Removes leading slash from paths, to normalize expressions (#32)

Paketo images have a mix of layers where some are prefixed with '/' and
others aren't. This normalizes to the default ( no prefix ).

Signed-off-by: Adrian Cole <[email protected]>

internal/car/car.go

@@ -82,6 +82,7 @@ func (c *car) do(ctx context.Context, readFile api.ReadFile, ref api.Reference,
 	}
 	pm := patternmatcher.New(c.filePatterns, c.fastRead)
 	rf := func(name string, size int64, mode os.FileMode, modTime time.Time, reader io.Reader) error {
+		name = stripLeadingSlash(name)
 		if !pm.MatchesPattern(name) {
 			return nil
 		}
@@ -187,3 +188,13 @@ func (c *car) getFilesystemLayers(ctx context.Context, ref api.Reference, platfo
 	}
 	return filteredLayers, nil
 }
+
+// stripLeadingSlash removes any leading slash from the input file name, to
+// normalize pattern matching. For example, paketo images have a combination of
+// relative and absolute paths in their squashed image.
+func stripLeadingSlash(name string) string {
+	if len(name) > 0 && name[0] == '/' {
+		return name[1:]
+	}
+	return name
+}

internal/registry/fake/fake.go

@@ -186,7 +186,8 @@ type fakeFile struct {
 // The fake data intentionally overlaps on "usr/local" for testing. Even if weird, it adds windows paths.
 var fakeFiles = [][]*fakeFile{
 	{
-		{"bin/apple.txt", 10, 0o640 & os.ModePerm, "2020-06-07T06:28:15Z"},
+		// intentionally leading slash
+		{"/bin/apple.txt", 10, 0o640 & os.ModePerm, "2020-06-07T06:28:15Z"},
 		{"usr/local/bin/boat", 20, 0o755 & os.ModePerm, "2021-04-16T22:53:09Z"},
 	},
 	{