	if len(config.Region) == 0 {
+		return errors.New("region cannot be empty")
+	}
+	if len(config.AccessKey) == 0 {
+		return errors.New("access_key cannot be empty")
+	}
+	if len(config.AccessSecret) == 0 {
+		return errors.New("access_secret cannot be empty")
+	}
+	return nil
+}
+
+// SetPathPermissions calls fs.Chown.
+// It does nothing for local filesystem on windows
+func SetPathPermissions(fs Fs, path string, uid int, gid int) {
+	if IsLocalOsFs(fs) {
+		if runtime.GOOS == "windows" {
+			return
+		}
+	}
+	if err := fs.Chown(path, uid, gid); err != nil {
+		fsLog(fs, logger.LevelWarn, "error chowning path %v: %v", path, err)
+	}
+}
+
+func fsLog(fs Fs, level logger.LogLevel, format string, v ...interface{}) {
+	logger.Log(level, fs.Name(), fs.ConnectionID(), format, v...)
+}