Support PostProcess for all file types

Not just HTML.

Fixes #10269

common/hugio/hasBytesWriter.go

@@ -0,0 +1,57 @@
+// Copyright 2022 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugio
+
+import (
+	"bytes"
+)
+
+// HasBytesWriter is a writer that will set Match to true if the given pattern
+// is found in the stream.
+type HasBytesWriter struct {
+	Match   bool
+	Pattern []byte
+
+	i    int
+	done bool
+	buff []byte
+}
+
+func (h *HasBytesWriter) Write(p []byte) (n int, err error) {
+	if h.done {
+		return len(p), nil
+	}
+
+	if len(h.buff) == 0 {
+		h.buff = make([]byte, len(h.Pattern)*2)
+	}
+
+	for i := range p {
+		h.buff[h.i] = p[i]
+		h.i++
+		if h.i == len(h.buff) {
+			// Shift left.
+			copy(h.buff, h.buff[len(h.buff)/2:])
+			h.i = len(h.buff) / 2
+		}
+
+		if bytes.Contains(h.buff, h.Pattern) {
+			h.Match = true
+			h.done = true
+			return len(p), nil
+		}
+	}
+
+	return len(p), nil
+}