templates: Implement placeholders function (#3324)

* caddyhttp, httpcaddyfile: Implement placeholders in template

* caddyhttp, httpcaddyfile: Remove support for placeholder shorthands in templates

* Update modules/caddyhttp/templates/templates.go

updates JSON doc

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

* Update modules/caddyhttp/templates/tplcontext.go

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

modules/caddyhttp/templates/templates.go

@@ -64,6 +64,16 @@ func init() {
 // {{env "VAR_NAME"}}
 // ```
 //
+// ##### `placeholder`
+//
+// Gets an [placeholder variable](/docs/conventions#placeholders).
+// The braces (`{}`) have to be omitted.
+//
+// ```
+// {{placeholder "http.request.uri.path"}}
+// {{placeholder "http.error.status_code"}}
+// ```
+//
 // ##### `.Host`
 //
 // Returns the hostname portion (no port) of the Host header of the HTTP request.

modules/caddyhttp/templates/tplcontext.go

@@ -29,6 +29,7 @@ import (
 
 	"github.com/Masterminds/sprig/v3"
 	"github.com/alecthomas/chroma/formatters/html"
+	"github.com/caddyserver/caddy/v2"
 	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
 	"github.com/yuin/goldmark"
 	highlighting "github.com/yuin/goldmark-highlighting"
@@ -152,6 +153,7 @@ func (c templateContext) executeTemplateInBuffer(tplName string, buf *bytes.Buff
 		"splitFrontMatter": c.funcSplitFrontMatter,
 		"listFiles":        c.funcListFiles,
 		"env":              c.funcEnv,
+		"placeholder":      c.placeholder,
 	})
 
 	parsedTpl, err := tpl.Parse(buf.String())
@@ -164,6 +166,12 @@ func (c templateContext) executeTemplateInBuffer(tplName string, buf *bytes.Buff
 	return parsedTpl.Execute(buf, c)
 }
 
+func (c templateContext) placeholder(name string) string {
+	repl := c.Req.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+	value, _ := repl.GetString(name)
+	return value
+}
+
 func (templateContext) funcEnv(varName string) string {
 	return os.Getenv(varName)
 }