caddyhttp: Accept placeholders in vars matcher key

Until now, the vars matcher has unintentionally lacked parity with the
map directive: the destination placeholders of the map directive would
be expressed as placeholders, i.e. {foo}. But the vars matcher would
not use { }: vars foo value

This looked weird, and was confusing, since it implied that the key
could be dynamic, which doesn't seem helpful here.

I think this is a proper bug fix, since we're not used to accessing
placeholders literally without { } in the Caddyfile.

modules/caddyhttp/vars.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/caddyserver/caddy/v2"
 	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
@@ -66,7 +67,6 @@ func (m VarsMiddleware) ServeHTTP(w http.ResponseWriter, r *http.Request, next H
 //	    <name> <val>
 //	    ...
 //	}
-//
 func (m *VarsMiddleware) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 	if *m == nil {
 		*m = make(VarsMiddleware)
@@ -109,14 +109,17 @@ func (m *VarsMiddleware) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 }
 
 // VarsMatcher is an HTTP request matcher which can match
-// requests based on variables in the context. The key is
-// the name of the variable, and the values are possible
-// values the variable can be in order to match (OR'ed).
+// requests based on variables in the context or placeholder
+// values. The key is the placeholder or name of the variable,
+// and the values are possible values the variable can be in
+// order to match (logical OR'ed).
 //
-// As a special case, this matcher can also match on
-// placeholders generally. If the key is not an HTTP chain
-// variable, it will be checked to see if it is a
-// placeholder name, and if so, will compare its value.
+// If the key is surrounded by `{ }` it is assumed to be a
+// placeholder. Otherwise, it will be considered a variable
+// name.
+//
+// Placeholders in the keys are not expanded, but
+// placeholders in the values are.
 type VarsMatcher map[string][]string
 
 // CaddyModule returns the Caddy module information.
@@ -160,13 +163,13 @@ func (m VarsMatcher) Match(r *http.Request) bool {
 	repl := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
 
 	for key, vals := range m {
-		// look up the comparison value we will check against with this key
-		matcherVarNameExpanded := repl.ReplaceAll(key, "")
-		varValue, ok := vars[matcherVarNameExpanded]
-		if !ok {
-			// as a special case, if it's not an HTTP variable,
-			// see if it's a placeholder name
-			varValue, _ = repl.Get(matcherVarNameExpanded)
+		var varValue any
+		if strings.HasPrefix(key, "{") &&
+			strings.HasSuffix(key, "}") &&
+			strings.Count(key, "{") == 1 {
+			varValue, _ = repl.Get(strings.Trim(key, "{}"))
+		} else {
+			varValue = vars[key]
 		}
 
 		// see if any of the values given in the matcher match the actual value

replacer.go

@@ -238,6 +238,8 @@ func toString(val any) string {
 		return v
 	case fmt.Stringer:
 		return v.String()
+	case error:
+		return v.Error()
 	case byte:
 		return string(v)
 	case []byte: