map: Coerce val to string, fix #4987

Also prevent infinite recursion, and enforce placeholder syntax.

modules/caddyhttp/map/caddyfile.go

@@ -76,7 +76,7 @@ func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error)
 				continue
 			}
 
-			// every other line maps one input to one or more outputs
+			// every line maps an input value to one or more outputs
 			in := h.Val()
 			var outs []any
 			for h.NextArg() {

modules/caddyhttp/map/map.go

@@ -62,6 +62,9 @@ func (Handler) CaddyModule() caddy.ModuleInfo {
 // Provision sets up h.
 func (h *Handler) Provision(_ caddy.Context) error {
 	for j, dest := range h.Destinations {
+		if strings.Count(dest, "{") != 1 || !strings.HasPrefix(dest, "{") {
+			return fmt.Errorf("destination must be a placeholder and only a placeholder")
+		}
 		h.Destinations[j] = strings.Trim(dest, "{}")
 	}
 
@@ -106,6 +109,16 @@ func (h *Handler) Validate() error {
 		}
 		seen[input] = i
 
+		// prevent infinite recursion
+		for _, out := range m.Outputs {
+			for _, dest := range h.Destinations {
+				if strings.Contains(caddy.ToString(out), dest) ||
+					strings.Contains(m.Input, dest) {
+					return fmt.Errorf("mapping %d requires value of {%s} to define value of {%s}: infinite recursion", i, dest, dest)
+				}
+			}
+		}
+
 		// ensure mappings have 1:1 output-to-destination correspondence
 		nOut := len(m.Outputs)
 		if nOut != nDest {
@@ -135,22 +148,23 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhtt
 			if output == nil {
 				continue
 			}
+			outputStr := caddy.ToString(output)
+
+			// evaluate regular expression if configured
 			if m.re != nil {
 				var result []byte
 				matches := m.re.FindStringSubmatchIndex(input)
 				if matches == nil {
 					continue
 				}
-				result = m.re.ExpandString(result, output.(string), input, matches)
+				result = m.re.ExpandString(result, outputStr, input, matches)
 				return string(result), true
 			}
+
+			// otherwise simple string comparison
 			if input == m.Input {
-				if outputStr, ok := output.(string); ok {
-					// NOTE: if the output has a placeholder that has the same key as the input, this is infinite recursion
 				return repl.ReplaceAll(outputStr, ""), true
 			}
-				return output, true
-			}
 		}
 
 		// fall back to default if no match or if matched nil value

modules/caddyhttp/templates/tplcontext.go

@@ -305,7 +305,7 @@ func (TemplateContext) funcStripHTML(s string) string {
 // funcMarkdown renders the markdown body as HTML. The resulting
 // HTML is NOT escaped so that it can be rendered as HTML.
 func (TemplateContext) funcMarkdown(input any) (string, error) {
-	inputStr := toString(input)
+	inputStr := caddy.ToString(input)
 
 	md := goldmark.New(
 		goldmark.WithExtensions(
@@ -341,7 +341,7 @@ func (TemplateContext) funcMarkdown(input any) (string, error) {
 // and returns the separated key-value pairs and the body/content. input
 // must be a "stringy" value.
 func (TemplateContext) funcSplitFrontMatter(input any) (parsedMarkdownDoc, error) {
-	meta, body, err := extractFrontMatter(toString(input))
+	meta, body, err := extractFrontMatter(caddy.ToString(input))
 	if err != nil {
 		return parsedMarkdownDoc{}, err
 	}
@@ -465,19 +465,6 @@ func (h WrappedHeader) Del(field string) string {
 	return ""
 }
 
-func toString(input any) string {
-	switch v := input.(type) {
-	case string:
-		return v
-	case fmt.Stringer:
-		return v.String()
-	case error:
-		return v.Error()
-	default:
-		return fmt.Sprintf("%v", input)
-	}
-}
-
 var bufPool = sync.Pool{
 	New: func() any {
 		return new(bytes.Buffer)

replacer.go

@@ -79,10 +79,10 @@ func (r *Replacer) Get(variable string) (any, bool) {
 }
 
 // GetString is the same as Get, but coerces the value to a
-// string representation.
+// string representation as efficiently as possible.
 func (r *Replacer) GetString(variable string) (string, bool) {
 	s, found := r.Get(variable)
-	return toString(s), found
+	return ToString(s), found
 }
 
 // Delete removes a variable with a static value
@@ -204,7 +204,7 @@ scan:
 		}
 
 		// convert val to a string as efficiently as possible
-		valStr := toString(val)
+		valStr := ToString(val)
 
 		// write the value; if it's empty, either return
 		// an error or write a default value
@@ -230,7 +230,9 @@ scan:
 	return sb.String(), nil
 }
 
-func toString(val any) string {
+// ToString returns val as a string, as efficiently as possible.
+// EXPERIMENTAL: may be changed or removed later.
+func ToString(val any) string {
 	switch v := val.(type) {
 	case nil:
 		return ""