Correct directory diffing test and algorithm

Two steps:

1. TestDiffDirectories did not check if the expected only return value
was correct; the intention was there to do so (judging by the
comment "change in order"), but my eye for detail failed me.

2. Reversing the directory comparison in the test revealed bugs in the
comparison code -- in general, it should skip any directory that is
not a directory in the comparator.

To make this easier, the code now keeps track of the expected files it
saw. That means the check for whether an actual file has an expected
counterpart only has two cases, yes or no (rather that trying to
account for whether it's a directory and so on). If a directory was
skipped while scanning the expected files, it won't be in the actual
files anyway.

Signed-off-by: Michael Bridgen <mikeb@weave.works>

pkg/test/files.go

@@ -83,16 +83,21 @@ func (d dirfile) FailedExpectation(g *WithT) {
 // `foo.yaml~`). It panics if it encounters any error apart from a
 // file not found.
 func DiffDirectories(actual, expected string) (actualonly []string, expectedonly []string, different []Diff) {
+	seen := make(map[string]struct{})
+
 	filepath.Walk(expected, func(expectedPath string, expectedInfo os.FileInfo, err error) error {
 		if err != nil {
 			panic(err)
 		}
+
+		relPath := expectedPath[len(expected):]
+		seen[relPath] = struct{}{}
+
 		// ignore emacs backups
 		if strings.HasSuffix(expectedPath, "~") {
 			return nil
 		}
-		relPath := expectedPath[len(expected):]
-		actualPath := filepath.Join(actual, relPath)
+
 		// ignore dotfiles
 		if strings.HasPrefix(filepath.Base(expectedPath), ".") {
 			if expectedInfo.IsDir() {
@@ -101,24 +106,30 @@ func DiffDirectories(actual, expected string) (actualonly []string, expectedonly
 			return nil
 		}
 
+		actualPath := filepath.Join(actual, relPath)
 		actualInfo, err := os.Stat(actualPath)
 		switch {
 		case err == nil:
 			break
 		case os.IsNotExist(err):
 			expectedonly = append(expectedonly, relPath)
+			if expectedInfo.IsDir() {
+				return filepath.SkipDir
+			}
 			return nil
 		default:
 			panic(err)
 		}
 
 		// file exists in both places
-
 		switch {
 		case actualInfo.IsDir() && expectedInfo.IsDir():
 			return nil // i.e., keep recursing
 		case actualInfo.IsDir() || expectedInfo.IsDir():
 			different = append(different, dirfile{path: relPath, abspath: actualPath, expectedRegularFile: actualInfo.IsDir()})
+			if expectedInfo.IsDir() {
+				return filepath.SkipDir
+			}
 			return nil
 		}
 
@@ -152,18 +163,12 @@ func DiffDirectories(actual, expected string) (actualonly []string, expectedonly
 		if actualInfo.IsDir() && strings.HasPrefix(filepath.Base(actualPath), ".") {
 			return filepath.SkipDir
 		}
-		// since I've already compared any file that exists in
-		// expected or both, I'm only concerned with files that appear
-		// in actual but not in expected.
-		expectedPath := filepath.Join(expected, relPath)
-		_, err = os.Stat(expectedPath)
-		switch {
-		case err == nil:
-			break
-		case os.IsNotExist(err):
+
+		if _, ok := seen[relPath]; !ok {
 			actualonly = append(actualonly, relPath)
-		default:
-			panic(err)
+			if actualInfo.IsDir() {
+				return filepath.SkipDir
+			}
 		}
 		return nil
 	})

pkg/test/files_test.go

@@ -51,13 +51,13 @@ func TestExpectMatchingDirectories(t *testing.T) {
 func TestDiffDirectories(t *testing.T) {
 	g := NewWithT(t)
 
-	// Finds files in expected from a/ but not in actual b/.
-	aonly, _, _ := DiffDirectories("testdata/diff/a", "testdata/diff/b")
-	g.Expect(aonly).To(Equal([]string{"/only", "/only/here.yaml", "/onlyhere.yaml"}))
-
 	// Finds files in actual a/ that weren't expected from b/.
-	bonly, _, _ := DiffDirectories("testdata/diff/a", "testdata/diff/b") // change in order
-	g.Expect(bonly).To(Equal([]string{"/only", "/only/here.yaml", "/onlyhere.yaml"}))
+	actualonly, _, _ := DiffDirectories("testdata/diff/a", "testdata/diff/b")
+	g.Expect(actualonly).To(Equal([]string{"/only", "/onlyhere.yaml"}))
+
+	// Finds files in expected from a/ but not in actual b/.
+	_, expectedonly, _ := DiffDirectories("testdata/diff/b", "testdata/diff/a") // NB change in order
+	g.Expect(expectedonly).To(Equal([]string{"/only", "/onlyhere.yaml"}))
 
 	// Finds files that are different in a and b.
 	_, _, diffs := DiffDirectories("testdata/diff/a", "testdata/diff/b")