check: implement "missing" failure type

If a file is missing, previously the check would simply fail. This
allows you to get information even if files are missing from the tree.
This also implements some of the wrappers required to add more failure
states.

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

check.go

@@ -1,6 +1,7 @@
 package mtree
 
 import (
+	"encoding/json"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -13,17 +14,108 @@ type Result struct {
 	Failures []Failure `json:"failures"`
 }
 
-// Failure of a particular keyword for a path
-type Failure struct {
-	Path     string `json:"path"`
-	Keyword  string `json:"keyword"`
-	Expected string `json:"expected"`
-	Got      string `json:"got"`
+// FailureType represents a type of Failure encountered when checking for
+// discrepancies between a manifest and a directory state.
+type FailureType string
+
+const (
+	// None means no discrepancy (unused).
+	None FailureType = "none"
+
+	// Missing represents a discrepancy where an object is present in the
+	// manifest but is not present in the directory state being checked.
+	Missing FailureType = "missing"
+
+	// Modified represents a discrepancy where one or more of the keywords
+	// present in the manifest do not match the keywords generated for the same
+	// object in the directory state being checked.
+	Modified FailureType = "modified"
+)
+
+// Failure represents a discrepancy between a manifest and the state it is
+// being compared against. The discrepancy may take the form of a missing,
+// erroneous or otherwise modified object.
+type Failure interface {
+	// String returns a "pretty" formatting for a Failure. It's based on the
+	// BSD mtree(8) format, so that we are compatible.
+	String() string
+
+	// MarshalJSON ensures that we fulfil the JSON Marshaler interface.
+	MarshalJSON() ([]byte, error)
+
+	// Path returns the path (relative to the root of the tree) that this
+	// discrepancy refers to.
+	Path() string
+
+	// Type returns the type of failure that occurred.
+	Type() FailureType
 }
 
-// String returns a "pretty" formatting for a Failure
-func (f Failure) String() string {
-	return fmt.Sprintf("%q: keyword %q: expected %s; got %s", f.Path, f.Keyword, f.Expected, f.Got)
+// An object modified from the manifest state.
+type modified struct {
+	path     string
+	keyword  string
+	expected string
+	got      string
+}
+
+func (m modified) String() string {
+	return fmt.Sprintf("%q: keyword %q: expected %s; got %s", m.path, m.keyword, m.expected, m.got)
+}
+
+func (m modified) MarshalJSON() ([]byte, error) {
+	// Because of Go's reflection policies, we have to make an anonymous struct
+	// with the fields exported.
+	return json.Marshal(struct {
+		Type     FailureType `json:"type"`
+		Path     string      `json:"path"`
+		Keyword  string      `json:"keyword"`
+		Expected string      `json:"expected"`
+		Got      string      `json:"got"`
+	}{
+		Type:     m.Type(),
+		Path:     m.path,
+		Keyword:  m.keyword,
+		Expected: m.expected,
+		Got:      m.got,
+	})
+}
+
+func (m modified) Path() string {
+	return m.path
+}
+
+func (m modified) Type() FailureType {
+	return Modified
+}
+
+// An object that is listed in the manifest but is not present in the state.
+type missing struct {
+	path string
+}
+
+func (m missing) String() string {
+	return fmt.Sprintf("%q: expected object missing", m.path)
+}
+
+func (m missing) MarshalJSON() ([]byte, error) {
+	// Because of Go's reflection policies, we have to make an anonymous struct
+	// with the fields exported.
+	return json.Marshal(struct {
+		Type FailureType `json:"type"`
+		Path string      `json:"path"`
+	}{
+		Type: m.Type(),
+		Path: m.path,
+	})
+}
+
+func (m missing) Path() string {
+	return m.path
+}
+
+func (m missing) Type() FailureType {
+	return Missing
 }
 
 // Check a root directory path against the DirectoryHierarchy, regarding only
@@ -53,6 +145,12 @@ func Check(root string, dh *DirectoryHierarchy, keywords []string) (*Result, err
 		case RelativeType, FullType:
 			info, err := os.Lstat(e.Path())
 			if err != nil {
+				if os.IsNotExist(err) {
+					result.Failures = append(result.Failures, missing{
+						path: e.Path(),
+					})
+					continue
+				}
 				return nil, err
 			}
 
@@ -76,8 +174,12 @@ func Check(root string, dh *DirectoryHierarchy, keywords []string) (*Result, err
 					return nil, err
 				}
 				if string(kv) != curKeyVal {
-					failure := Failure{Path: e.Path(), Keyword: kv.Keyword(), Expected: kv.Value(), Got: KeyVal(curKeyVal).Value()}
-					result.Failures = append(result.Failures, failure)
+					result.Failures = append(result.Failures, modified{
+						path:     e.Path(),
+						keyword:  kv.Keyword(),
+						expected: kv.Value(),
+						got:      KeyVal(curKeyVal).Value(),
+					})
 				}
 			}
 		}

cmd/gomtree/main.go

@@ -45,7 +45,7 @@ var formats = map[string]func(*mtree.Result) string{
 	"text": func(r *mtree.Result) string {
 		var buffer bytes.Buffer
 		for _, fail := range r.Failures {
-			fmt.Fprintf(&buffer, "modified\t%s\n", fail.Path)
+			fmt.Fprintf(&buffer, "%s\t%s\n", fail.Type(), fail.Path())
 		}
 		return buffer.String()
 	},