Implement most of static file server; refactor and improve Replacer

replacer.go

@@ -0,0 +1,104 @@
+package caddy2
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// Replacer can replace values in strings.
+type Replacer interface {
+	Set(variable, value string)
+	Delete(variable string)
+	Map(func() map[string]string)
+	ReplaceAll(input, empty string) string
+}
+
+// NewReplacer returns a new Replacer.
+func NewReplacer() Replacer {
+	rep := &replacer{
+		static: make(map[string]string),
+	}
+	rep.providers = []ReplacementsFunc{
+		defaultReplacements,
+		func() map[string]string { return rep.static },
+	}
+	return rep
+}
+
+type replacer struct {
+	providers []ReplacementsFunc
+	static    map[string]string
+}
+
+// Map augments the map of replacements with those returned
+// by the given replacements function. The function is only
+// executed at replace-time.
+func (r *replacer) Map(replacements func() map[string]string) {
+	r.providers = append(r.providers, replacements)
+}
+
+// Set sets a custom variable to a static value.
+func (r *replacer) Set(variable, value string) {
+	r.static[variable] = value
+}
+
+// Delete removes a variable with a static value
+// that was created using Set.
+func (r *replacer) Delete(variable string) {
+	delete(r.static, variable)
+}
+
+// ReplaceAll replaces placeholders in input with their values.
+// Values that are empty string will be substituted with the
+// empty parameter.
+func (r *replacer) ReplaceAll(input, empty string) string {
+	if !strings.Contains(input, phOpen) {
+		return input
+	}
+	for _, replacements := range r.providers {
+		for key, val := range replacements() {
+			if val == "" {
+				val = empty
+			}
+			input = strings.ReplaceAll(input, phOpen+key+phClose, val)
+		}
+	}
+	return input
+}
+
+// ReplacementsFunc is a function that returns replacements,
+// which is variable names mapped to their values. The
+// function will be evaluated only at replace-time to ensure
+// the most current values are mapped.
+type ReplacementsFunc func() map[string]string
+
+var defaultReplacements = func() map[string]string {
+	m := map[string]string{
+		"system.hostname": func() string {
+			// OK if there is an error; just return empty string
+			name, _ := os.Hostname()
+			return name
+		}(),
+		"system.slash": string(filepath.Separator),
+		"system.os":    runtime.GOOS,
+		"system.arch":  runtime.GOARCH,
+	}
+
+	// add environment variables
+	for _, keyval := range os.Environ() {
+		parts := strings.SplitN(keyval, "=", 2)
+		if len(parts) != 2 {
+			continue
+		}
+		m["env."+strings.ToUpper(parts[0])] = parts[1]
+	}
+
+	return m
+}
+
+// ReplacerCtxKey is the context key for a replacer.
+const ReplacerCtxKey CtxKey = "replacer"
+
+const phOpen, phClose = "{", "}"