headers: Support deleting all headers as first op (#5464)

* Delete all existing fields when fieldName is `*`

* Rearrange deletion before addition in headers

* Revert "Rearrange deletion before addition in headers"

This reverts commit 1b50eeeccc92ccd660c7896d8283c7d9e5d1fcb0.

* Treat deleting all headers as a special case

* Apply suggestions from code review

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

---------

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

modules/caddyhttp/headers/headers.go

@@ -192,6 +192,19 @@ type RespHeaderOps struct {
 
 // ApplyTo applies ops to hdr using repl.
 func (ops HeaderOps) ApplyTo(hdr http.Header, repl *caddy.Replacer) {
+	// before manipulating headers in other ways, check if there
+	// is configuration to delete all headers, and do that first
+	// because if a header is to be added, we don't want to delete
+	// it also
+	for _, fieldName := range ops.Delete {
+		fieldName = repl.ReplaceKnown(fieldName, "")
+		if fieldName == "*" {
+			for existingField := range hdr {
+				delete(hdr, existingField)
+			}
+		}
+	}
+
 	// add
 	for fieldName, vals := range ops.Add {
 		fieldName = repl.ReplaceKnown(fieldName, "")
@@ -215,6 +228,9 @@ func (ops HeaderOps) ApplyTo(hdr http.Header, repl *caddy.Replacer) {
 	// delete
 	for _, fieldName := range ops.Delete {
 		fieldName = strings.ToLower(repl.ReplaceKnown(fieldName, ""))
+		if fieldName == "*" {
+			continue // handled above
+		}
 		switch {
 		case strings.HasPrefix(fieldName, "*") && strings.HasSuffix(fieldName, "*"):
 			for existingField := range hdr {