walk: implement FsEval hooks

In certain circumstances (such as the manifest generation of a
filesystem as an unprivileged user) it is important to provide hooks
that override the default os.* implementation of filesystem-related
functions.

In order to avoid merging too much code from outside projects (such as
umoci) this is implemented by providing FsEval hooks to Walk() and
Check(). This allows for users of go-mtree to modify how filesystem
checks are done, without compromising the simplicity of go-mtree's code.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

fseval.go

@@ -0,0 +1,54 @@
+package mtree
+
+import "os"
+
+// FsEval is a mock-friendly method of specifying to go-mtree how to carry out
+// filesystem operations such as opening files and the like. The semantics of
+// all of these wrappers MUST be identical to the semantics described here.
+type FsEval interface {
+	// Open must have the same semantics as os.Open.
+	Open(path string) (*os.File, error)
+
+	// Lstat must have the same semantics as os.Lstat.
+	Lstat(path string) (os.FileInfo, error)
+
+	// Readdir must have the same semantics as calling os.Open on the given
+	// path and then returning the result of (*os.File).Readdir(-1).
+	Readdir(path string) ([]os.FileInfo, error)
+
+	// KeywordFunc must return a wrapper around the provided function (in other
+	// words, the returned function must refer to the same keyword).
+	KeywordFunc(fn KeywordFunc) KeywordFunc
+}
+
+// DefaultFsEval is the default implementation of FsEval (and is the default
+// used if a nil interface is passed to any mtree function). It does not modify
+// or wrap any of the methods (they all just call out to os.*).
+type DefaultFsEval struct{}
+
+// Open must have the same semantics as os.Open.
+func (fs DefaultFsEval) Open(path string) (*os.File, error) {
+	return os.Open(path)
+}
+
+// Lstat must have the same semantics as os.Lstat.
+func (fs DefaultFsEval) Lstat(path string) (os.FileInfo, error) {
+	return os.Lstat(path)
+}
+
+// Readdir must have the same semantics as calling os.Open on the given
+// path and then returning the result of (*os.File).Readdir(-1).
+func (fs DefaultFsEval) Readdir(path string) ([]os.FileInfo, error) {
+	fh, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer fh.Close()
+	return fh.Readdir(-1)
+}
+
+// KeywordFunc must return a wrapper around the provided function (in other
+// words, the returned function must refer to the same keyword).
+func (fs DefaultFsEval) KeywordFunc(fn KeywordFunc) KeywordFunc {
+	return fn
+}