Add Files action to template context. (closes #1198) (#1226)

* Add Files action to template context. (#1198)

* Fixes to testFiles().

- Set os.ModePerm on directories created during test.
- Use filepath.Join() to create directory path.
- Use Fatalf, not Fatal.

* Make additional fixes to test cases.

* Fix test cases to use correct path format.

Dir.Open() in net/http requires '/'-separated paths while
filepath.Join() may produce paths with different separator.

* Remove directory created by test at end of loop.

* Close the FileSystem before returning.

* Initialize names slice to the number of entries.

Also, do not call os.RemoveAll() unless the path to the directory
is a valid one.

caddyhttp/httpserver/context.go

@@ -288,3 +288,33 @@ func (c Context) Map(values ...interface{}) (map[string]interface{}, error) {
 	}
 	return dict, nil
 }
+
+// Files reads and returns a slice of names from the given directory
+// relative to the root of Context c.
+func (c Context) Files(name string) ([]string, error) {
+	dir, err := c.Root.Open(path.Clean(name))
+	if err != nil {
+		return nil, err
+	}
+	defer dir.Close()
+
+	stat, err := dir.Stat()
+	if err != nil {
+		return nil, err
+	}
+	if !stat.IsDir() {
+		return nil, fmt.Errorf("%v is not a directory", name)
+	}
+
+	dirInfo, err := dir.Readdir(0)
+	if err != nil {
+		return nil, err
+	}
+
+	names := make([]string, len(dirInfo))
+	for i, fileInfo := range dirInfo {
+		names[i] = fileInfo.Name()
+	}
+
+	return names, nil
+}